0001178879-21-000004 | 10-K | XBRL Viewer

Cover - USD ($)	12 Months Ended
Cover - USD ($)	Dec. 31, 2020	Feb. 19, 2021	Jun. 30, 2020
Cover [Abstract]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2020
Current Fiscal Year End Date	--12-31
Document Transition Report	false
Entity File Number	001-33497
Entity Registrant Name	Amicus Therapeutics, Inc.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	71-0869350
Entity Address, Address Line One	3675 Market Street,
Entity Address, City or Town	Philadelphia,
Entity Address, State or Province	PA
Entity Address, Postal Zip Code	19104
City Area Code	(215)
Local Phone Number	921-7600
Title of 12(b) Security	Common Stock, par value $0.01 per share
Trading Symbol	FOLD
Security Exchange Name	NASDAQ
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	true
Entity Shell Company	false
Entity Public Float			$ 3,851,129,284
Entity Common Stock, Shares Outstanding		264,316,074
Documents Incorporated by Reference	Portions of the Proxy Statement for the registrant's 2021 Annual Meeting of Stockholders which is to be filed subsequent to the date hereof are incorporated by reference into Part III of this Annual Report on Form 10-K.
Entity Central Index Key	0001178879
Amendment Flag	false
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 163,240	$ 142,837
Investments in marketable securities	320,029	309,903
Accounts receivable	46,923	33,284
Inventories	19,556	14,041
Prepaid expenses and other current assets	29,721	20,008
Total current assets	579,469	520,073
Operating lease right-of-use assets, less accumulated amortization of $7,574 and $5,342 at December 31, 2020 and December 31, 2019, respectively	23,296	33,315
Property and equipment, less accumulated depreciation of $14,487 and $17,604 at December 31, 2020 and December 31, 2019, respectively	43,863	47,705
In-process research & development	23,000	23,000
Goodwill	197,797	197,797
Other non-current assets	19,095	28,317
Total Assets	886,520	850,207
Current liabilities:
Accounts payable	17,063	21,722
Accrued expenses and other current liabilities	96,841	99,901
Contingent consideration payable	8,900	0
Operating lease liabilities	6,872	7,189
Total current liabilities	129,676	128,812
Deferred reimbursements	7,406	8,906
Long-term debt	389,254	149,505
Contingent consideration payable	16,925	22,681
Deferred income taxes	4,896	5,051
Operating lease liabilities	45,604	53,531
Other non-current liabilities	6,379	5,296
Total liabilities	600,140	373,782
Commitments and contingencies
Stockholders' equity:
Common stock, $0.01 par value, 500,000,000 shares authorized, 262,063,461 and 255,417,869 shares issued and outstanding at December 31, 2020 and December 31, 2019, respectively	2,650	2,598
Additional paid-in capital	2,308,578	2,227,225
Accumulated other comprehensive loss:
Foreign currency translation adjustment	8,412	2,785
Unrealized (loss) gain on available-for securities	(185)	40
Warrants	12,387	12,387
Accumulated deficit	(2,045,462)	(1,768,610)
Total stockholders' equity	286,380	476,425
Total Liabilities and Stockholders' Equity	$ 886,520	$ 850,207

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Statement of Financial Position [Abstract]
Accumulated depreciation and amortization, operating lease right-of-use assets	$ 7,574	$ 5,342
Accumulated depreciation of property and equipment	$ 14,487	$ 17,604
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	500,000,000	500,000,000
Common stock, shares issued (in shares)	262,063,461	255,417,869
Common stock, shares outstanding (in shares)	262,063,461	255,417,869

Consolidated Statements of Operations - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Statement [Abstract]
Net product sales	$ 260,886	$ 182,237	$ 91,245
Cost of goods sold	31,044	21,963	14,404
Gross profit	229,842	160,274	76,841
Operating expenses:
Research and development	308,443	286,378	270,902
Selling, general, and administrative	156,407	169,861	127,200
Changes in fair value of contingent consideration payable	3,144	3,297	3,300
Depreciation and amortization	8,846	4,775	4,216
Total operating expenses	476,840	464,311	405,618
Loss from operations	(246,998)	(304,037)	(328,777)
Other (expense) income:
Interest income	3,226	10,249	10,461
Interest expense	(22,425)	(18,872)	(22,402)
Loss on exchange of convertible notes	0	(40,624)	0
Loss on extinguishment of debt	(7,276)	0	0
Change in fair value of derivatives	0	0	(2,739)
Other expense	(781)	(2,626)	(5,632)
Loss before income tax	(274,254)	(355,910)	(349,089)
Income tax (expense) benefit	(2,598)	(478)	94
Net loss attributable to common stockholders	$ (276,852)	$ (356,388)	$ (348,995)
Net loss attributable to common stockholders per common share - basic and diluted (in dollars per share)	$ (1.07)	$ (1.48)	$ (1.88)
Weighted-average common shares outstanding - basic and diluted (in shares)	258,867,380	240,421,001	185,790,021

Consolidated Statements of Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement of Comprehensive Income [Abstract]
Net loss	$ (276,852)	$ (356,388)	$ (348,995)
Other comprehensive gain:
Foreign currency translation adjustment gain, net of tax impact of $1,552, $889, and $0, respectively	5,627	2,290	2,537
Unrealized (loss) gain on available-for-sale securities, net of tax impact of $(50), $182, and $0, respectively	(225)	467	9
Other comprehensive income	5,402	2,757	2,546
Comprehensive loss	$ (271,450)	$ (353,631)	$ (346,449)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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+ Details

Consolidated Statements of Comprehensive Loss (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement of Comprehensive Income [Abstract]
Foreign currency translation gain (loss), tax impact	$ 1,552	$ 889	$ 0
Available-for-sale securities, tax expense (benefit)	$ (50)	$ 182	$ 0

Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Cumulative Effect, Period of Adoption, Adjustment	Common Stock	Additional Paid-In Capital	Warrants	Other Comprehensive Gain (Loss)	Other Comprehensive Gain (Loss) Cumulative Effect, Period of Adoption, Adjustment	Accumulated Deficit	Accumulated Deficit Cumulative Effect, Period of Adoption, Adjustment
Balance (in shares) at Dec. 31, 2017			166,989,790
Balance at Dec. 31, 2017	$ 352,850	$ 0	$ 1,721	$ 1,400,758	$ 16,076	$ (2,095)	$ (383)	$ (1,063,610)	$ 383
Increase (Decrease) in Stockholders' Equity
Stock issued from exercise of stock options, net (in shares)			1,397,908
Stock issued from exercise of stock options, net	9,144		$ 14	9,130
Stock issued from equity financing (in shares)			20,239,839
Stock issued from equity financing	294,583		$ 202	294,381
Restricted stock tax vesting (in shares)			303,173
Restricted stock tax vesting	(2,832)			(2,832)
Stock-based compensation	29,260			29,260
Warrants exercised (in shares)			453,214
Warrants exercised	3,617		$ 5	6,625	(3,013)
Change in fair value of derivatives	2,739			2,739
Unrealized holding gain on available-for-sale securities	9					9
Foreign currency translation adjustment	2,537					2,537
Net loss	(348,995)							(348,995)
Balance (in shares) at Dec. 31, 2018			189,383,924
Balance at Dec. 31, 2018	342,912		$ 1,942	1,740,061	13,063	68		(1,412,222)
Increase (Decrease) in Stockholders' Equity
Stock issued from exercise of stock options, net (in shares)			1,967,087
Stock issued from exercise of stock options, net	11,476		$ 20	11,456
Stock issued from equity financing (in shares)			18,720,930
Stock issued from equity financing	188,994		$ 187	188,807
Restricted stock tax vesting (in shares)			477,198
Restricted stock tax vesting	(3,235)			(3,235)
Stock issued for contingent consideration (in shares)			771,804
Stock issued for contingent consideration	9,316		$ 8	9,308
Stock-based compensation	44,430			44,430
Warrants exercised (in shares)			101,787
Warrants exercised	812		$ 1	1,487	(676)
Equity component of the convertible notes (in shares)			43,995,139
Equity component of the convertible notes	215,476		$ 440	215,036
Termination of capped call confirmations	19,875			19,875
Unrealized holding gain on available-for-sale securities	467					467
Foreign currency translation adjustment	2,290					2,290
Net loss	$ (356,388)							(356,388)
Balance (in shares) at Dec. 31, 2019	255,417,869		255,417,869
Balance at Dec. 31, 2019	$ 476,425		$ 2,598	2,227,225	12,387	2,825		(1,768,610)
Increase (Decrease) in Stockholders' Equity
Stock issued from exercise of stock options, net (in shares)			5,233,672
Stock issued from exercise of stock options, net	42,282		$ 52	42,230
Restricted stock tax vesting (in shares)			1,411,920
Restricted stock tax vesting	(10,028)			(10,028)
Stock-based compensation	49,151			49,151
Unrealized holding gain on available-for-sale securities	(225)					(225)
Foreign currency translation adjustment	5,627					5,627
Net loss	$ (276,852)							(276,852)
Balance (in shares) at Dec. 31, 2020	262,063,461		262,063,461
Balance at Dec. 31, 2020	$ 286,380		$ 2,650	$ 2,308,578	$ 12,387	$ 8,227		$ (2,045,462)

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating activities
Net loss	$ (276,852)	$ (356,388)	$ (348,995)
Adjustments to reconcile net loss to net cash used in operating activities:
Amortization of debt discount and deferred financing	1,794	2,546	10,976
Depreciation and amortization	8,846	4,775	4,216
Stock-based compensation	49,151	44,430	29,260
Loss on exchange of convertible notes	0	40,624	0
Loss on extinguishment of debt	7,276	0	0
Change in fair value of derivatives	0	0	2,739
Non-cash changes in the fair value of contingent consideration payable	3,144	3,297	3,300
Foreign currency remeasurement loss	5,471	254	3,217
Non-cash deferred taxes	(155)	(1,415)	0
Other	99	(149)	59
Changes in operating assets and liabilities:
Accounts receivable	(11,219)	(11,114)	(13,294)
Inventories	(4,639)	(5,114)	(4,205)
Prepaid expenses and other current assets	(8,766)	(3,287)	2,488
Accounts payable, accrued expenses, and other current liabilities	(10,610)	46,742	17,115
Other non-current assets and liabilities	4,420	(10,117)	(581)
Deferred reimbursements	(1,250)	(5,500)	(6,250)
Net cash used in operating activities	(233,290)	(250,416)	(299,955)
Investing activities
Sale and redemption of marketable securities	354,826	499,047	463,502
Purchases of marketable securities	(365,178)	(383,882)	(578,394)
Capital expenditures	(3,227)	(20,012)	(6,308)
Net cash (used in) provided by investing activities	(13,579)	95,153	(121,200)
Financing activities
Proceeds from issuance of common stock, net of issuance costs	0	188,994	294,584
Payment of long-term debt	(155,249)	0	0
Proceeds from long-term debt, net of issuance costs	385,929	0	146,596
Payment of finance leases	(76)	(420)	(334)
Purchase of vested restricted stock units	(10,028)	(3,235)	(2,832)
Proceeds from termination of capped call confirmations	0	19,875	0
Proceeds from exercise of stock options	42,282	11,476	9,144
Proceeds from exercise of warrants	0	812	3,617
Net cash provided by financing activities	262,858	217,502	450,775
Effect of exchange rate changes on cash, cash equivalents, and restricted cash	3,830	1,729	1,518
Net increase in cash, cash equivalents, and restricted cash	19,819	63,968	31,138
Cash, cash equivalents, and restricted cash at the beginning of the year	146,343	82,375	51,237
Cash, cash equivalents, and restricted cash at the end of the year	166,162	146,343	82,375
Supplemental disclosures of cash flow information
Tenant improvements paid through lease incentive	470	19,388	0
Cash paid during the period for interest	24,683	16,966	7,500
Contingent consideration paid in shares	0	9,316	0
Capital expenditures unpaid at the end of period	985	1,865	106
Cash paid for taxes	$ 10,371	$ 691	$ 1,062

Description of Business	12 Months Ended
Description of Business	Dec. 31, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Description of Business	Description of Business Amicus Therapeutics, Inc. (the "Company") is a global, patient-dedicated biotechnology company focused on discovering, developing, and delivering novel medicines for rare diseases. The Company has a portfolio of product opportunities led by the first, oral monotherapy for Fabry disease that has achieved widespread global approval, a differentiated biologic for Pompe disease, and an industry leading rare disease gene therapy portfolio. The cornerstone of the Company's portfolio is Galafold® (also referred to as "migalastat"), the first and only approved oral precision medicine for people living with Fabry disease who have amenable genetic variants. Migalastat is currently approved under the trade name Galafold® in the United States ("U.S."), European Union ("E.U."), United Kingdom ("U.K."), and Japan, with multiple additional approvals granted and applications pending in several other geographies around the world. The lead biologics program of the Company's pipeline is Amicus Therapeutics GAA ("AT-GAA", also known as ATB200/AT2221, or cipaglucosidase alfa/miglustat), a novel, clinical-stage, potential best-in-class treatment paradigm for Pompe disease. In February 2019, the FDA granted Breakthrough Therapy designation ("BTD") to AT-GAA for the treatment of late onset Pompe disease. The Company initiated the rolling Biologic License Application ("BLA") submission to the FDA in the fourth quarter of 2020. The Company has established an industry leading gene therapy portfolio of potential therapies for people living with rare metabolic diseases, through a license with Nationwide Children's Hospital ("Nationwide Children's") and an expanded collaboration with the University of Pennsylvania ("Penn"). The Company's pipeline includes gene therapy programs in rare, neurologic lysosomal disorders ("LDs"), specifically: CLN6, CLN3, and CLN1 Batten disease, Pompe disease, Fabry disease, CDKL5 deficiency disorder ("CDD"), Mucopolysaccharidosis Type IIIB ("MPS IIIB"), as well as a next generation program in Mucopolysaccharidosis Type IIIA ("MPS IIIA"). This expanded collaboration with Penn also provides the Company with exclusive disease-specific access and option rights to develop potentially disruptive new gene therapy platform technologies and programs for most LDs and a broader portfolio of more prevalent rare diseases, including Rett Syndrome, Angelman Syndrome, Myotonic Dystrophy, and select other muscular dystrophies. In the first quarter of 2020, the FDA granted Fast Track designation to the CLN3 Batten disease gene therapy, AT-GTX-502, for the treatment of pediatric patients less than 18 years of age. In September 2020 and February 2021, the European Medicines Agency granted Priority Medicines designation and the FDA granted Fast Track Designation, respectively, to the CLN6 Batten disease gene therapy, AT-GTX-501, for the treatment of patients with variant late infantile neuronal ceroid lipofuscinosis 6 ("vLINCL6"). The Company's operations have not been significantly impacted from the novel coronavirus (“COVID-19”) pandemic in 2020; however, the Company observed increased lag times between patient identification and Galafold® initiation due to the resurgence of COVID-19 in the fourth quarter. The Company has maintained operations in all geographies, secured its global supply chain for its commercial and clinical products, and maintained the operational integrity of its clinical trials, with minimal disruption. The Company believes its ability to continue to operate without any significant disruptions will depend on the continued health of its employees, the ongoing demand for Galafold® and the continued operation of its global supply chain. The Company has continued to provide uninterrupted access to medicines for those in need of treatment, while prioritizing the health and safety of its global workforce. However, the Company's results of operations in future periods may be negatively impacted by unknown future impacts from the COVID-19 pandemic. In July 2020, the Company entered into a definitive agreement for a $400 million credit facility with Hayfin Capital Management (“Senior Secured Term Loan due 2026”) with an interest rate equal to 3-month LIBOR, subject to a 1% floor, plus 6.5% per annum and requires interest-only payments until mid-2024 and matures in six years in 2026. This transaction resulted in net proceeds of $385.9 million, after deducting fees and expenses. There were no warrants or equity conversion features associated with the Senior Secured Term Loan due 2026. Additionally, the Company used $156.3 million of the proceeds to voluntarily settle the principal amount, accrued interest, and early settlement premiums of the Senior Secured Term Loan with BioPharma Credit PLC that was due in 2023 ("Senior Secured Term Loan due 2023"). The Company had an accumulated deficit of $2.0 billion as of December 31, 2020 and anticipates incurring losses through the fiscal year ending December 31, 2021 and beyond. The Company has historically funded its operations through stock offerings, debt issuances, Galafold® revenues, collaborations, and other financing arrangements. Based on current operating models, the Company believes that the current cash position, which includes the net proceeds from the Senior Secured Term Loan due 2026 and expected revenues, is sufficient to fund the Company's operations and ongoing research programs to achieve self-sustainability. Potential impacts of the COVID-19 pandemic, business development collaborations, pipeline expansion, and investment in manufacturing capabilities could impact the Company's future capital requirements.

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2020

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

Basis of Presentation

The Company has prepared the accompanying Consolidated Financial Statements in accordance with U.S. generally accepted accounting principles ("U.S. GAAP") and include all adjustments necessary for the fair presentation of the Company's financial position for the periods presented.

Consolidation

The Consolidated Financial Statements include the accounts of the Company and its subsidiaries. Intercompany accounts and transactions are eliminated in consolidation.

Foreign Currency Transactions

The functional currency for most of the Company's foreign subsidiaries is their local currency. For non-U.S. subsidiaries that transact in a functional currency other than the U.S. dollar, assets and liabilities are translated at current rates of exchange at the balance sheet date. Income and expense items are translated at the average foreign exchange rates for the period. Adjustments resulting from the translation of the financial statements of the Company's foreign operations into U.S. dollars are excluded from the determination of net income and are recorded in accumulated other comprehensive income, a separate component of stockholders' equity.

Use of Estimates

The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements, and the reported amounts of revenues and expenses during the reporting periods. Actual results could differ from those estimates.

Additionally, the Company assessed the impact COVID-19 pandemic has had on its operations and financial results as of December 31, 2020 and through the issuance of this report. The Company’s analysis was informed by the facts and circumstances as they were known to the Company. This assessment considered the impact COVID-19 may have on financial estimates and assumptions that affect the reported amounts of assets and liabilities and revenue and expenses.

Cash, Cash Equivalents, Marketable Securities, and Restricted Cash

The Company considers all highly liquid investments purchased with a maturity of three months or less at the date of acquisition to be cash equivalents. Marketable securities consist of fixed income investments with a maturity of greater than three months and other highly liquid investments that can be readily purchased or sold using established markets. These investments are classified as available-for-sale and are reported at fair value on the Company's Consolidated Balance Sheets. Unrealized holding gains and losses are reported within comprehensive income (loss) in the Statements of Comprehensive Loss. Fair value is based on available market information including quoted market prices, broker or dealer quotations, or other observable inputs.

Restricted cash consists primarily of funds held to satisfy the requirements of certain agreements that are restricted in their use and is included in other current assets and other non-current assets on the Company's Consolidated Balance Sheets.

Concentration of Credit Risk

The Company's financial instruments that are exposed to concentration of credit risk consist primarily of cash, cash equivalents, and marketable securities. The Company maintains its cash and cash equivalents in bank accounts, which, at times, exceed federally insured limits. The Company invests its marketable securities in high-quality commercial financial instruments. The Company has not recognized any losses from credit risks on such accounts during any of the periods presented. The Company believes it is not exposed to significant credit risk on its cash, cash equivalents, or marketable securities.

The Company is subject to credit risk from its accounts receivable related to its product sales of Galafold®. The Company's accounts receivable at December 31, 2020 have arisen from product sales primarily in Europe and the U.S. The Company will periodically assess the financial strength of its customers to establish allowances for anticipated losses, if any. For accounts receivable that have arisen from named patient sales, the payment terms are predetermined, and the Company evaluates the creditworthiness of each customer on a regular basis. As of December 31, 2020, the Company recorded an allowance for doubtful accounts of $0.1 million.

Property and Equipment

Property and equipment are stated at cost, less accumulated depreciation. Depreciation is calculated over the estimated useful lives of the respective assets, which range from three to five years, or the lesser of the related initial term of the lease or useful life for leasehold improvements.

The initial cost of property and equipment consists of its purchase price and any directly attributable costs of bringing the asset to its working condition and location for its intended use. Expenditures incurred after the fixed assets have been put into operation, such as repairs and maintenance, are charged to income in the period in which the costs are incurred. Major replacements, improvements, and additions are capitalized in accordance with Company policy.

Revenue Recognition

The Company’s net product sales consist of sales of Galafold® for the treatment of Fabry disease. The Company has recorded revenue on sales where Galafold® is available either on a commercial basis or through a reimbursed early access program ("EAP"). Orders for Galafold® are generally received from distributors and pharmacies, with the ultimate payor often a government authority. In 2020, one customer accounted for 25% of net product sales and 12% of accounts receivable from product sales. In 2019, one customer accounted for 22% of net product sales and 11% of accounts receivable from product sales.

The Company recognizes revenue when its performance obligations to its customers have been satisfied, which occurs at a point in time when the pharmacies or distributors obtain control of Galafold®. The transaction price is determined based on fixed consideration in the Company's customer contracts and is recorded net of estimates for variable consideration, which are third party discounts and rebates. The identified variable consideration is recorded as a reduction of revenue at the time revenue from the sale of Galafold® is recognized. The Company recognizes revenue to the extent that it is probable that a significant revenue reversal will not occur in a future period. These estimates may differ from actual consideration received. The Company evaluates these estimates each reporting period to reflect known changes.

The following table summarizes the Company's net product sales from Galafold® disaggregated by geographic area:


	For the Year
(in thousands)	2020		2019		2018
U.S.	$	80,046	$	52,478	$	7,223
Ex-U.S.	180,840		129,759		84,022
Total net product sales	$	260,886	$	182,237	$	91,245

Inventories and Cost of Goods Sold

Inventories are stated at the lower of cost and net realizable value, determined by the first-in, first-out method. Inventories are reviewed periodically to identify slow-moving or obsolete inventory based on projected sales activity as well as product shelf-life. In evaluating the recoverability of inventories produced, the probability that revenue will be obtained from the future sale of the related inventory is considered and inventory value is written down for inventory quantities in excess of expected requirements. Expired inventory is disposed of and the related costs are recognized as cost of goods sold in the Consolidated Statements of Operations.

Cost of goods sold includes the cost of inventory sold, manufacturing and supply chain costs, product shipping and handling costs, provisions for excess and obsolete inventory, as well as royalties payable.

Fair Value Measurements

The Company records certain asset and liability balances under the fair value measurements as defined by the FASB guidance. Current FASB fair value guidance emphasizes that fair value is a market-based measurement, not an entity-specific measurement. Therefore, a fair value measurement should be determined based on the assumptions that market participants would use in pricing the asset or liability. As a basis for considering market participant assumptions in fair value measurements, current FASB guidance establishes a fair value hierarchy that distinguishes between market participant assumptions based on market data obtained from sources independent of the reporting entity (observable inputs that are classified within Levels 1 and 2 of the hierarchy) and the reporting entity's own assumptions that market participants assumptions would use in pricing assets or liabilities (unobservable inputs classified within Level 3 of the hierarchy).

Level 1 inputs utilize quoted prices (unadjusted) in active markets for identical assets or liabilities that the Company has the ability to access at measurement date. Level 2 inputs are inputs other than quoted prices included in Level 1 that are observable for the asset or liability, either directly or indirectly. Level 2 inputs may include quoted prices for similar assets and liabilities in active markets, as well as inputs that are observable for the asset or liability (other than quoted prices), such as interest rates, foreign exchange rates, and yield curves that are observable at commonly quoted intervals. Level 3 inputs are unobservable inputs for the asset or liability, which is typically based on an entity's own assumptions, as there is little, if any, related market activity. In instances where the determination of the fair value measurement is based on inputs from different levels of the fair value hierarchy, the level in the fair value hierarchy within which the entire fair value measurement falls is based on the lowest level input that is significant to the fair value measurement in its entirety. The Company's assessment of the significance of a particular input to the fair value measurement in its entirety requires judgment and considers factors specific to the asset or liability.

Contingent Liabilities

On an ongoing basis, the Company may be involved in various claims and legal proceedings. On a quarterly basis, the Company reviews the status of each significant matter and assesses its potential financial exposure. If the potential loss from any claim, asserted or unasserted, or legal proceeding is considered probable and the amount can be reasonably estimated, the Company will accrue a liability for the estimated loss. Because of uncertainties related to claims and litigation, accruals will be based on the Company's best estimates based on available information. On a periodic basis, as additional information becomes available, or based on specific events such as the outcome of litigation or settlement of claims, the Company may reassess the potential liability related to these matters and may revise these estimates, which could result in material adverse adjustments to the Company's operating results.

Research and Development Costs

Research and development costs are expensed as incurred. Research and development expense consist primarily of costs related to personnel, including salaries and other personnel related expenses, consulting fees, and the cost of facilities and support services used in drug development. Assets acquired that are used for research and development and have no future alternative use are expensed as in-process research and development.

Interest Income and Interest Expense

Interest income consists of interest earned on the Company's cash, cash equivalents, and marketable securities. Interest expense consists of interest incurred on debt and finance leases.

Income Taxes

The Company accounts for income taxes under the liability method. Under this method deferred income tax liabilities and assets are determined based on the difference between the financial statement carrying amounts and tax basis of assets and liabilities and for operating losses and tax credit carry forwards, using enacted tax rates in effect in the years in which the differences are expected to reverse. A valuation allowance is recorded if it is "more likely than not" that a portion or all of a deferred tax asset will not be realized.

Other Comprehensive Income (Loss)

Components of other comprehensive income (loss) include unrealized gains and losses on available-for-sale securities and gain (loss) on foreign currency transactions and are included in the Statements of Comprehensive Loss.

Leases

The Company primarily enters into lease agreements for office space, equipment, and vehicles. The leases have varying terms, some of which could include options to renew, extend, and early terminate. The Company determines if an arrangement is a lease at contract inception. Operating leases are included in right-of-use ("ROU") assets and lease liabilities on the Consolidated Balance Sheets.

ROU assets represent the Company's right to control the use of an explicitly or implicitly identified fixed asset for a period of time and lease liabilities represent the Company's obligation to make lease payments arising from the lease. Control of an underlying asset is conveyed to the Company if the Company obtains the rights to direct the use of and to obtain substantially all of the economic benefits from using the underlying asset. ROU assets and liabilities are recognized at commencement date based on the present value of lease payments over the lease term. The Company uses its incremental borrowing rate based on the information available at commencement date in determining the present value of lease payments.

Lease payments included in the measurement of the lease liability are comprised of fixed payments. Variable lease payments are excluded from the ROU asset and lease liability and are recognized in the period in which the obligation for those payments is incurred. Variable lease payments are presented in the Consolidated Statements of Operations in the same line item as expenses arising from fixed lease payments for operating leases. The Company has lease agreements that include lease and non-lease components, which the Company accounts for as a single lease component for all underlying asset categories.

The lease term for all of the Company's leases include the non-cancellable period of the lease plus any additional periods covered by either a Company option to extend (or not to terminate) the lease that the Company is reasonably certain to exercise, or an option to extend (or not to terminate) the lease controlled by the lessor.

Leases with an initial term of 12 months or less are not recorded on the Consolidated Balance Sheets. The Company recognizes lease expense for these leases on a straight-line basis over the lease term. The Company applies this policy to all underlying asset categories.

The information presented for the periods prior to January 1, 2019 has not been restated and is reported under the accounting standard in effect for those periods. For additional information, see " -Note 13. Leases".

Nonqualified Cash Deferral Plan

The Company's Cash Deferral Plan (the "Deferral Plan"), provides certain key employees and members of the Board of Directors as selected by the Compensation Committee of the Board of Directors of the Company (the "Compensation Committee"), with an opportunity to defer the receipt of such participant's base salary, bonus, and director's fees, as applicable. The Deferral Plan is intended to be a nonqualified deferred compensation plan that complies with the provisions of Section 409A of the Internal Revenue Code (the "Code"). All of the investments held in the Deferral Plan are classified as investments held-to-maturity and recorded at fair value with changes in the investments' fair value recognized as earnings in the period they occur. The corresponding liability for the Deferral Plan is included in other non-current liability in the Consolidated Balance Sheets.

Equity-based Compensation

At December 31, 2020, the Company had one equity-based employee compensation plan, which is described more fully in "— Note 9. Stockholders' Equity." The Company applies the fair value method of measuring equity-based compensation, which requires a public entity to measure the cost of employee services received in exchange for an award of equity instruments based on the grant-date fair value of the award.

Loss per Common Share

The Company calculates net loss per share as a measurement of the Company's performance while giving effect to all dilutive potential common shares that were outstanding during the reporting period. The Company had a net loss for all periods presented; accordingly, the inclusion of common stock options, unvested RSUs, and warrants would be anti-dilutive. Therefore, the weighted average shares used to calculate both basic and diluted earnings per share are the same. See "— Note 17. Basic and Diluted Net Loss per Common Share" for further discussion on net loss per share.

Segment Information

The Company currently operates in one business segment focused on the discovery, development, and commercialization of advanced therapies to treat a range of devastating rare and orphan diseases. The Company is not organized by market and is managed and operated as one business. A single management team reports to the chief operating decision maker who comprehensively manages the entire business. The Company does not operate any separate lines of business or separate business entities with respect to its products. Accordingly, the Company does not accumulate discrete financial information with respect to separate service lines, and thus there is one reporting unit.

Business Combinations

The Company assigns fair value to the tangible and intangible assets acquired and liabilities assumed based upon their estimated fair values on the acquisition date from acquired businesses. The purchase price allocation process requires management to make significant estimates and assumptions, especially at the acquisition date with respect to intangible assets and in-process research and development ("IPR&D"). In connection with the purchase price allocations for acquisitions, the Company estimates the fair value of contingent payments utilizing a probability-based income approach inclusive of an estimated discount rate.

Contingent Consideration Payable

Contingent consideration payments in asset acquisitions are recognized when the contingency is resolved and the consideration is paid or becomes payable. This does not apply in circumstances when the contingent consideration meets the definition of a derivative, in which case the amount becomes part of the basis in the asset acquired. Upon recognition of the contingent consideration payment, the amount is included in the cost of the acquired asset or group of assets. The Company determines the fair value of contingent acquisition consideration payable on the acquisition date using a probability-based income approach utilizing an appropriate discount rate. The short term and long term portions of the contingent consideration payable is shown on the Company's Consolidated Balance Sheets. The fair value of the contingent consideration payable will be determined each period end and the resulting change will be recorded on the Consolidated Statements of Operations.

Intangible Assets and Goodwill

The Company records goodwill in a business combination when the total consideration exceeds the fair value of the net tangible and identifiable intangible assets acquired. Purchased IPR&D is accounted for as an indefinite lived intangible asset until the underlying project is completed, at which point the intangible asset will be accounted for as a definite lived intangible asset, or abandoned, at which point the intangible asset will be written off or partially impaired. Goodwill and indefinite lived intangible assets are assessed annually for impairment and whenever events or circumstances indicate that the carrying amount of an asset may not be recoverable. If it is determined that the full carrying amount of an asset is not recoverable, an impairment loss is recorded in the amount by which the carrying amount of the asset exceeds its fair value. No indicators of impairment were noted during the years ended December 31, 2020 and December 31, 2019.

Recent Accounting Developments - Guidance Adopted in 2020

ASU 2018-15 - In August 2018, the Financial Accounting Standards Board ("FASB") issued Accounting Standards Update ("ASU") 2018-15, Intangibles-Goodwill and Other-Internal-Use Software (Subtopic 350-40): ("ASU 2018-15"), relating to a customer’s accounting for implementation, set-up, and other upfront costs incurred in a cloud computing arrangement that is hosted by a vendor. Under the new guidance, a customer will apply the same criteria for capitalizing implementation costs as it would for an arrangement that has a software license. The new guidance does not affect the accounting for the service element of a hosting arrangement that is a service contract. The new guidance also prescribes the balance sheet, income statement and cash flow classification of the capitalized software costs and related amortization expense and requires additional quantitative and qualitative disclosures. ASU 2018-15 is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019 for public companies. The Company adopted this guidance on January 1, 2020. The adoption did not have a material impact on the Company's Consolidated Financial Statements or related disclosures.

ASU 2018-13 - In August 2018, the FASB issued ASU 2018-03, Fair Value Measurement (Topic 820): Disclosure Framework-Changes to the Disclosure Requirements for Fair Value Measurement ("ASU 2018-13"). The amendments modify the disclosure requirements in Topic 820. ASU 2018-13 is effective for all entities for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. The amendments on (i) changes in unrealized gains and losses, (ii) the range and weighted average of significant unobservable inputs used to develop Level 3 fair value measurements, and (iii) the narrative description of measurement uncertainty should be applied prospectively for only the most recent interim or annual period presented in the initial fiscal year of adoption. All other amendments should be applied retrospectively to all periods presented upon their effective date. The Company adopted this guidance on January 1, 2020. The adoption did not have a material impact on the Company's Consolidated Financial Statements or related disclosures.

ASU 2017-04 - In January 2017, the FASB issued ASU 2017-04, Intangibles - Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment ("ASU 2017-04"). ASU 2017-04 simplifies the recognition and measurement of a goodwill impairment loss by eliminating Step 2 of the quantitative goodwill impairment test. The guidance requires a one-step impairment test in which an entity compares the fair value of a reporting unit with its carrying amount and recognizes an impairment charge for the amount by which the carrying amount exceeds the reporting unit's fair value, if any. ASU 2017-04 is effective for fiscal years beginning after December 15, 2019 and should be applied on a prospective basis. The Company adopted this guidance on January 1, 2020. The adoption did not have a material impact on the Company's Consolidated Financial Statements or related disclosures.

ASU 2016-13 - In June 2016, the FASB issued ASU 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments ("ASU 2016-13"). ASU 2016-13 requires financial assets measured at amortized cost basis to be presented at the net amount expected to be collected and amends guidance on the impairment of financial instruments. ASU 2016-13 is effective for public companies who are SEC filers for fiscal years beginning after December 15, 2019, including interim periods within those years. The Company adopted this guidance on January 1, 2020. The adoption did not have a material impact on the Company's Consolidated Financial Statements or related disclosures.

Recent Accounting Developments - Guidance Not Yet AdoptedASU 2019-12 - In December 2019, the FASB issued ASU 2019-15, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes ("ASU 2019-12"). This new guidance removes specific exceptions to the general principles in Topic 740. It eliminates the need for an organization to analyze whether the following apply in a given period: (i) exception to the incremental approach for intraperiod tax allocation; (ii) exceptions to accounting for basis differences when there are ownership changes in foreign investments; and (iii) exception in interim period income tax accounting for year-to-date losses that exceed anticipated losses. ASU 2019-12 also improves financial statement preparers’ application of income tax-related guidance and simplifies the following: (i) franchise taxes that are partially based on income; (ii) transactions with a government that result in a step up in the tax basis of goodwill; (iii) separate financial statements of legal entities that are not subject to tax; and (iv) enacted changes in tax laws in interim periods. ASU 2019 is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2020. Early adoption is permitted for public business entities for periods for which financial statements have not yet been issued. The Company will adopt this guidance prospectively on January 1, 2021, and it believes the adoption of this guidance will not have a material impact on its Consolidated Financial Statements or related disclosures

Acquisitions	12 Months Ended
Acquisitions	Dec. 31, 2020
Business Combinations [Abstract]
Acquisitions	Acquisitions Acquisition of Celenex In September 2018, the Company expanded its pipeline by acquiring the rights and related intellectual property of ten gene therapy programs through its acquisition of Celenex, Inc., ("Celenex"). Celenex is a private, clinical stage gene therapy company whose lead programs are ten gene therapy programs including CLN6 and CLN3, which are in clinical stage, and several programs in pre-clinical stage. Pursuant to the terms of the agreement, the Company agreed to pay up to an additional $10 million in connection with the achievement of certain development milestones, $220 million in connection with the achievement of certain regulatory approval milestones across multiple programs and up to $75 million in tiered sales milestone payments. Celenex has an exclusive license agreement with Nationwide Children’s Hospital ("Nationwide Children’s"). Under this license agreement, Nationwide Children’s is eligible to receive development and sales-based milestones of up to $7.8 million for each product. The Company evaluated the Celenex transaction and concluded that the transaction did not meet the definition of a business and was an asset acquisition. Given the fact that the license has no alternative future use, the $100 million upfront payment was expensed to research and development expense in the Consolidated Statements of Operations for the year ended December 31, 2018.

Goodwill and Intangible Assets

12 Months Ended

Dec. 31, 2020

Goodwill and Intangible Assets Disclosure [Abstract]

Goodwill and Intangible Assets

As of December 31, 2020, in connection with the acquisitions, the Company had goodwill of $197.8 million and IPR&D of $23.0 million, which are reported at fair value on the Company's Consolidated Balance Sheets. Intangible assets related to IPR&D assets are considered to be indefinite-lived until the completion or abandonment of the associated research and development efforts. During the period the assets are considered indefinite-lived, they will not be amortized but will be tested for impairment on an annual basis and between annual tests if the Company becomes aware of any events occurring or changes in circumstances that would indicate a reduction in the fair value of the IPR&D assets below their respective carrying amounts.

Goodwill and intangible assets are assessed annually for impairment on October 1 and whenever events or circumstances indicate that the carrying amount of an asset may not be recoverable. If it is determined that the full carrying amount of an asset is not recoverable, an impairment loss is recorded in the amount by which the carrying amount of the asset exceeds its fair value.

The following table represents the changes in IPR&D for the years ended December 31, 2020 and 2019, respectively:


	(in millions)
Balance at December 31, 2018	$	23.0
Change in IPR&D	—
Balance at December 31, 2019	$	23.0
Change in IPR&D	—
Balance at December 31, 2020	$	23.0

The following table represents the changes in Goodwill for the years ended December 31, 2020 and 2019, respectively:


	(in millions)
Balance at December 31, 2018	$	197.8
Change in goodwill	—
Balance at December 31, 2019	$	197.8
Change in goodwill	—
Balance at December 31, 2020	$	197.8

Cash, Cash Equivalents, Marketable Securities, and Restricted Cash

12 Months Ended

Dec. 31, 2020

Cash, Cash Equivalents, and Short-term Investments [Abstract]

Cash, Cash Equivalents, Marketable Securities, and Restricted Cash

As of December 31, 2020, the Company held $163.2 million in cash and cash equivalents and $320.0 million of marketable securities which are reported at fair value on the Company's Consolidated Balance Sheets. Unrealized holding gains and losses are generally reported within accumulated other comprehensive loss in the Statements of Comprehensive Loss. If a decline in the fair value of a marketable security below the Company's cost basis is determined to be other-than-temporary or if an available-for-sale debt security’s fair value is determined to less than the amortized cost and the Company intends or is more than likely to sell the security before recovery and it is not considered a credit loss, such security is written down to its estimated fair value as a new cost basis and the amount of the write-down is included in earnings as an impairment charge. If the unrealized loss of an available-for-sale debt security is determined to be a result of credit loss the Company would recognize an allowance and the corresponding credit loss would be included in earnings.

The Company regularly invests excess operating cash in deposits with major financial institutions, money market funds, notes issued by the U.S. government, as well as fixed income investments and U.S. bond funds, both of which can be readily purchased and sold using established markets. The Company believes that the market risk arising from its holdings of these financial instruments is mitigated as many of these securities are either government backed or of the highest credit rating. Investments that have original maturities greater than three months but less than one year are classified as current.

Cash, cash equivalents, and marketable securities are classified as current unless mentioned otherwise below and consisted of the following:


	As of December 31, 2020
(in thousands)	Cost		Gross Unrealized Gain		Gross Unrealized Loss		Fair Value
Cash and cash equivalents	$	163,240	$	—	$	—	$	163,240
Corporate debt securities	39,525		4		(16)		39,513
Commercial paper	217,087		14		(6)		217,095
Asset-backed securities	9,420		18		—		9,438
U.S. government agency bonds	53,583		3		(4)		53,582
Money market	350		—		—		350
Certificates of deposit	51		—		—		51
	$	483,256	$	39	$	(26)	$	483,269
Included in cash and cash equivalents	$	163,240	$	—	$	—	$	163,240
Included in marketable securities	320,016		39		(26)		320,029
Total cash, cash equivalents, and marketable securities	$	483,256	$	39	$	(26)	$	483,269


	As of December 31, 2019
(in thousands)	Cost		Gross Unrealized Gain		Gross Unrealized Loss		Fair Value
Cash and cash equivalents	$	142,837	$	—	$	—	$	142,837
Corporate debt securities	145,875		121		(5)		145,991
Commercial paper	73,659		53		(2)		73,710
Asset-backed securities	77,731		79		—		77,810
U.S. government agency bonds	11,999		2		(10)		11,991
Money market	350		—		—		350
Certificates of deposit	51		—		—		51
	$	452,502	$	255	$	(17)	$	452,740
Included in cash and cash equivalents	$	142,837	$	—	$	—	$	142,837
Included in marketable securities (1)	309,665		255		(17)		309,903
Total cash, cash equivalents, and marketable securities	$	452,502	$	255	$	(17)	$	452,740

______________________________

(1) As of December 31, 2019, $9.5 million of marketable securities had maturity dates greater than 12 months and were available to convert into cash, if needed.

For the year ended December 31, 2020 there were nominal realized gains. For the fiscal year ended December 31, 2019, there were no realized gains or losses. The cost of securities sold is based on the specific identification method.

Unrealized loss positions in the marketable securities as of December 31, 2020 and 2019 reflect temporary impairments and are not a result of credit loss. Additionally, as these positions have been in a loss position for less than twelve months and the Company does not intend to sell these securities before recovery, the losses are recognized in other comprehensive gain (loss). The fair value of these marketable securities in unrealized loss positions was $124.9 million and $42.6 million as of December 31, 2020 and December 31, 2019, respectively.

The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the Consolidated Balance Sheets that sum to the total of the same such amounts shown in the Consolidated Statements of Cash Flows.


(in thousands)	December 31, 2020		December 31, 2019		December 31, 2018
Cash and cash equivalents	$	163,240	$	142,837	$	79,749
Restricted cash	2,922		3,506		2,626
Cash, cash equivalents, and restricted cash shown in the Consolidated Statements of Cash Flows	$	166,162	$	146,343	$	82,375

Inventories

12 Months Ended

Dec. 31, 2020

Inventory Disclosure [Abstract]

Inventories

Inventories consist of raw materials, work in process, and finished goods related to the manufacture of Galafold®. The following table summarizes the components of inventories:


(in thousands)	December 31, 2020		December 31, 2019
Raw materials	$	5,547	$	6,544
Work-in-process	7,693		3,660
Finished goods	6,316		3,837
Total inventories	$	19,556	$	14,041

The Company recorded a reserve for inventory of $0.1 million and $0.2 million as of December 31, 2020 and December 31, 2019, respectively.

Property and Equipment

12 Months Ended

Dec. 31, 2020

Property, Plant and Equipment [Abstract]

Property and Equipment

Property and equipment consist of the following:


	December 31,
(in thousands)	2020		2019
Property and equipment consist of the following:
Computer equipment	$	5,546	$	6,185
Computer software	1,207		1,607
Research equipment	15,387		14,568
Furniture and fixtures	2,880		5,157
Leasehold improvements	24,146		30,294
Vehicles	124		124
Land	3,190		3,190
Construction in progress	5,870		4,184
Gross property and equipment	58,350		65,309
Less accumulated depreciation	(14,487)		(17,604)
Net property and equipment	$	43,863	$	47,705

Depreciation expense was $8.4 million and $4.5 million for the years ended December 31, 2020 and December 31, 2019, respectively.

Accrued Expenses and Other Current Liabilities

12 Months Ended

Dec. 31, 2020

Payables and Accruals [Abstract]

Accrued Expenses and Other Current Liabilities

Accrued expenses and other current liabilities consist of the following:


	December 31,
(in thousands)	2020		2019
Accrued professional fees	$	3,700	$	7,832
Accrued contract manufacturing & contract research costs	19,617		17,800
Accrued compensation and benefits	23,025		27,156
Accrued program fees	16,599		17,016
Accrued royalties	6,550		5,196
Accrued interest	—		3,679
Accrued milestones	—		4,000
Accrued sales rebates and discounts	12,079		5,900
Accrued taxes	11,177		7,300
Deferred reimbursements	1,500		1,250
Other	2,594		2,772
	$	96,841	$	99,901

Stockholders' Equity	12 Months Ended
Stockholders' Equity	Dec. 31, 2020
Equity [Abstract]
Stockholders' Equity	Stockholders' Equity Common Stock and Warrants As of December 31, 2020, the Company was authorized to issue 500 million shares of common stock. Dividends on common stock will be paid when, and if, declared by the board of directors. Each holder of common stock is entitled to vote on all matters that are appropriate for stockholder voting and is entitled to one vote for each share held. In February 2021, 1,050,000 warrants were exercised at $7.06 per share of common stock resulting in gross cash proceeds of $7.4 million. During the second quarter of 2019, the Company completed an underwritten equity offering and issued 18.7 million shares of its common stock at $10.75 per share, inclusive of the fully exercised option to purchase additional shares from the initial offering. This transaction resulted in net proceeds of $189.0 million, after deducting underwriting discounts and commissions and offering expenses. In March 2019, 101,787 warrants were exercised at $7.98 per share of common stock resulting in gross cash proceeds of $0.8 million. As discussed in "— Note 11. Assets and Liabilities Measured at Fair Value", during the fourth quarter of 2018, the Company reached a clinical milestone, which was the dosing of the first patient in a Phase 3 study, related to the contingent consideration from the acquisition of Callidus Biopharma, Inc. ("Callidus"). The milestone for this event was $9.0 million, which was paid in Company common stock in the first quarter of 2019 and resulted in a $9.3 million impact on stockholder's equity. As discussed in ''— Note 12. Debt'', during the first and second quarters of 2019, the Company entered into separate, privately negotiated Exchange Agreements with a limited number of holders of the Convertible Notes. Under the terms of the Exchange Agreements, the holders of the Convertible Notes agreed to exchange an aggregate principal amount of $247.2 million of Convertible Notes held by them in exchange for an aggregate of approximately 44.0 million shares of Company's common stock, par value $0.01 per share. In February 2018, the Company completed an underwritten offering of 20.2 million shares of its common stock at $15.50 per share, inclusive of the fully exercised option to purchase additional shares from the initial offering. This transaction resulted in net proceeds of $294.6 million, after deducting underwriting discounts and commissions and offering expenses. In April 2018, 453,214 warrants were exercised at $7.98 per share of common stock resulting in gross cash proceeds of $3.6 million. In June 2018, the Company’s stockholders approved an amendment to the Company’s Restated Certificate of Incorporation to increase the number of shares of common stock, par value $0.01 per share, that the Company is authorized to issue from 250 million shares to 500 million shares. Nonqualified Cash Plan The Company's Deferral Plan, (the "Deferral Plan") provides certain key employees and members of the Board of Directors as selected by the Compensation Committee, with an opportunity to defer the receipt of such participant's base salary, bonus, and director's fees, as applicable. The Deferral Plan is intended to be a nonqualified deferred compensation plan that complies with the provisions of Section 409A of the Internal Revenue Code of 1986 as amended. The Company had a deferred compensation investment balance of $4.1 million and $4.4 million as of December 31, 2020 and December 31, 2019, respectively, with corresponding approximate amounts of liability. Deferral Plan investment assets are classified as trading securities and are recorded at fair value with changes in the investments' fair value recognized in AOCI in the period they occur. Deferred compensation liability amounts under the Deferral Plan are included in other long-term liabilities. Equity Incentive Plan The Company's Amended and Restated 2007 Equity Incentive Plan (the "Plan") provides for the granting of restricted stock units and options to purchase common stock in the Company to employees, directors, advisors, and consultants at a price to be determined by the Company's Board of Directors. The Plan is intended to encourage ownership of stock by employees and consultants of the Company and to provide additional incentives for them to promote the success of the Company's business. Under the provisions of the Plan, no option will have a term in excess of 10 years. The Board of Directors, or its committee, is responsible for determining the individuals to be granted options, the number of options each individual will receive, the option price per share, and the exercise period of each option. Options granted pursuant to the Plan generally vest 25% on the first year anniversary date of grant plus an additional 1/48th for each month thereafter and may be exercised in whole or in part for 100% of the shares vested at any time after the date of grant. As of December 31, 2020, the Company has reserved up to 15,018,368 shares for issuance under the Plan.

Share based Compensation

12 Months Ended

Dec. 31, 2020

Share-based Payment Arrangement [Abstract]

Share based Compensation

Share based Compensation The Company’s Equity Incentive Plan provides for the granting of restricted stock units and options to purchase common stock in the Company to employees, directors. advisors, and consultants at a price to be determined by the Company’s Board of Directors. The Plan is intended to encourage ownership of stock by employees and consultants of the Company and to provide additional incentives for them to promote the success of the Company’s business. The Board of Directors, or its committee, is responsible for determining the individuals to be granted options, the number of options each individual will receive, the option price per share, and the exercise period of each option.

On December 21, 2018, the Board of Directors of the Company approved an amendment (the "Amendment") to the Plan. The Amendment provides for certain benefits to qualifying Plan Participants who separate from service with the Company due to death, disability or "Retirement" (as such term is defined under the Plan) ("Qualified Participants"). Options granted under the Plan ("Options") to a Qualified Participant shall continue to vest until the 2nd anniversary of the Qualified Participant’s separation and all vested Options held by such Qualified Participant shall remain exercisable until the earlier of the 4th anniversary of the Qualified Participant’s separation or the original expiration date of the Option. Options that are not exercised during this exercise period shall be forfeited. Time-based restricted stock units and restricted stock granted to a Qualified Participant under the Plan that was scheduled to vest within the two year period following the Qualified Participant's separation shall accelerate and be delivered upon such separation. Any time-based restricted stock units or restricted stock that would have vested after such two year period will be forfeited upon the Qualified Participant's separation. Also, per the Amendment, any performance-based restricted stock units under the Plan ("PRSUs") received by the Qualified Participant, shall remain eligible to vest after the Qualified Participant’s separation based on the actual performance of the Company through the end of the performance period applicable to any such PRSUs.

Stock Option Grants

The Company uses the fair value method of measuring stock-based compensation, using the fair value of each equity award granted. The Company chose the ‘‘straight-line’’ attribution method for allocating compensation costs and recognized the fair value of each stock option on a straight-line basis over the vesting period of the related awards.

The Company uses the Black-Scholes option pricing model when estimating the grant date fair value for stock-based awards. Use of a valuation model requires management to make certain assumptions with respect to selected model inputs. Expected volatility was based on our historical volatility since our initial public offering in May 2007. The average expected life is determined using our actual historical data. The risk-free interest rate is based on U.S. Treasury, zero-coupon issues with a remaining term equal to the expected life assumed at the date of grant. Forfeitures are estimated based on voluntary termination behavior, as well as a historical analysis of actual option forfeitures.

The fair value of the stock options granted is estimated on the date of grant using a Black-Scholes option pricing model with the following weighted-average assumptions:


	Years Ended December 31,
	2020			2019			2018
Expected stock price volatility	75.1		%	74.1		%	78.6		%
Risk free interest rate	1.6		%	2.4		%	2.4		%
Expected life of options (years)	5.67			5.68			5.62
Expected annual dividend per share	$	0.00		$	0.00		$	0.00

The weighted average grant-date fair value per share of options granted during 2020, 2019, and 2018 were $6.40, $6.67, and $10.19, respectively.

A summary of the Company's stock options for the year ended December 31, 2020 were as follows:


	Number of Shares	Weighted Average Exercise Price		Weighted Average Remaining Years	Aggregate Intrinsic Value
	(in thousands)				(in millions)
Options outstanding, December 31, 2017	15,181	$	7.48
Granted	2,348	$	14.96
Exercised	(1,398)	$	6.54
Forfeited	(313)	$	9.55
Expired	(8)	$	10.76
Options outstanding, December 31, 2018	15,810	$	8.63
Granted	4,091	$	10.29
Exercised	(1,967)	$	5.83
Forfeited	(879)	$	11.07
Expired	(331)	$	13.38
Options outstanding, December 31, 2019	16,724	$	9.15
Granted	4,362	$	9.98
Exercised	(5,243)	$	8.11
Forfeited	(1,376)	$	10.42
Expired	(435)	$	13.33
Options outstanding, December 31, 2020	14,032	$	9.54	6.5	$	190.1
Vested and unvested expected to vest, December 31, 2020	13,267	$	9.49	6.4	$	180.5
Exercisable at December 31, 2020	8,130	$	8.86	5.1	$	115.7

The aggregate intrinsic value of options exercised during the years ended December 31, 2020, 2019 and 2018 was $40.9 million, $11.8 million, and $11.9 million respectively. Cash proceeds from stock options exercised during the years ended December 31, 2020, 2019, and 2018 were $42.3 million, $11.5 million, and $9.1 million, respectively. As of December 31, 2020, the total unrecognized compensation cost related to non-vested stock options granted was $29.4 million and is expected to be recognized over a weighted average period of three years.

Restricted Stock Units and Performance-Based Restricted Stock Units (collectively "RSUs")

RSUs awarded under the Plan are generally subject to graded vesting and are contingent on an employee's continued service. RSUs are generally subject to forfeiture if employment terminates prior to the release of vesting restrictions. The Company expenses the cost of the RSUs, which is determined to be the fair market value of the shares of common stock underlying the RSUs at the date of grant, ratably over the period during which the vesting restrictions lapse. A summary of non-vested RSU activity under the Plan for the year ended December 31, 2020 is as follows:


	Number of Shares	Weighted Average Grant Date Fair Value		Weighted Average Remaining Years	Aggregate Intrinsic Value
	(in thousands)				(in millions)
Non-vested units as of December 31, 2017	2,575	$	5.85
Granted	1,812	$	16.11
Vested	(530)	$	6.01
Forfeited	(145)	$	9.65
Non-vested units as of December 31, 2018	3,712	$	10.59
Granted	3,526	$	10.92
Vested	(921)	$	8.49
Forfeited	(525)	$	10.52
Non-vested units as of December 31, 2019	5,792	$	11.18
Granted	4,692	$	11.29
Vested	(2,164)	$	10.70
Forfeited	(1,240)	$	11.14
Non-vested units as of December 31, 2020	7,080	$	11.35	2.3	$	158.0

For the year ended December 31, 2020, 2,163,732 RSUs have vested and all non-vested units are expected to vest over their normal term. As of December 31, 2020, there was $59.4 million of total unrecognized compensation cost related to unvested RSUs with service-based vesting conditions. These costs are expected to be recognized over a weighted average period of two years.

Compensation Expense Related to Equity Awards

The following table summarizes information related to compensation expense recognized in the Consolidated Statements of Operations related to the equity awards:


	Years Ended December 31,
(in thousands)	2020		2019		2018
Equity compensation expense recognized in:
Research and development expense	$	20,817	$	17,575	$	11,740
Selling, general, and administrative expense	28,334		26,855		17,520
Total equity compensation expense	$	49,151	$	44,430	$	29,260

Assets and Liabilities Measured at Fair Value

12 Months Ended

Dec. 31, 2020

Fair Value Disclosures [Abstract]

Assets and Liabilities Measured at Fair Value

The Company's financial assets and liabilities are measured at fair value and classified within the fair value hierarchy which is defined as follows:

Level 1 — Quoted prices in active markets for identical assets or liabilities that the Company has the ability to access at the measurement date.

Level 2 — Inputs other than quoted prices in active markets that are observable for the asset or liability, either directly or indirectly.

Level 3 — Inputs that are unobservable for the asset or liability.

A summary of the fair value of the Company's recurring assets and liabilities aggregated by the level in the fair value hierarchy within which those measurements fall as of December 31, 2020 are identified in the following tables:


(in thousands)	Level 2		Total
Assets:
Commercial paper	$	217,095	$	217,095
Asset-backed securities	9,438		9,438
Corporate debt securities	39,513		39,513
U.S. government agency bonds	53,582		53,582
Money market funds	4,427		4,427
	$	324,055	$	324,055


(in thousands)	Level 2		Level 3		Total
Liabilities:
Contingent consideration payable	$	—	$	25,825	$	25,825
Deferred compensation plan liability	4,078		—		4,078
	$	4,078	$	25,825	$	29,903


(in thousands)	Level 2		Total
Assets:
Commercial paper	$	73,710	$	73,710
Asset-backed securities	77,810		77,810
Corporate debt securities	145,991		145,991
U.S. government agency bonds	11,991		11,991
Money market funds	4,768		4,768
	$	314,270	$	314,270


(in thousands)	Level 2		Level 3		Total
Liabilities:
Contingent consideration payable	$	—	$	22,681	$	22,681
Deferred compensation plan liability	4,419		—		4,419
	$	4,419	$	22,681	$	27,100

The Company’s Convertible Notes fall into the Level 2 category within the fair value level hierarchy. The fair value was determined using broker quotes in a non-active market for valuation. The fair value of the Convertible Notes at December 31, 2020 was $10.9 million.

The Company’s Senior Secured Term Loan due 2026 falls into the Level 2 category within the fair value level hierarchy and the fair value was determined using quoted prices for similar liabilities in active markets, as well as inputs that are observable for the liability (other than quoted prices), such as interest rates that are observable at commonly quoted intervals. The carrying value of the Senior Secured Term Loan due 2026 approximates the fair value.

The Company did not have any Level 3 assets as of December 31, 2020 or December 31, 2019.

Cash, Money Market Funds and Marketable Securities

The Company classifies its cash within the fair value hierarchy as Level 1 as these assets are valued using quoted prices in an active market for identical assets at the measurement date. The Company considers its investments in marketable securities as available-for-sale and classifies these assets and the money market funds within the fair value hierarchy as Level 2 primarily utilizing broker quotes in a non-active market for valuation of these securities. No changes in valuation techniques or inputs occurred during the year ended December 31, 2020. No transfers of assets between Level 1 and Level 2 of the fair value measurement hierarchy occurred during the year ended December 31, 2020.

Contingent Consideration Payable

The contingent consideration payable resulted from the acquisition of Callidus. The most recent valuation was determined using a probability weighted discounted cash flow valuation approach. Gains and losses are included in the Consolidated Statements of Operations.

The contingent consideration payable for Callidus has been classified as a Level 3 recurring liability as its valuation requires substantial judgment and estimation of factors that are not currently observable in the market. If different assumptions were used for the various inputs to the valuation approach, the estimated fair value could be significantly higher or lower than the fair value the Company determined.

The following significant unobservable inputs were used in the valuation of the contingent consideration payable of Callidus for the ATB-200 Pompe program:

Contingent Consideration Liability

Fair Value as of December 31, 2020

Valuation Technique

Unobservable Input

Range

(in thousands)

Discount rate

7.5%

Clinical and regulatory milestones

25,101

Probability weighted discounted cash flow

Probability of achievement of milestones

75%-78%

Projected year of payments

2021-2022

Contingent consideration liabilities are remeasured to fair value each reporting period using discount rates, probabilities of payment, and projected payment dates. Projected contingent payment amounts related to clinical and regulatory based milestones are discounted back to the current period using a discounted cash flow model. Increases in discount rates and the time to payment may result in lower fair value measurements. Increases or decreases in any of those inputs together, or in isolation, may result in a significantly lower or higher fair value measurement. There is no assurance that any of the conditions for the milestone payments will be met.

During the fourth quarter of 2018, the Company reached a clinical milestone, which was the dosing of the first patient in a Phase 3 study, related to the contingent consideration from the acquisition of Callidus. The milestone for this event was $9.0 million, which was paid in Company common stock in the first quarter of 2019 and resulted in a $9.3 million impact on stockholder's equity.

The following table shows the change in the balance of contingent consideration payable for the year ended December 31, 2020 and December 31, 2019, respectively:


	Years ended December 31,
(in thousands)	2020		2019
Balance, beginning of the period	$	22,681	$	19,700
Changes in fair value during the period, included in the Consolidated Statements of Operations	3,144		3,297
Adjustment for contingent consideration paid in stock	—		(316)
Balance, end of the period	$	25,825	$	22,681

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Debt

12 Months Ended

Dec. 31, 2020

Debt Disclosure [Abstract]

Debt

The Company's debt consists of the following:


(in thousands)	December 31, 2020		December 31, 2019
Senior Secured Term Loan due 2026:
Principal	$	400,000	$	—
Less: debt discount (1)	(7,434)		—
Less: deferred financing (1)	(5,592)		—
Net carrying value of the Senior Secured Term Loan	$	386,974	$	—

Senior Secured Term Loan due 2023 (2):
Principal	$	—	$	150,000
Less: debt discount (1)	—		(2,315)
Less: deferred financing (1)	—		(311)
Net carrying value of the Senior Secured Term Loan	$	—	$	147,374

Convertible Notes (3):
Principal	$	2,825	$	2,825
Less: debt discount (1)	(518)		(659)
Less: deferred financing (1)	(27)		(35)
Net carrying value of the Convertible Notes	$	2,280	$	2,131

Net carrying value of Long-term debt	$	389,254	$	149,505

______________________________

(1) Included in the Consolidated Balance Sheets within long-term debt and amortized to interest expense over the remaining life of the Convertible Notes and Senior Secured Term Loans using the effective interest rate method.

(2) The principle, accrued interest and early settlement premiums associated with the Senior Secured Term Loan due 2023 were fully paid and settled in July 2020.

(3) The Convertible Notes are currently convertible as the last reported sale price of the Company's common stock was equal to or more than 130% of the conversion price for at least 20 trading days of the 30 consecutive trading days ending on the last day of the quarter.

Senior Secured Term Loan due 2026

In July 2020, the Company entered into a definitive agreement for a $400 million credit facility with Hayfin Capital Management (“Senior Secured Term Loan due 2026”) with an interest rate equal to 3-month LIBOR, subject to a 1% floor, plus 6.5% per annum and requires interest-only payments until mid-2024. The Senior Secured Term Loan due 2026 will be repaid in nine quarterly payments of $44.4 million, starting on July 2024 with the final balance due on the maturity date in July 2026. This transaction resulted in net proceeds of $385.9 million, after deducting fees and expenses. There were no warrants or equity conversion features associated with the Senior Secured Term Loan due 2026. The Company used the proceeds to voluntarily settle the Senior Secured Term Loan with BioPharma Credit PLC that was due in 2023 ("Senior Secured Term Loan due 2023").

The Senior Secured Term Loan due 2026 is subject to mandatory prepayment provisions that require prepayment upon a change of control, the incurrence of certain additional indebtedness, asset sale, or an event of loss, subject to certain conditions set forth in the Senior Secured Term Loan due 2026. The Company may prepay the Senior Secured Term Loan due 2026 in whole, at its option at any time. Any prepayment of the Senior Secured Term Loan due 2026 is subject to certain make-whole premiums and prepayment premiums, the latter of which decrease until the fourth anniversary of the transaction date at which point no prepayment penalty shall exist. The obligations under the Senior Secured Term Loan due 2026 are secured by a first lien security interest in certain assets of the Company. The Senior Secured Term Loan due 2026 contains certain customary representations and warranties, affirmative and negative covenants and events of default applicable to the Company. The Senior Secured Term Loan due 2026 also contains a minimum liquidity covenant of $75 million, and an incremental minimum consolidated revenue covenant, measured as of the previous four consecutive fiscal quarters. The minimum consolidated

revenue covenant ranges from $140 million, beginning March 31, 2021, and peaks at $225 million by June 30, 2023, continuing at that level until the Senior Secured Term Loan due 2026 is repaid. If an event of default occurs and is continuing, Hayfin Capital Management may declare all amounts outstanding under the Senior Secured Term Loan due 2026 to be immediately due and payable.

Senior Secured Term Loan due 2023

In September 2018, the Company entered into a loan agreement with BioPharma Credit PLC (“Senior Secured Term Loan due 2023") as the lender. The loan agreement provided for a $150 million senior secured term loan with an interest rate equal to the 3-month LIBOR plus 7.5% per annum and would mature in 5 years from the maturity date. The Senior Secured Term Loan due 2023 would be repaid in four quarterly payments equal to 12.5% thereof starting on the forty-eight month anniversary of the date of the first credit extension with the balance due on the Maturity Date. Interest was payable quarterly in arrears. The Senior Secured Term Loan due 2023 contained certain customary representations and warranties, affirmative and negative covenants, and events of default applicable to the Company and certain of its subsidiaries but did not include any financial covenants relating to the achievement or maintenance of revenue or cash flow. If an event of default occurred and was continuing, the lender could declare all amounts outstanding under the Senior Secured Term Loan due 2023 to be immediately due and payable. The Company received net proceeds of $146.6 million in September 2018, after deducting fees and expenses.

In July 2020, the Company used $156.3 million of the proceeds from the Senior Secured Term Loan due 2026 to voluntarily settle the principal amount, $1.1 million accrued interest, and $5.2 million in early settlement premiums. As a result of this early extinguishment, the Company recognized a loss on extinguishment of debt of $7.3 million in the Consolidated Statements of Operations.

Convertible Notes

In December 2016, the Company issued at par value $250 million aggregate principal amount of Convertible Notes, which included the exercise in full of the $25 million over-allotment option granted to the initial purchasers of the Convertible Notes in a private offering to qualified institutional buyers pursuant to Rule 144A under the Securities Act (the "Note Offering"). Interest is payable semiannually on June 15 and December 15 of each year, beginning on June 15, 2017. The Convertible Notes will mature on December 15, 2023, unless earlier repurchased, redeemed, or converted in accordance with their terms. The Convertible Notes are convertible at the option of the holders of the Convertible Notes (the "Holders"), under certain circumstances and during certain periods, into cash, shares of the Company’s common stock or a combination thereof. The net proceeds from the Note Offering were $243.0 million, after deducting fees and expenses. In addition, the Company used approximately $13.5 million of the net proceeds from the issuance of the Convertible Notes to pay the cost of the capped call transactions ("Capped Call Confirmations") that the Company entered into in connection with the issuance of the Convertible Notes. In accounting for the issuance of the Convertible Notes, the Company separated the Convertible Notes into liability and equity components based on their relative values. The Convertible Notes were initially convertible into approximately 40.8 million shares of the Company’s common stock under certain circumstances prior to maturity at a conversion rate of 163.3987 shares per $1,000 principal amount of Convertible Notes, which represents a conversion price of approximately $6.12 per share of common stock, subject to adjustment under certain conditions. Holders may convert their Convertible Notes at their option at specified times prior to the maturity date of December 15, 2023, only if:

•during any fiscal quarter commencing after March 31, 2017, if the last reported sale price of the Company’s common stock for at least 20 trading days in the period of 30 consecutive trading days ending on the last trading day of the immediately preceding fiscal quarter is equal to or more than 130% of the conversion price of the Convertible Notes on the last day of such preceding fiscal quarter;

•a holder submits its Convertible Notes for conversion during the five business day period following any five consecutive trading day period in which the trading price for the Convertible Notes, per $1,000 principal amount of the Convertible Notes, for each such trading day was less than 98% of the product of the last reported sale price of the Company’s common stock and the conversion rate of the Convertible Notes on such date;

•the Company issues to all or substantially all of the holders of common stock rights options or warrants entitling then them for a period of not more than 60 calendar days after the date of such issuance to subscribe for or purchase shares of the common stock, at a price per share less than the average of the Last Reported Sale Prices of the common stock for the 10 consecutive Trading Day period ending on, and including, the Trading Day immediately preceding the date of announcement of such issuance or distributes to all or substantially all holders of the common stock the Company’s assets, debt securities or rights to purchase the Company’s securities which distribution has a per share value of exceeding 10% of the Last Reported Sale Price of the common stock on the Trading Day immediately preceding the date of announcement of such distribution;

•the Company enters into specified corporate transactions; or

•the Company has had a call for redemption, the holder can convert up until the second trading day immediately preceding the redemption date.

The Convertible Notes will be convertible, at the option of the note holders, regardless of whether any of the foregoing conditions have been satisfied, on or after September 15, 2023 at any time prior to the close of business on the second scheduled trading day immediately preceding the stated maturity date of December 15, 2023.

During the first and second quarters of 2019, the Company entered into separate, privately negotiated Exchange Agreements with a limited number of holders of the Convertible Notes. Under the terms of the Exchange Agreements, the limited number of Holders agreed to exchange an aggregate principal amount of $247.2 million of Convertible Notes held by them in exchange for an aggregate of approximately 44.0 million shares of Company common stock, par value $0.01 per share. In addition, pursuant to the Exchange Agreements, the Company made aggregate cash payments of $1.3 million to the limited number of Holders to satisfy accrued and unpaid interest to the closing date of the transactions, along with cash in lieu of fractional shares. These transactions resulted in $215.0 million in additional paid-in-capital and common stock of $0.4 million on the Consolidated Balance Sheets as of December 31, 2019. Additionally, the Company recognized a loss on the exchange of debt of $40.6 million on the Consolidated Statements of Operations during the year ended December 31, 2019. During the second half of the year ended December 31, 2019, there were no additional debt conversion transactions.

During the first and second quarter of 2019, the Company terminated the Capped Call Confirmations related to the exchange of the Convertible Notes for proceeds of $19.9 million.

Interest Expense

The following table sets forth interest expense recognized related to the Company's debt for the years ended December 31, 2020 and 2019 respectively:


(in thousands)	December 31, 2020			December 31, 2019
Contractual interest expense	$	20,933		$	16,483
Amortization of debt discount	$	1,267		$	2,381
Amortization of deferred financing	$	527		$	165
Effective interest rate of the liability component, Convertible debt	10.6		%	10.6		%
Effective interest rate of the liability component, Senior Secured Term Loan due 2026	8.4		%	—		%
Effective interest rate of the liability component, Senior Secured Term Loan due 2023	—		%	10.2		%

Leases

12 Months Ended

Dec. 31, 2020

Leases [Abstract]

Leases

The Company currently has operating leases for office and research laboratory space, equipment, and vehicles under agreements expiring at various dates through 2044, which include renewal options on leases which the Company is reasonably certain to exercise.

For the years ended December 31, 2020, and 2019, the operating lease expense was $11.1 million and $10.8 million, respectively. For the years ended December 31, 2020, and 2019, the Company paid $9.6 million and $4.9 million, respectively,

for amounts included in the measurement of operating lease liabilities and recorded $1.7 million and $21.8 million, respectively, of right-of-use assets and $0.5 million and $19.4 million, respectively of tenant improvements paid through lease incentives in exchange for new operating lease liabilities.

Commitments under finance leases are not significant for the years ended December 31, 2020, and 2019.

Supplemental balance sheet information related to operating leases were as follows:


(in thousands, except year and discount rate amounts)	December 31, 2020			December 31, 2019
Operating lease ROU asset	$	23,296		$	33,315

Current portion of the operating lease liabilities	$	6,872		$	7,189
Non-current portion of the operating lease liabilities	45,604			53,531
Total operating lease liability	$	52,476		$	60,720

Weighted-average remaining lease terms (years)	19.8			18.2
Weighted-average discount rate	13.0		%	13.1		%

At December 31, 2020, the future minimum operating lease payments were as follows:


(in thousands)	Operating Lease
2021	$	8,549
2022	8,494
2023	8,691
2024	8,404
2025	8,371
Thereafter	153,241
Total lease payments	195,750
Less lease incentives	(28,939)
Less imputed interest	(114,335)
Total operating lease liability	$	52,476

At December 31, 2019, the future minimum operating lease payments were as follows:


(in thousands)	Operating Lease
2020	$	9,738
2021	10,787
2022	10,459
2023	10,809
2024	10,582
Thereafter	163,266
Total lease payments	$	215,641
Less lease incentives	(28,939)
Less imputed interest	(125,982)
Total operating lease liability	$	60,720

Rent expense, including fees for utilities and common area maintenances under ASC 840 for the year ended December 31, 2018 was $5.7 million.

Income Taxes

12 Months Ended

Dec. 31, 2020

Income Tax Disclosure [Abstract]

Income Taxes

For financial reporting purposes, loss before income taxes includes the following components:


	Years Ended December 31,
(in thousands)	2020		2019		2018
United States	$	(365,332)	$	(393,955)	$	(309,183)
Foreign	91,078		38,045		(39,906)
Total	$	(274,254)	$	(355,910)	$	(349,089)

Following were the components of income tax expense (benefit) for the years ended December 31, 2020, 2019, and 2018:


(in thousands)	2020		2019		2018
Current
Federal	$	—	$	—	$	—
State	—		—		6
Foreign	4,163		2,877		(100)
Deferred
Federal	(1,410)		(984)		—
State	(155)		(1,415)		—
Foreign	—		—		—
Total	$	2,598	$	478	$	(94)

A reconciliation of the statutory tax rates and the effective tax rates for the years ended December 31, 2020, 2019, and 2018 are as follows:


	Years Ended December 31,
	2020		2019		2018
Statutory rate	(21)	%	(21)	%	(21)	%
State taxes, net of federal benefit	—		—		(4)
Nondeductible IPR&D	—		—		6
Contingent consideration	—		—		1
Tax credits	(7)		(16)		(10)
Impact of foreign operations	4		9		2
Nondeductible executive compensation	2		1		—
Nondeductible debt conversion	—		(1)		—
Other	—		(1)		—
Valuation allowance	23		29		26
Net	1	%	—	%	—	%

On December 22, 2017, the U.S. government enacted the Tax Cuts and Jobs Act ("Tax Act"). The Tax Act significantly revises U.S. tax law by, among other provisions, lowering the U.S. federal statutory income tax rate to 21%, imposing a mandatory one-time transition tax on previously deferred foreign earnings, and eliminating or reducing certain income tax deductions. The Tax Act also introduced an additional U.S. tax on certain non-U.S. subsidiaries’ earnings which are considered to be Global Intangible Low Taxed Income (referred to as "GILTI"). After consideration of the relevant guidance and completing the accounting for the tax effects of the Tax Act, the Company has elected to treat GILTI as a period cost.

The Company recorded an income tax expense of $4.2 million in 2020 for taxes in foreign jurisdictions and a $1.6 million tax benefit in 2020 for taxes in U.S. federal and state jurisdictions.

The Company did not recognize interest or penalties related to income tax during the period ended December 31, 2020 and did not accrue for interest or penalties as of December 31, 2020. The Company does not have an accrual for uncertain tax positions as of December 31, 2020. Tax returns for years 2015 through 2019 are open to examination by tax authorities. The Company is also subject to examination in any period for which it has net operating losses.

Deferred income taxes reflect the net effect of temporary difference between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts used for income tax purposes. The significant components of the deferred tax assets and liabilities are as follows:


	Years Ended December 31,
(in thousands)	2020		2019
Deferred tax assets
Intellectual property	$	68,209	$	46,521
Amortization/depreciation	489		2,329
Research tax credit	182,651		141,669
Net operating loss carry forwards	305,876		286,850
Deferred reimbursements	1,912		2,230
Non-cash stock issue	17,553		19,058
Interest carry forward limitation	5,655		2,224
Lease liability	9,235		6,983
Other	10,855		13,921
Gross deferred tax assets	602,435		521,785
Deferred tax liabilities
Business acquisition	(4,937)		(5,051)
Royalty payable	(68,209)		(46,521)
Convertible notes	(94)		(653)
Advanced R&D payments	(1,776)		(2,611)
Right of use asset	(3,420)		(5,643)
Total net deferred tax assets	523,999		461,306
Less: valuation allowance	(528,895)		(466,357)
Net deferred tax liability	$	(4,896)	$	(5,051)

The Company records a valuation allowance for temporary differences for which it is more likely than not that the Company will not receive future tax benefits. At December 31, 2020 and 2019, the Company recorded valuation allowances of $528.9 million and $466.4 million, respectively, representing an increase in the valuation allowance of $62.5 million in 2020, due to the uncertainty regarding the realization of such deferred tax assets, to offset the benefits of net operating losses generated during those years. The deferred tax liability related to business acquisitions pertains to the basis difference in IPR&D acquired by the Company. The Company's policy is to record a deferred tax liability related to acquired IPR&D that may eventually be realized either upon amortization of the asset when the research is completed, and a product is successfully launched or the write-off of the asset if it is abandoned or unsuccessful.

As of December 31, 2020, the Company had federal and state net operating loss carry forwards ("NOLs") of approximately $1,156 million and $889.4 million, respectively. The federal carry forward for losses generated prior to 2018 will expire in 2029 through 2037. Federal net operating losses incurred in 2018 and onward have an indefinite expiration under the 2017 Tax Act. Most of the state carry forwards generated prior to 2009 have expired through 2016. The remaining state carry forwards including those generated in 2009 through 2020 will expire in 2029 through 2040. Utilization of NOLs may be subject to a substantial limitation pursuant to Section 382 of the Internal Revenue Code of 1986, as amended ( the "Code") as well as similar state statutes in the event of an ownership change. Such ownership changes have occurred in the past and could occur again in the future. Under Section 382 of the Internal Revenue Code of 1986, as amended, or Section 382, if a corporation undergoes an "ownership change," generally defined as a greater than 50% change (by value) in its equity ownership over a three-year period, the corporation's ability to use its pre-change NOLs and other pre-change tax attributes (such as research and development tax credits) to offset its post-change income may be limited. The Company may experience ownership changes in the future as a result of shifts in the stock ownership some of which are outside the Company's control. The Company completed a detailed study of the NOLs and determined that there was not an ownership change in excess of 50%. Ownership changes in future periods may place additional limits on the Company's ability to utilize net operating loss and tax credit carry forwards. In addition, at the state level, there may be periods during which the use of NOLs is suspended or otherwise limited, which could accelerate or permanently decrease the amount of state attributes and increase state taxes owed.

The Company also has research and experimentation and orphan drug credit carryforwards of approximately $30.4 million and $141.8 million, respectively, which will expire in the years 2021 through 2040. Deferred tax assets for these carryforwards are subject to a full valuation allowance.

- Definition

The entire disclosure for income taxes. Disclosures may include net deferred tax liability or asset recognized in an enterprise's statement of financial position, net change during the year in the total valuation allowance, approximate tax effect of each type of temporary difference and carryforward that gives rise to a significant portion of deferred tax liabilities and deferred tax assets, utilization of a tax carryback, and tax uncertainties information.

+ References + Details

Collaborative Agreements	12 Months Ended
Collaborative Agreements	Dec. 31, 2020
Collaborative Agreements
Collaborative Agreements	Collaborative Agreements University of Pennsylvania In October 2018, as amended, the Company entered into a collaboration agreement with Penn to pursue research and development of novel gene therapies. The Company's gene therapy portfolio pipeline expanded to include Pompe disease, Fabry disease, CDD, CLN1, MPS IIIB, as well as a next generation program in MPS IIIA. This expanded collaboration with Penn also provides the Company with exclusive disease-specific access and option rights to develop potentially disruptive new gene therapy platform technologies and programs for most LDs and a broader portfolio of rare diseases, including Rett Syndrome, Angelman Syndrome, Myotonic Dystrophy, and select other muscular dystrophies. Under the expanded collaboration agreement with Penn, Penn is eligible to receive certain milestone, royalty and discovery research payments with respect to licensed products for each indication. Milestone payments are payable following the achievement of certain development and commercial milestone events in each indication, up to an aggregate of $88.0 million per indication. Royalty payments are based on net sales of licensed products on a licensed product-by-licensed product and country- by-country basis. The Company will provide $10.0 million each year during the five-year agreement to fund the discovery research program. In connection with the collaboration agreement, the Company made an upfront payment of $7 million in cash to Penn in 2018 that was expensed to research and development expense in the Consolidated Statements of Operations. GlaxoSmithKline In July 2012, as amended in November 2013, the Company entered into an agreement with GlaxoSmithKline ("GSK"), pursuant to which Amicus obtained global rights to develop and commercialize Galafold® as a monotherapy and in combination with ERT for Fabry disease (“Collaboration Agreement”). Under the terms of the Collaboration Agreement, GSK is eligible to receive post-approval and sales-based milestones up to $40 million, as well as tiered royalties in the mid-teens in eight major markets outside the U.S. The contingent milestone payments due to GSK are recorded within the short term and long term deferred reimbursements accounts on the Consolidated Balance Sheets. Sales based tiered royalties due to GSK are recorded within the cost of goods sold within the Consolidated Statements of Operations. For the year ended December 31, 2020, under the GSK collaboration agreements, the Company paid $1.3 million in sales-based milestones and the Company incurred approximately $21.7 million of royalty expenses. As of December 31, 2020, $8.9 million was recorded as deferred reimbursements and the Company recognized a liability of $6.6 million related to royalties payable to GSK in accounts payable and accrued expenses in the Consolidated Balance Sheets.

Basic and Diluted Net Loss per Common Share

12 Months Ended

Dec. 31, 2020

Earnings Per Share [Abstract]

Basic and Diluted Net Loss per Common Share

The following table provides a reconciliation of the numerator and denominator used in computing basic and diluted net loss attributable to common stockholders per common share:


	Years Ended December 31,
(in thousands, except per share amounts)	2020		2019		2018
Numerator:
Net loss attributable to common stockholders	$	(276,852)	$	(356,388)	$	(348,995)
Denominator:
Weighted average common shares outstanding — basic and diluted	258,867,380		240,421,001		185,790,021

Dilutive common stock equivalents would include the dilutive effect of common stock options, convertible debt units, RSUs and warrants for common stock equivalents. Potentially dilutive common stock equivalents were excluded from the diluted earnings per share denominator for all periods because of their anti-dilutive effect.

The table below presents potential shares of common stock that were excluded from the computation as they were anti-dilutive using the treasury stock method:


	Years ended December 31,
(in thousands)	2020	2019	2018
Options to purchase common stock	14,032	16,724	15,810
Convertible notes	462	462	40,850
Outstanding warrants, convertible to common stock	2,555	2,555	2,657
Unvested restricted stock units	7,080	5,792	3,712
Total number of potentially issuable shares	24,129	25,533	63,029

Selected Quarterly Financial Data (Unaudited)

12 Months Ended

Dec. 31, 2020

Quarterly Financial Information Disclosure [Abstract]

Selected Quarterly Financial Data (Unaudited)


	Quarters Ended
(in thousands except per share data)	March 31		June 30		September 30		December 31
2020
Net product sales	$	60,525	$	62,353	$	67,437	$	70,571
Net loss	$	(88,948)	$	(52,492)	$	(64,011)	$	(71,401)
Basic and diluted net loss per common share (1)	$	(0.35)	$	(0.20)	$	(0.25)	$	(0.27)
2019
Net product sales	$	34,046	$	44,130	$	48,768	$	55,293
Net loss	$	(120,299)	$	(84,551)	$	(61,809)	$	(89,729)
Basic and diluted net loss per common share (1)	$	(0.56)	$	(0.36)	$	(0.24)	$	(0.35)

_______________________________

(1) Per common share amounts for the quarters and full years have been calculated separately. Accordingly, quarterly amounts do not add to the annual amounts because of differences on the weighted-average common shares outstanding during each period principally due to the effect of the Company issuing shares of its common stock during the year.

Summary of Significant Accounting Policies (Policies)	12 Months Ended
Summary of Significant Accounting Policies (Policies)	Dec. 31, 2020
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The Company has prepared the accompanying Consolidated Financial Statements in accordance with U.S. generally accepted accounting principles ("U.S. GAAP") and include all adjustments necessary for the fair presentation of the Company's financial position for the periods presented.
Consolidation	Consolidation The Consolidated Financial Statements include the accounts of the Company and its subsidiaries. Intercompany accounts and transactions are eliminated in consolidation.
Foreign Currency Transactions	Foreign Currency Transactions The functional currency for most of the Company's foreign subsidiaries is their local currency. For non-U.S. subsidiaries that transact in a functional currency other than the U.S. dollar, assets and liabilities are translated at current rates of exchange at the balance sheet date. Income and expense items are translated at the average foreign exchange rates for the period. Adjustments resulting from the translation of the financial statements of the Company's foreign operations into U.S. dollars are excluded from the determination of net income and are recorded in accumulated other comprehensive income, a separate component of stockholders' equity.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements, and the reported amounts of revenues and expenses during the reporting periods. Actual results could differ from those estimates. Additionally, the Company assessed the impact COVID-19 pandemic has had on its operations and financial results as of December 31, 2020 and through the issuance of this report. The Company’s analysis was informed by the facts and circumstances as they were known to the Company. This assessment considered the impact COVID-19 may have on financial estimates and assumptions that affect the reported amounts of assets and liabilities and revenue and expenses.
Cash, Cash Equivalents, Marketable Securities, and Restricted Cash	Cash, Cash Equivalents, Marketable Securities, and Restricted Cash The Company considers all highly liquid investments purchased with a maturity of three months or less at the date of acquisition to be cash equivalents. Marketable securities consist of fixed income investments with a maturity of greater than three months and other highly liquid investments that can be readily purchased or sold using established markets. These investments are classified as available-for-sale and are reported at fair value on the Company's Consolidated Balance Sheets. Unrealized holding gains and losses are reported within comprehensive income (loss) in the Statements of Comprehensive Loss. Fair value is based on available market information including quoted market prices, broker or dealer quotations, or other observable inputs. Restricted cash consists primarily of funds held to satisfy the requirements of certain agreements that are restricted in their use and is included in other current assets and other non-current assets on the Company's Consolidated Balance Sheets.
Concentration of Credit Risk	Concentration of Credit Risk The Company's financial instruments that are exposed to concentration of credit risk consist primarily of cash, cash equivalents, and marketable securities. The Company maintains its cash and cash equivalents in bank accounts, which, at times, exceed federally insured limits. The Company invests its marketable securities in high-quality commercial financial instruments. The Company has not recognized any losses from credit risks on such accounts during any of the periods presented. The Company believes it is not exposed to significant credit risk on its cash, cash equivalents, or marketable securities. The Company is subject to credit risk from its accounts receivable related to its product sales of Galafold®. The Company's accounts receivable at December 31, 2020 have arisen from product sales primarily in Europe and the U.S. The Company will periodically assess the financial strength of its customers to establish allowances for anticipated losses, if any. For accounts receivable that have arisen from named patient sales, the payment terms are predetermined, and the Company evaluates the creditworthiness of each customer on a regular basis.
Property and Equipment	Property and Equipment Property and equipment are stated at cost, less accumulated depreciation. Depreciation is calculated over the estimated useful lives of the respective assets, which range from three to five years, or the lesser of the related initial term of the lease or useful life for leasehold improvements. The initial cost of property and equipment consists of its purchase price and any directly attributable costs of bringing the asset to its working condition and location for its intended use. Expenditures incurred after the fixed assets have been put into operation, such as repairs and maintenance, are charged to income in the period in which the costs are incurred. Major replacements, improvements, and additions are capitalized in accordance with Company policy.
Revenue Recognition	Revenue Recognition The Company’s net product sales consist of sales of Galafold® for the treatment of Fabry disease. The Company has recorded revenue on sales where Galafold® is available either on a commercial basis or through a reimbursed early access program ("EAP"). Orders for Galafold® are generally received from distributors and pharmacies, with the ultimate payor often a government authority. In 2020, one customer accounted for 25% of net product sales and 12% of accounts receivable from product sales. In 2019, one customer accounted for 22% of net product sales and 11% of accounts receivable from product sales. The Company recognizes revenue when its performance obligations to its customers have been satisfied, which occurs at a point in time when the pharmacies or distributors obtain control of Galafold®. The transaction price is determined based on fixed consideration in the Company's customer contracts and is recorded net of estimates for variable consideration, which are third party discounts and rebates. The identified variable consideration is recorded as a reduction of revenue at the time revenue from the sale of Galafold® is recognized. The Company recognizes revenue to the extent that it is probable that a significant revenue reversal will not occur in a future period. These estimates may differ from actual consideration received. The Company evaluates these estimates each reporting period to reflect known changes.
Inventories and Cost of Goods Sold	Inventories and Cost of Goods Sold Inventories are stated at the lower of cost and net realizable value, determined by the first-in, first-out method. Inventories are reviewed periodically to identify slow-moving or obsolete inventory based on projected sales activity as well as product shelf-life. In evaluating the recoverability of inventories produced, the probability that revenue will be obtained from the future sale of the related inventory is considered and inventory value is written down for inventory quantities in excess of expected requirements. Expired inventory is disposed of and the related costs are recognized as cost of goods sold in the Consolidated Statements of Operations. Cost of goods sold includes the cost of inventory sold, manufacturing and supply chain costs, product shipping and handling costs, provisions for excess and obsolete inventory, as well as royalties payable.
Fair Value Measurements	Fair Value Measurements The Company records certain asset and liability balances under the fair value measurements as defined by the FASB guidance. Current FASB fair value guidance emphasizes that fair value is a market-based measurement, not an entity-specific measurement. Therefore, a fair value measurement should be determined based on the assumptions that market participants would use in pricing the asset or liability. As a basis for considering market participant assumptions in fair value measurements, current FASB guidance establishes a fair value hierarchy that distinguishes between market participant assumptions based on market data obtained from sources independent of the reporting entity (observable inputs that are classified within Levels 1 and 2 of the hierarchy) and the reporting entity's own assumptions that market participants assumptions would use in pricing assets or liabilities (unobservable inputs classified within Level 3 of the hierarchy). Level 1 inputs utilize quoted prices (unadjusted) in active markets for identical assets or liabilities that the Company has the ability to access at measurement date. Level 2 inputs are inputs other than quoted prices included in Level 1 that are observable for the asset or liability, either directly or indirectly. Level 2 inputs may include quoted prices for similar assets and liabilities in active markets, as well as inputs that are observable for the asset or liability (other than quoted prices), such as interest rates, foreign exchange rates, and yield curves that are observable at commonly quoted intervals. Level 3 inputs are unobservable inputs for the asset or liability, which is typically based on an entity's own assumptions, as there is little, if any, related market activity. In instances where the determination of the fair value measurement is based on inputs from different levels of the fair value hierarchy, the level in the fair value hierarchy within which the entire fair value measurement falls is based on the lowest level input that is significant to the fair value measurement in its entirety. The Company's assessment of the significance of a particular input to the fair value measurement in its entirety requires judgment and considers factors specific to the asset or liability.
Contingent Liabilities	Contingent Liabilities On an ongoing basis, the Company may be involved in various claims and legal proceedings. On a quarterly basis, the Company reviews the status of each significant matter and assesses its potential financial exposure. If the potential loss from any claim, asserted or unasserted, or legal proceeding is considered probable and the amount can be reasonably estimated, the Company will accrue a liability for the estimated loss. Because of uncertainties related to claims and litigation, accruals will be based on the Company's best estimates based on available information. On a periodic basis, as additional information becomes available, or based on specific events such as the outcome of litigation or settlement of claims, the Company may reassess the potential liability related to these matters and may revise these estimates, which could result in material adverse adjustments to the Company's operating results.
Research and Development Costs	Research and Development Costs Research and development costs are expensed as incurred. Research and development expense consist primarily of costs related to personnel, including salaries and other personnel related expenses, consulting fees, and the cost of facilities and support services used in drug development. Assets acquired that are used for research and development and have no future alternative use are expensed as in-process research and development.
Interest Income and Interest Expense	Interest Income and Interest Expense Interest income consists of interest earned on the Company's cash, cash equivalents, and marketable securities. Interest expense consists of interest incurred on debt and finance leases.
Income Taxes	Income Taxes The Company accounts for income taxes under the liability method. Under this method deferred income tax liabilities and assets are determined based on the difference between the financial statement carrying amounts and tax basis of assets and liabilities and for operating losses and tax credit carry forwards, using enacted tax rates in effect in the years in which the differences are expected to reverse. A valuation allowance is recorded if it is "more likely than not" that a portion or all of a deferred tax asset will not be realized.
Other Comprehensive Income (Loss)	Other Comprehensive Income (Loss) Components of other comprehensive income (loss) include unrealized gains and losses on available-for-sale securities and gain (loss) on foreign currency transactions and are included in the Statements of Comprehensive Loss.
Leases	Leases The Company primarily enters into lease agreements for office space, equipment, and vehicles. The leases have varying terms, some of which could include options to renew, extend, and early terminate. The Company determines if an arrangement is a lease at contract inception. Operating leases are included in right-of-use ("ROU") assets and lease liabilities on the Consolidated Balance Sheets. ROU assets represent the Company's right to control the use of an explicitly or implicitly identified fixed asset for a period of time and lease liabilities represent the Company's obligation to make lease payments arising from the lease. Control of an underlying asset is conveyed to the Company if the Company obtains the rights to direct the use of and to obtain substantially all of the economic benefits from using the underlying asset. ROU assets and liabilities are recognized at commencement date based on the present value of lease payments over the lease term. The Company uses its incremental borrowing rate based on the information available at commencement date in determining the present value of lease payments. Lease payments included in the measurement of the lease liability are comprised of fixed payments. Variable lease payments are excluded from the ROU asset and lease liability and are recognized in the period in which the obligation for those payments is incurred. Variable lease payments are presented in the Consolidated Statements of Operations in the same line item as expenses arising from fixed lease payments for operating leases. The Company has lease agreements that include lease and non-lease components, which the Company accounts for as a single lease component for all underlying asset categories. The lease term for all of the Company's leases include the non-cancellable period of the lease plus any additional periods covered by either a Company option to extend (or not to terminate) the lease that the Company is reasonably certain to exercise, or an option to extend (or not to terminate) the lease controlled by the lessor. Leases with an initial term of 12 months or less are not recorded on the Consolidated Balance Sheets. The Company recognizes lease expense for these leases on a straight-line basis over the lease term. The Company applies this policy to all underlying asset categories. The information presented for the periods prior to January 1, 2019 has not been restated and is reported under the accounting standard in effect for those periods. For additional information, see " -Note 13. Leases".
Nonqualified Cash Deferral Plan	Nonqualified Cash Deferral Plan The Company's Cash Deferral Plan (the "Deferral Plan"), provides certain key employees and members of the Board of Directors as selected by the Compensation Committee of the Board of Directors of the Company (the "Compensation Committee"), with an opportunity to defer the receipt of such participant's base salary, bonus, and director's fees, as applicable. The Deferral Plan is intended to be a nonqualified deferred compensation plan that complies with the provisions of Section 409A of the Internal Revenue Code (the "Code"). All of the investments held in the Deferral Plan are classified as investments held-to-maturity and recorded at fair value with changes in the investments' fair value recognized as earnings in the period they occur. The corresponding liability for the Deferral Plan is included in other non-current liability in the Consolidated Balance Sheets.
Equity-based Compensation	Equity-based Compensation At December 31, 2020, the Company had one equity-based employee compensation plan, which is described more fully in "— Note 9. Stockholders' Equity." The Company applies the fair value method of measuring equity-based compensation, which requires a public entity to measure the cost of employee services received in exchange for an award of equity instruments based on the grant-date fair value of the award.
Loss per Common Share	Loss per Common Share The Company calculates net loss per share as a measurement of the Company's performance while giving effect to all dilutive potential common shares that were outstanding during the reporting period. The Company had a net loss for all periods presented; accordingly, the inclusion of common stock options, unvested RSUs, and warrants would be anti-dilutive. Therefore, the weighted average shares used to calculate both basic and diluted earnings per share are the same. See "— Note 17. Basic and Diluted Net Loss per Common Share" for further discussion on net loss per share.
Segment Information	Segment Information The Company currently operates in one business segment focused on the discovery, development, and commercialization of advanced therapies to treat a range of devastating rare and orphan diseases. The Company is not organized by market and is managed and operated as one business. A single management team reports to the chief operating decision maker who comprehensively manages the entire business. The Company does not operate any separate lines of business or separate business entities with respect to its products. Accordingly, the Company does not accumulate discrete financial information with respect to separate service lines, and thus there is one reporting unit.
Business Combinations	Business Combinations The Company assigns fair value to the tangible and intangible assets acquired and liabilities assumed based upon their estimated fair values on the acquisition date from acquired businesses. The purchase price allocation process requires management to make significant estimates and assumptions, especially at the acquisition date with respect to intangible assets and in-process research and development ("IPR&D"). In connection with the purchase price allocations for acquisitions, the Company estimates the fair value of contingent payments utilizing a probability-based income approach inclusive of an estimated discount rate.
Contingent Consideration Payable	Contingent Consideration Payable Contingent consideration payments in asset acquisitions are recognized when the contingency is resolved and the consideration is paid or becomes payable. This does not apply in circumstances when the contingent consideration meets the definition of a derivative, in which case the amount becomes part of the basis in the asset acquired. Upon recognition of the contingent consideration payment, the amount is included in the cost of the acquired asset or group of assets. The Company determines the fair value of contingent acquisition consideration payable on the acquisition date using a probability-based income approach utilizing an appropriate discount rate. The short term and long term portions of the contingent consideration payable is shown on the Company's Consolidated Balance Sheets. The fair value of the contingent consideration payable will be determined each period end and the resulting change will be recorded on the Consolidated Statements of Operations.
Intangible Assets and Goodwill	Intangible Assets and GoodwillThe Company records goodwill in a business combination when the total consideration exceeds the fair value of the net tangible and identifiable intangible assets acquired. Purchased IPR&D is accounted for as an indefinite lived intangible asset until the underlying project is completed, at which point the intangible asset will be accounted for as a definite lived intangible asset, or abandoned, at which point the intangible asset will be written off or partially impaired. Goodwill and indefinite lived intangible assets are assessed annually for impairment and whenever events or circumstances indicate that the carrying amount of an asset may not be recoverable. If it is determined that the full carrying amount of an asset is not recoverable, an impairment loss is recorded in the amount by which the carrying amount of the asset exceeds its fair value.
Recent Accounting Developments	Recent Accounting Developments - Guidance Adopted in 2020 ASU 2018-15 - In August 2018, the Financial Accounting Standards Board ("FASB") issued Accounting Standards Update ("ASU") 2018-15, Intangibles-Goodwill and Other-Internal-Use Software (Subtopic 350-40): ("ASU 2018-15"), relating to a customer’s accounting for implementation, set-up, and other upfront costs incurred in a cloud computing arrangement that is hosted by a vendor. Under the new guidance, a customer will apply the same criteria for capitalizing implementation costs as it would for an arrangement that has a software license. The new guidance does not affect the accounting for the service element of a hosting arrangement that is a service contract. The new guidance also prescribes the balance sheet, income statement and cash flow classification of the capitalized software costs and related amortization expense and requires additional quantitative and qualitative disclosures. ASU 2018-15 is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019 for public companies. The Company adopted this guidance on January 1, 2020. The adoption did not have a material impact on the Company's Consolidated Financial Statements or related disclosures. ASU 2018-13 - In August 2018, the FASB issued ASU 2018-03, Fair Value Measurement (Topic 820): Disclosure Framework-Changes to the Disclosure Requirements for Fair Value Measurement ("ASU 2018-13"). The amendments modify the disclosure requirements in Topic 820. ASU 2018-13 is effective for all entities for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. The amendments on (i) changes in unrealized gains and losses, (ii) the range and weighted average of significant unobservable inputs used to develop Level 3 fair value measurements, and (iii) the narrative description of measurement uncertainty should be applied prospectively for only the most recent interim or annual period presented in the initial fiscal year of adoption. All other amendments should be applied retrospectively to all periods presented upon their effective date. The Company adopted this guidance on January 1, 2020. The adoption did not have a material impact on the Company's Consolidated Financial Statements or related disclosures. ASU 2017-04 - In January 2017, the FASB issued ASU 2017-04, Intangibles - Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment ("ASU 2017-04"). ASU 2017-04 simplifies the recognition and measurement of a goodwill impairment loss by eliminating Step 2 of the quantitative goodwill impairment test. The guidance requires a one-step impairment test in which an entity compares the fair value of a reporting unit with its carrying amount and recognizes an impairment charge for the amount by which the carrying amount exceeds the reporting unit's fair value, if any. ASU 2017-04 is effective for fiscal years beginning after December 15, 2019 and should be applied on a prospective basis. The Company adopted this guidance on January 1, 2020. The adoption did not have a material impact on the Company's Consolidated Financial Statements or related disclosures. ASU 2016-13 - In June 2016, the FASB issued ASU 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments ("ASU 2016-13"). ASU 2016-13 requires financial assets measured at amortized cost basis to be presented at the net amount expected to be collected and amends guidance on the impairment of financial instruments. ASU 2016-13 is effective for public companies who are SEC filers for fiscal years beginning after December 15, 2019, including interim periods within those years. The Company adopted this guidance on January 1, 2020. The adoption did not have a material impact on the Company's Consolidated Financial Statements or related disclosures. Recent Accounting Developments - Guidance Not Yet AdoptedASU 2019-12 - In December 2019, the FASB issued ASU 2019-15, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes ("ASU 2019-12"). This new guidance removes specific exceptions to the general principles in Topic 740. It eliminates the need for an organization to analyze whether the following apply in a given period: (i) exception to the incremental approach for intraperiod tax allocation; (ii) exceptions to accounting for basis differences when there are ownership changes in foreign investments; and (iii) exception in interim period income tax accounting for year-to-date losses that exceed anticipated losses. ASU 2019-12 also improves financial statement preparers’ application of income tax-related guidance and simplifies the following: (i) franchise taxes that are partially based on income; (ii) transactions with a government that result in a step up in the tax basis of goodwill; (iii) separate financial statements of legal entities that are not subject to tax; and (iv) enacted changes in tax laws in interim periods. ASU 2019 is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2020. Early adoption is permitted for public business entities for periods for which financial statements have not yet been issued. The Company will adopt this guidance prospectively on January 1, 2021, and it believes the adoption of this guidance will not have a material impact on its Consolidated Financial Statements or related disclosures

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2020

Accounting Policies [Abstract]

Revenue Disaggregated by Geographical Area

The following table summarizes the Company's net product sales from Galafold® disaggregated by geographic area:


	For the Year
(in thousands)	2020		2019		2018
U.S.	$	80,046	$	52,478	$	7,223
Ex-U.S.	180,840		129,759		84,022
Total net product sales	$	260,886	$	182,237	$	91,245

Goodwill and Intangible Assets (Tables)

12 Months Ended

Dec. 31, 2020

Goodwill and Intangible Assets Disclosure [Abstract]

Schedule of Changes in IPR&D

The following table represents the changes in IPR&D for the years ended December 31, 2020 and 2019, respectively:


	(in millions)
Balance at December 31, 2018	$	23.0
Change in IPR&D	—
Balance at December 31, 2019	$	23.0
Change in IPR&D	—
Balance at December 31, 2020	$	23.0

Schedule of Changes in Goodwill

The following table represents the changes in Goodwill for the years ended December 31, 2020 and 2019, respectively:


	(in millions)
Balance at December 31, 2018	$	197.8
Change in goodwill	—
Balance at December 31, 2019	$	197.8
Change in goodwill	—
Balance at December 31, 2020	$	197.8

Cash, Cash Equivalents, Marketable Securities, and Restricted Cash (Tables)

12 Months Ended

Dec. 31, 2020

Cash, Cash Equivalents, and Short-term Investments [Abstract]

Schedule of Cash and Available for Sale Securities

Cash, cash equivalents, and marketable securities are classified as current unless mentioned otherwise below and consisted of the following:


	As of December 31, 2020
(in thousands)	Cost		Gross Unrealized Gain		Gross Unrealized Loss		Fair Value
Cash and cash equivalents	$	163,240	$	—	$	—	$	163,240
Corporate debt securities	39,525		4		(16)		39,513
Commercial paper	217,087		14		(6)		217,095
Asset-backed securities	9,420		18		—		9,438
U.S. government agency bonds	53,583		3		(4)		53,582
Money market	350		—		—		350
Certificates of deposit	51		—		—		51
	$	483,256	$	39	$	(26)	$	483,269
Included in cash and cash equivalents	$	163,240	$	—	$	—	$	163,240
Included in marketable securities	320,016		39		(26)		320,029
Total cash, cash equivalents, and marketable securities	$	483,256	$	39	$	(26)	$	483,269


	As of December 31, 2019
(in thousands)	Cost		Gross Unrealized Gain		Gross Unrealized Loss		Fair Value
Cash and cash equivalents	$	142,837	$	—	$	—	$	142,837
Corporate debt securities	145,875		121		(5)		145,991
Commercial paper	73,659		53		(2)		73,710
Asset-backed securities	77,731		79		—		77,810
U.S. government agency bonds	11,999		2		(10)		11,991
Money market	350		—		—		350
Certificates of deposit	51		—		—		51
	$	452,502	$	255	$	(17)	$	452,740
Included in cash and cash equivalents	$	142,837	$	—	$	—	$	142,837
Included in marketable securities (1)	309,665		255		(17)		309,903
Total cash, cash equivalents, and marketable securities	$	452,502	$	255	$	(17)	$	452,740

______________________________

(1) As of December 31, 2019, $9.5 million of marketable securities had maturity dates greater than 12 months and were available to convert into cash, if needed.

Schedule of Reconciliation of Cash and Cash Equivalents and Restricted Cash


(in thousands)	December 31, 2020		December 31, 2019		December 31, 2018
Cash and cash equivalents	$	163,240	$	142,837	$	79,749
Restricted cash	2,922		3,506		2,626
Cash, cash equivalents, and restricted cash shown in the Consolidated Statements of Cash Flows	$	166,162	$	146,343	$	82,375

Inventories (Tables)

12 Months Ended

Dec. 31, 2020

Inventory Disclosure [Abstract]

Schedule of Inventories for the Period

The following table summarizes the components of inventories:


(in thousands)	December 31, 2020		December 31, 2019
Raw materials	$	5,547	$	6,544
Work-in-process	7,693		3,660
Finished goods	6,316		3,837
Total inventories	$	19,556	$	14,041

Property and Equipment (Tables)

12 Months Ended

Dec. 31, 2020

Property, Plant and Equipment [Abstract]

Schedule of Property and Equipment

Property and equipment consist of the following:


	December 31,
(in thousands)	2020		2019
Property and equipment consist of the following:
Computer equipment	$	5,546	$	6,185
Computer software	1,207		1,607
Research equipment	15,387		14,568
Furniture and fixtures	2,880		5,157
Leasehold improvements	24,146		30,294
Vehicles	124		124
Land	3,190		3,190
Construction in progress	5,870		4,184
Gross property and equipment	58,350		65,309
Less accumulated depreciation	(14,487)		(17,604)
Net property and equipment	$	43,863	$	47,705

Accrued Expenses and Other Current Liabilities (Tables)

12 Months Ended

Dec. 31, 2020

Payables and Accruals [Abstract]

Schedule of Accounts Payable and Accrued Expenses

Accrued expenses and other current liabilities consist of the following:


	December 31,
(in thousands)	2020		2019
Accrued professional fees	$	3,700	$	7,832
Accrued contract manufacturing & contract research costs	19,617		17,800
Accrued compensation and benefits	23,025		27,156
Accrued program fees	16,599		17,016
Accrued royalties	6,550		5,196
Accrued interest	—		3,679
Accrued milestones	—		4,000
Accrued sales rebates and discounts	12,079		5,900
Accrued taxes	11,177		7,300
Deferred reimbursements	1,500		1,250
Other	2,594		2,772
	$	96,841	$	99,901

Share based Compensation (Tables)

12 Months Ended

Dec. 31, 2020

Share-based Payment Arrangement [Abstract]

Schedule of Fair Value of Options

The fair value of the stock options granted is estimated on the date of grant using a Black-Scholes option pricing model with the following weighted-average assumptions:


	Years Ended December 31,
	2020			2019			2018
Expected stock price volatility	75.1		%	74.1		%	78.6		%
Risk free interest rate	1.6		%	2.4		%	2.4		%
Expected life of options (years)	5.67			5.68			5.62
Expected annual dividend per share	$	0.00		$	0.00		$	0.00

Schedule of Stock Options Activity

A summary of the Company's stock options for the year ended December 31, 2020 were as follows:


	Number of Shares	Weighted Average Exercise Price		Weighted Average Remaining Years	Aggregate Intrinsic Value
	(in thousands)				(in millions)
Options outstanding, December 31, 2017	15,181	$	7.48
Granted	2,348	$	14.96
Exercised	(1,398)	$	6.54
Forfeited	(313)	$	9.55
Expired	(8)	$	10.76
Options outstanding, December 31, 2018	15,810	$	8.63
Granted	4,091	$	10.29
Exercised	(1,967)	$	5.83
Forfeited	(879)	$	11.07
Expired	(331)	$	13.38
Options outstanding, December 31, 2019	16,724	$	9.15
Granted	4,362	$	9.98
Exercised	(5,243)	$	8.11
Forfeited	(1,376)	$	10.42
Expired	(435)	$	13.33
Options outstanding, December 31, 2020	14,032	$	9.54	6.5	$	190.1
Vested and unvested expected to vest, December 31, 2020	13,267	$	9.49	6.4	$	180.5
Exercisable at December 31, 2020	8,130	$	8.86	5.1	$	115.7

Schedule of Non-Vested RSU Activity under the Plan

A summary of non-vested RSU activity under the Plan for the year ended December 31, 2020 is as follows:


	Number of Shares	Weighted Average Grant Date Fair Value		Weighted Average Remaining Years	Aggregate Intrinsic Value
	(in thousands)				(in millions)
Non-vested units as of December 31, 2017	2,575	$	5.85
Granted	1,812	$	16.11
Vested	(530)	$	6.01
Forfeited	(145)	$	9.65
Non-vested units as of December 31, 2018	3,712	$	10.59
Granted	3,526	$	10.92
Vested	(921)	$	8.49
Forfeited	(525)	$	10.52
Non-vested units as of December 31, 2019	5,792	$	11.18
Granted	4,692	$	11.29
Vested	(2,164)	$	10.70
Forfeited	(1,240)	$	11.14
Non-vested units as of December 31, 2020	7,080	$	11.35	2.3	$	158.0

Schedule of Equity Compensation Expenses

The following table summarizes information related to compensation expense recognized in the Consolidated Statements of Operations related to the equity awards:


	Years Ended December 31,
(in thousands)	2020		2019		2018
Equity compensation expense recognized in:
Research and development expense	$	20,817	$	17,575	$	11,740
Selling, general, and administrative expense	28,334		26,855		17,520
Total equity compensation expense	$	49,151	$	44,430	$	29,260

Assets and Liabilities Measured at Fair Value (Tables)

12 Months Ended

Dec. 31, 2020

Fair Value Disclosures [Abstract]

Summary of Assets and Liabilities Subject to Fair Value Measurements


(in thousands)	Level 2		Total
Assets:
Commercial paper	$	217,095	$	217,095
Asset-backed securities	9,438		9,438
Corporate debt securities	39,513		39,513
U.S. government agency bonds	53,582		53,582
Money market funds	4,427		4,427
	$	324,055	$	324,055


(in thousands)	Level 2		Level 3		Total
Liabilities:
Contingent consideration payable	$	—	$	25,825	$	25,825
Deferred compensation plan liability	4,078		—		4,078
	$	4,078	$	25,825	$	29,903


(in thousands)	Level 2		Total
Assets:
Commercial paper	$	73,710	$	73,710
Asset-backed securities	77,810		77,810
Corporate debt securities	145,991		145,991
U.S. government agency bonds	11,991		11,991
Money market funds	4,768		4,768
	$	314,270	$	314,270


(in thousands)	Level 2		Level 3		Total
Liabilities:
Contingent consideration payable	$	—	$	22,681	$	22,681
Deferred compensation plan liability	4,419		—		4,419
	$	4,419	$	22,681	$	27,100

Schedule of Significant Unobservable Inputs Used in the Valuation of the Contingent Consideration Payable

The following significant unobservable inputs were used in the valuation of the contingent consideration payable of Callidus for the ATB-200 Pompe program:

Contingent Consideration Liability

Fair Value as of December 31, 2020

Valuation Technique

Unobservable Input

Range

(in thousands)

Discount rate

7.5%

Clinical and regulatory milestones

25,101

Probability weighted discounted cash flow

Probability of achievement of milestones

75%-78%

Projected year of payments

2021-2022

Schedule of Changes in Contingent Consideration Payable

The following table shows the change in the balance of contingent consideration payable for the year ended December 31, 2020 and December 31, 2019, respectively:


	Years ended December 31,
(in thousands)	2020		2019
Balance, beginning of the period	$	22,681	$	19,700
Changes in fair value during the period, included in the Consolidated Statements of Operations	3,144		3,297
Adjustment for contingent consideration paid in stock	—		(316)
Balance, end of the period	$	25,825	$	22,681

- Definition

Tabular disclosure of financial instruments measured at fair value, including those classified in shareholders' equity measured on a recurring or nonrecurring basis. Disclosures include, but are not limited to, fair value measurements recorded and the reasons for the measurements, level within the fair value hierarchy in which the fair value measurements are categorized and transfers between levels 1 and 2. Nonrecurring fair value measurements are those that are required or permitted in the statement of financial position in particular circumstances.

+ References + Details

Debt (Tables)

12 Months Ended

Dec. 31, 2020

Debt Disclosure [Abstract]

Schedule of Liability Components of the Convertible Notes

The Company's debt consists of the following:


(in thousands)	December 31, 2020		December 31, 2019
Senior Secured Term Loan due 2026:
Principal	$	400,000	$	—
Less: debt discount (1)	(7,434)		—
Less: deferred financing (1)	(5,592)		—
Net carrying value of the Senior Secured Term Loan	$	386,974	$	—

Senior Secured Term Loan due 2023 (2):
Principal	$	—	$	150,000
Less: debt discount (1)	—		(2,315)
Less: deferred financing (1)	—		(311)
Net carrying value of the Senior Secured Term Loan	$	—	$	147,374

Convertible Notes (3):
Principal	$	2,825	$	2,825
Less: debt discount (1)	(518)		(659)
Less: deferred financing (1)	(27)		(35)
Net carrying value of the Convertible Notes	$	2,280	$	2,131

Net carrying value of Long-term debt	$	389,254	$	149,505

______________________________

(2) The principle, accrued interest and early settlement premiums associated with the Senior Secured Term Loan due 2023 were fully paid and settled in July 2020.

Components of Total Interest Expense Recognized Related to the Convertible Notes

The following table sets forth interest expense recognized related to the Company's debt for the years ended December 31, 2020 and 2019 respectively:


(in thousands)	December 31, 2020			December 31, 2019
Contractual interest expense	$	20,933		$	16,483
Amortization of debt discount	$	1,267		$	2,381
Amortization of deferred financing	$	527		$	165
Effective interest rate of the liability component, Convertible debt	10.6		%	10.6		%
Effective interest rate of the liability component, Senior Secured Term Loan due 2026	8.4		%	—		%
Effective interest rate of the liability component, Senior Secured Term Loan due 2023	—		%	10.2		%

Leases (Tables)

12 Months Ended

Dec. 31, 2020

Leases [Abstract]

Lease Cost

Supplemental balance sheet information related to operating leases were as follows:


(in thousands, except year and discount rate amounts)	December 31, 2020			December 31, 2019
Operating lease ROU asset	$	23,296		$	33,315

Current portion of the operating lease liabilities	$	6,872		$	7,189
Non-current portion of the operating lease liabilities	45,604			53,531
Total operating lease liability	$	52,476		$	60,720

Weighted-average remaining lease terms (years)	19.8			18.2
Weighted-average discount rate	13.0		%	13.1		%

Future Minimum Operating Lease Payments

At December 31, 2020, the future minimum operating lease payments were as follows:


(in thousands)	Operating Lease
2021	$	8,549
2022	8,494
2023	8,691
2024	8,404
2025	8,371
Thereafter	153,241
Total lease payments	195,750
Less lease incentives	(28,939)
Less imputed interest	(114,335)
Total operating lease liability	$	52,476

At December 31, 2019, the future minimum operating lease payments were as follows:


(in thousands)	Operating Lease
2020	$	9,738
2021	10,787
2022	10,459
2023	10,809
2024	10,582
Thereafter	163,266
Total lease payments	$	215,641
Less lease incentives	(28,939)
Less imputed interest	(125,982)
Total operating lease liability	$	60,720

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2020

Income Tax Disclosure [Abstract]

Schedule of loss before Income Taxes

For financial reporting purposes, loss before income taxes includes the following components:


	Years Ended December 31,
(in thousands)	2020		2019		2018
United States	$	(365,332)	$	(393,955)	$	(309,183)
Foreign	91,078		38,045		(39,906)
Total	$	(274,254)	$	(355,910)	$	(349,089)

Schedule of Income Tax Expense (Benefit)

Following were the components of income tax expense (benefit) for the years ended December 31, 2020, 2019, and 2018:


(in thousands)	2020		2019		2018
Current
Federal	$	—	$	—	$	—
State	—		—		6
Foreign	4,163		2,877		(100)
Deferred
Federal	(1,410)		(984)		—
State	(155)		(1,415)		—
Foreign	—		—		—
Total	$	2,598	$	478	$	(94)

Schedule of Reconciliation of the Statutory Tax Rates and the Effective Tax Rates

A reconciliation of the statutory tax rates and the effective tax rates for the years ended December 31, 2020, 2019, and 2018 are as follows:


	Years Ended December 31,
	2020		2019		2018
Statutory rate	(21)	%	(21)	%	(21)	%
State taxes, net of federal benefit	—		—		(4)
Nondeductible IPR&D	—		—		6
Contingent consideration	—		—		1
Tax credits	(7)		(16)		(10)
Impact of foreign operations	4		9		2
Nondeductible executive compensation	2		1		—
Nondeductible debt conversion	—		(1)		—
Other	—		(1)		—
Valuation allowance	23		29		26
Net	1	%	—	%	—	%

Schedule of Significant Components of the Deferred Tax Assets and Liabilities

The significant components of the deferred tax assets and liabilities are as follows:


	Years Ended December 31,
(in thousands)	2020		2019
Deferred tax assets
Intellectual property	$	68,209	$	46,521
Amortization/depreciation	489		2,329
Research tax credit	182,651		141,669
Net operating loss carry forwards	305,876		286,850
Deferred reimbursements	1,912		2,230
Non-cash stock issue	17,553		19,058
Interest carry forward limitation	5,655		2,224
Lease liability	9,235		6,983
Other	10,855		13,921
Gross deferred tax assets	602,435		521,785
Deferred tax liabilities
Business acquisition	(4,937)		(5,051)
Royalty payable	(68,209)		(46,521)
Convertible notes	(94)		(653)
Advanced R&D payments	(1,776)		(2,611)
Right of use asset	(3,420)		(5,643)
Total net deferred tax assets	523,999		461,306
Less: valuation allowance	(528,895)		(466,357)
Net deferred tax liability	$	(4,896)	$	(5,051)

Basic and Diluted Net Loss per Common Share (Tables)

12 Months Ended

Dec. 31, 2020

Earnings Per Share [Abstract]

Schedule of Reconciliation of the Numerator and Denominator Used in Computing Basic and Diluted Net Loss Per Common Share

The following table provides a reconciliation of the numerator and denominator used in computing basic and diluted net loss attributable to common stockholders per common share:


	Years Ended December 31,
(in thousands, except per share amounts)	2020		2019		2018
Numerator:
Net loss attributable to common stockholders	$	(276,852)	$	(356,388)	$	(348,995)
Denominator:
Weighted average common shares outstanding — basic and diluted	258,867,380		240,421,001		185,790,021

Schedule of Potential Shares of Common Stock that were Excluded from the Computation as they were Anti-Dilutive Using the Treasury Stock Method

The table below presents potential shares of common stock that were excluded from the computation as they were anti-dilutive using the treasury stock method:


	Years ended December 31,
(in thousands)	2020	2019	2018
Options to purchase common stock	14,032	16,724	15,810
Convertible notes	462	462	40,850
Outstanding warrants, convertible to common stock	2,555	2,555	2,657
Unvested restricted stock units	7,080	5,792	3,712
Total number of potentially issuable shares	24,129	25,533	63,029

Selected Quarterly Financial Data (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2020

Quarterly Financial Information Disclosure [Abstract]

Schedule of Selected Quarterly Financial Data


	Quarters Ended
(in thousands except per share data)	March 31		June 30		September 30		December 31
2020
Net product sales	$	60,525	$	62,353	$	67,437	$	70,571
Net loss	$	(88,948)	$	(52,492)	$	(64,011)	$	(71,401)
Basic and diluted net loss per common share (1)	$	(0.35)	$	(0.20)	$	(0.25)	$	(0.27)
2019
Net product sales	$	34,046	$	44,130	$	48,768	$	55,293
Net loss	$	(120,299)	$	(84,551)	$	(61,809)	$	(89,729)
Basic and diluted net loss per common share (1)	$	(0.56)	$	(0.36)	$	(0.24)	$	(0.35)

_______________________________

Description of Business (Details) - USD ($)	1 Months Ended
Description of Business (Details) - USD ($)	Jul. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Accumulated deficit		$ 2,045,462,000	$ 1,768,610,000
Senior Secured Term Loan Due2023 \| Senior Loans
Debt Instrument [Line Items]
Early repayment of senior debt	$ 156,300,000
Secured Debt \| Senior Secured Term Loan due 2026
Debt Instrument [Line Items]
Maximum borrowing capacity	400,000,000
Proceeds from secured lines of credit	385,900,000
Early repayment of senior debt	$ 156,300,000
Secured Debt \| Senior Secured Term Loan due 2026 \| LIBOR
Debt Instrument [Line Items]
Variable rate	1.00%
Basis spread on variable rate	6.50%
Secured Debt \| Senior Secured Term Loan due 2026 \| Senior Loans
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 400,000,000
Debt instrument term	6 years
Proceeds from secured lines of credit	$ 385,900,000
Secured Debt \| Senior Secured Term Loan due 2026 \| Senior Loans \| Minimum \| LIBOR
Debt Instrument [Line Items]
Variable rate	1.00%
Secured Debt \| Senior Secured Term Loan due 2026 \| Senior Loans \| Maximum \| LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	6.50%

Summary of Significant Accounting Policies - Concentration of Credit Risk (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Concentration Risk [Line Items]
Allowance for doubtful accounts receivable	$ 0.1
Net product sales \| Customer Concentration Risk
Concentration Risk [Line Items]
Concentration risk, percentage	25.00%	22.00%
Accounts receivable \| Customer Concentration Risk
Concentration Risk [Line Items]
Concentration risk, percentage	12.00%	11.00%

Summary of Significant Accounting Policies - Property and Equipment (Details)	12 Months Ended
	Dec. 31, 2020
Minimum
Property and Equipment
Estimated useful lives	3 years
Maximum
Property and Equipment
Estimated useful lives	5 years

Summary of Significant Accounting Policies - Revenue by Geographical Area (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Net product sales	$ 70,571	$ 67,437	$ 62,353	$ 60,525	$ 55,293	$ 48,768	$ 44,130	$ 34,046	$ 260,886	$ 182,237	$ 91,245
U.S.
Disaggregation of Revenue [Line Items]
Net product sales									80,046	52,478	7,223
Ex-U.S.
Disaggregation of Revenue [Line Items]
Net product sales									$ 180,840	$ 129,759	$ 84,022

- Definition

Amount, including tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value-added and excise.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 41
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e9038-108599

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=121604090&loc=SL49130545-203045

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 32
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8933-108599

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 32
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8933-108599

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 30
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8906-108599

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 22
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8736-108599

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 22
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8736-108599

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121604090&loc=SL49130543-203045

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 42
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e9054-108599

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 40
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e9031-108599

Reference 11: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 924
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 11.L)
-URI http://asc.fasb.org/extlink&oid=6472922&loc=d3e499488-122856

+ Details

Summary of Significant Accounting Policies - Equity Incentive Plan and Stock-Based Compensation (Details)	Dec. 31, 2020 plan
Equity-based Compensation
Number of equity-based employee compensation plans	1

Summary of Significant Accounting Policies - Segment Information and Recent Accounting Policies (Details)	12 Months Ended
	Dec. 31, 2020 segment unit
Accounting Policies [Abstract]
Number of business segments \| segment	1
Number of reporting units \| unit	1

Acquisitions (Details) $ in Millions	1 Months Ended	12 Months Ended
Acquisitions (Details) $ in Millions	Sep. 30, 2018 USD ($) program	Dec. 31, 2018 USD ($)
Acquisitions
Number of gene therapy programs for neurologic LSDs developed \| program	10
Celenex
Acquisitions
Contingent consideration based on the achievement of certain milestones	$ 10.0
Contingent consideration based on the achievement of regulatory milestones	220.0
Maximum payment under contingent consideration	75.0
Research and development expense		$ 100.0
Celenex \| Nationwide Children's Hospital
Acquisitions
Contingent consideration based on the achievement of certain milestones	$ 7.8

Goodwill and Intangible Assets - Narrative (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill	$ 197,797	$ 197,797	$ 197,800
In-process research & development	$ 23,000	$ 23,000

Goodwill and Intangible Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Changes in IPR&D
Beginning balance	$ 23,000	$ 23,000
Change in IPR&D	0	0
Ending balance	23,000	23,000
Changes in goodwill
Beginning balance	197,797	197,800
Change in goodwill	0	0
Ending balance	$ 197,797	$ 197,797

Cash, Cash Equivalents, Marketable Securities, and Restricted Cash - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash, Cash Equivalents, and Short-term Investments [Abstract]
Cash and cash equivalents	$ 163,240,000	$ 142,837,000	$ 79,749,000
Debt securities, available-for-sale	320,029,000	309,903,000
Debt securities, realized gain (loss)	0	0
Fair value of available-for-sale debt securities in unrealized loss positions	$ 124,900,000	$ 42,600,000

Cash, Cash Equivalents, Marketable Securities, and Restricted Cash - Cash and Available for Sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Cash and cash equivalents	$ 163,240	$ 142,837	$ 79,749
Cost, debt securities	320,016	309,665
Cost, cash balances and debt securities	483,256	452,502
Gross Unrealized Gain	39	255
Gross Unrealized Loss	(26)	(17)
Fair value, cash balances	163,240	142,837
Debt securities, available-for-sale	320,029	309,903
Fair value, cash balances and debt securities	483,269	452,740
Marketable securities with maturity date greater than 12 months		9,500
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Cost, debt securities	39,525	145,875
Gross Unrealized Gain	4	121
Gross Unrealized Loss	(16)	(5)
Debt securities, available-for-sale	39,513	145,991
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Cost, debt securities	217,087	73,659
Gross Unrealized Gain	14	53
Gross Unrealized Loss	(6)	(2)
Debt securities, available-for-sale	217,095	73,710
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Cost, debt securities	9,420	77,731
Gross Unrealized Gain	18	79
Gross Unrealized Loss	0	0
Debt securities, available-for-sale	9,438	77,810
U.S. government agency bonds
Debt Securities, Available-for-sale [Line Items]
Cost, debt securities	53,583	11,999
Gross Unrealized Gain	3	2
Gross Unrealized Loss	(4)	(10)
Debt securities, available-for-sale	53,582	11,991
Money market
Debt Securities, Available-for-sale [Line Items]
Cost, debt securities	350	350
Gross Unrealized Gain	0	0
Gross Unrealized Loss	0	0
Debt securities, available-for-sale	350	350
Certificates of deposit
Debt Securities, Available-for-sale [Line Items]
Cost, debt securities	51	51
Gross Unrealized Gain	0	0
Gross Unrealized Loss	0	0
Debt securities, available-for-sale	$ 51	$ 51

Cash, Cash Equivalents, Marketable Securities, and Restricted Cash - Reconciliation of Cash and Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash, Cash Equivalents, and Short-term Investments [Abstract]
Cash and cash equivalents	$ 163,240	$ 142,837	$ 79,749
Restricted cash	2,922	3,506	2,626
Cash, cash equivalents, and restricted cash shown in the Consolidated Statements of Cash Flows	$ 166,162	$ 146,343	$ 82,375	$ 51,237

Inventories (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Inventory Disclosure [Abstract]
Raw materials	$ 5,547	$ 6,544
Work-in-process	7,693	3,660
Finished goods	6,316	3,837
Total inventories	19,556	14,041
Reserve for inventory	$ 100	$ 200

Property and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
Property and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Property and Equipment
Property and equipment, gross	$ 58,350	$ 65,309
Less accumulated depreciation	(14,487)	(17,604)
Net property and equipment	43,863	47,705
Depreciation and amortization	8,400	4,500
Computer equipment
Property and Equipment
Property and equipment, gross	5,546	6,185
Computer software
Property and Equipment
Property and equipment, gross	1,207	1,607
Research equipment
Property and Equipment
Property and equipment, gross	15,387	14,568
Furniture and fixtures
Property and Equipment
Property and equipment, gross	2,880	5,157
Leasehold improvements
Property and Equipment
Property and equipment, gross	24,146	30,294
Vehicles
Property and Equipment
Property and equipment, gross	124	124
Land
Property and Equipment
Property and equipment, gross	3,190	3,190
Construction in progress
Property and Equipment
Property and equipment, gross	$ 5,870	$ 4,184

Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Accounts payable and accrued expenses
Accrued professional fees	$ 3,700	$ 7,832
Accrued contract manufacturing & contract research costs	19,617	17,800
Accrued compensation and benefits	23,025	27,156
Accrued program fees	16,599	17,016
Accrued royalties	6,550	5,196
Accrued interest	0	3,679
Accrued milestones	0	4,000
Accrued sales rebates and discounts	12,079	5,900
Accrued taxes	11,177	7,300
Deferred reimbursements	1,500	1,250
Other	2,594	2,772
Accrued expenses and other current liabilities	$ 96,841	$ 99,901

Stockholders' Equity - Common Stock and Warrants (Details) $ / shares in Units, $ in Thousands	1 Months Ended				3 Months Ended		12 Months Ended
	Feb. 28, 2021 USD ($) $ / shares shares	Mar. 31, 2019 USD ($) $ / shares shares	Apr. 30, 2018 USD ($) $ / shares shares	Feb. 28, 2018 USD ($) $ / shares shares	Jun. 30, 2019 USD ($) $ / shares shares	Mar. 31, 2019 USD ($) $ / shares	Dec. 31, 2020 USD ($) vote $ / shares shares	Dec. 31, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) shares	Jun. 30, 2018 $ / shares shares	May 31, 2018 shares
Convertible Debt
Common stock, shares authorized (in shares) \| shares							500,000,000	500,000,000		500,000,000	250,000,000
Voting right for each share held, number \| vote							1
Stock issued from equity financing (in shares) \| shares					18,700,000
Gross cash proceeds from warrant exercises							$ 0	$ 812	$ 3,617
Net proceeds from the issuance of common stock after deducting underwriting discounts and commissions					$ 189,000		$ 0	$ 188,994	$ 294,584
Contingent consideration, milestone payment						$ 9,000
Milestone payment, stock issued as consideration						9,000
Exchange agreement, aggregate principal amount					$ 247,200
Exchange agreement, authorized (in shares) \| shares					44,000,000.0
Common stock, par value (in dollars per share) \| $ / shares					$ 0.01		$ 0.01	$ 0.01		$ 0.01
Common Stock
Convertible Debt
Stock issued from equity financing (in shares) \| shares								18,720,930	20,239,839
Milestone payment, stock issued as consideration						$ 9,300
Underwritten Offering
Convertible Debt
Stock issued (in dollars per share) \| $ / shares					$ 10.75
Common Stock
Convertible Debt
Stock issued from equity financing (in shares) \| shares				20,200,000
Net proceeds from the issuance of common stock after deducting underwriting discounts and commissions				$ 294,600
Price per share of common stock issued (in dollars per share) \| $ / shares				$ 15.50
Warrants
Convertible Debt
Stock issued from equity financing (in shares) \| shares		101,787	453,214
Gross cash proceeds from warrant exercises		$ 800	$ 3,600
Warrants \| Common Stock
Convertible Debt
Exercise price of warrants (in dollars per share) \| $ / shares		$ 7.98	$ 7.98			$ 7.98
Warrants \| Subsequent Event
Convertible Debt
Stock issued from equity financing (in shares) \| shares	1,050,000
Gross cash proceeds from warrant exercises	$ 7,400
Warrants \| Subsequent Event \| Common Stock
Convertible Debt
Exercise price of warrants (in dollars per share) \| $ / shares	$ 7.06

Stockholders' Equity - Nonqualified Cash Plan and Equity Incentive Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Stockholders' Equity
Deferred compensation investment	$ 4.1	$ 4.4
2007 Plan and 2007 Director Plan
Stockholders' Equity
Expiration period	10 years
Vesting percentage	100.00%
2007 Plan and 2007 Director Plan \| Maximum \| Options to purchase common stock
Stockholders' Equity
Number of shares reserved for issuance (in shares)	15,018,368
2007 Plan and 2007 Director Plan \| First Anniversary
Stockholders' Equity
Monthly vesting percentage	2.08%
Vesting percentage	25.00%

Share based Compensation - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
	Dec. 21, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Proceeds from exercise of stock options		$ 42,282	$ 11,476	$ 9,144
Unvested restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Period following separation for accelerated vesting	2 years
Unvested restricted stock units \| Stock Option Plan Amended Restated 2007 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation costs, period for recognition (in years)		2 years
RSUs vested and non-vested units expected to vest (in shares)		2,163,732	921,000	530,000
Unrecognized compensation cost		$ 59,400
Options to purchase common stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average grant-date fair value, options (in dollars per share)		$ 6.40	$ 6.67	$ 10.19
Aggregate intrinsic value of options exercised		$ 40,900	$ 11,800	$ 11,900
Proceeds from exercise of stock options		42,300	$ 11,500	$ 9,100
Total unrecognized compensation costs		$ 29,400
Unrecognized compensation costs, period for recognition (in years)		3 years

Share based Compensation - Weighted-average Assumptions (Details) - Options to purchase common stock - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected stock price volatility	75.10%	74.10%	78.60%
Risk free interest rate	1.60%	2.40%	2.40%
Expected life of options (years)	5 years 8 months 1 day	5 years 8 months 4 days	5 years 7 months 13 days
Expected annual dividend per share	$ 0.00	$ 0.00	$ 0.00

Share based Compensation - Stock Option Activity (Details) - Options to purchase common stock - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Number of Shares
Balance at the beginning of the period (in shares)	16,724,000	15,810,000	15,181,000
Options, Granted (in shares)	4,362,000	4,091,000	2,348,000
Options, Exercised (in shares)	(5,243,000)	(1,967,000)	(1,398,000)
Options, Forfeited (in shares)	(1,376,000)	(879,000)	(313,000)
Options, Expired (in shares)	(435,000)	(331,000)	(8,000)
Balance at the end of the period (in shares)	14,032,000	16,724,000	15,810,000
Vested and unvested expected to vest as of the end of the period (in shares)	13,267,000
Exercisable at the end of the period (in shares)	8,130,000
Weighted Average Exercise Price
Balance at the beginning of the period (in dollars per share)	$ 9.15	$ 8.63	$ 7.48
Options, Granted (in dollars per share)	9.98	10.29	14.96
Options, Exercised (in dollars per share)	8.11	5.83	6.54
Options, Forfeited (in dollars per share)	10.42	11.07	9.55
Options, Expired (in dollars per share)	13.33	13.38	10.76
Balance at the end of the period (in dollars per share)	9.54	$ 9.15	$ 8.63
Vested and unvested expected to vest as of the end of the period (in dollars per share)	9.49
Exercisable at the end of the period (in dollars per share)	$ 8.86
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Balance at the end of the period	6 years 6 months
Vested and unvested expected to vest at the end of the period	6 years 4 months 24 days
Exercisable at the end of the period	5 years 1 month 6 days
Aggregate intrinsic value of options outstanding	$ 190.1
Aggregate intrinsic value of options vested and unvested expected to vest	180.5
Aggregate intrinsic value of options exercisable	$ 115.7

Share based Compensation - RSUs and PBRSUs Summary (Details) - Stock Option Plan Amended Restated 2007 Equity Incentive Plan - Unvested restricted stock units - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Number of Shares
Non-vested units as of the beginning of the period (in shares)	5,792,000	3,712,000	2,575,000
Granted (in shares)	4,692,000	3,526,000	1,812,000
Vested (in shares)	(2,163,732)	(921,000)	(530,000)
Forfeited (in shares)	(1,240,000)	(525,000)	(145,000)
Non-vested units as of the end of the period (in shares)	7,080,000	5,792,000	3,712,000
Weighted Average Grant Date Fair Value
Non-vested units as of the beginning of the period (in dollars per share)	$ 11.18	$ 10.59	$ 5.85
Granted (in dollars per share)	11.29	10.92	16.11
Vested (in dollars per share)	10.70	8.49	6.01
Forfeited (in dollars per share)	11.14	10.52	9.65
Non-vested units as of the end of the period (in dollars per share)	$ 11.35	$ 11.18	$ 10.59
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Additional Disclosures [Abstract]
Non-vested units, weighted average remaining years	2 years 3 months 18 days
Non-vested units, aggregate intrinsic value	$ 158.0

Share based Compensation - Expense Summary (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total equity compensation expense	$ 49,151	$ 44,430	$ 29,260
Research and development expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total equity compensation expense	20,817	17,575	11,740
Selling, general, and administrative expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total equity compensation expense	$ 28,334	$ 26,855	$ 17,520

Assets and Liabilities Measured at Fair Value (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Assets:
Fair value of assets		$ 324,055	$ 314,270
Liabilities:
Contingent consideration payable		25,825	22,681
Deferred compensation plan liability		4,078	4,419
Fair value of liabilities		29,903	27,100
Milestone payment, stock issued as consideration	$ 9,000
Common Stock
Liabilities:
Milestone payment, stock issued as consideration	$ 9,300
ATB-200 Pompe Program \| Clinical and Regulatory Approval Milestones \| Callidus
Liabilities:
Contingent consideration payable		$ 25,101
ATB-200 Pompe Program \| Clinical and Regulatory Approval Milestones \| Callidus \| Probability of Milestone Achievement \| Minimum
Liabilities:
Contingent consideration, measurement input		0.75
ATB-200 Pompe Program \| Clinical and Regulatory Approval Milestones \| Callidus \| Probability of Milestone Achievement \| Maximum
Liabilities:
Contingent consideration, measurement input		0.78
Convertible Notes \| 2016 Convertible Notes
Liabilities:
Fair value of the debt		$ 10,900
Commercial paper
Assets:
Fair value of assets		217,095	73,710
Asset-backed securities
Assets:
Fair value of assets		9,438	77,810
Corporate debt securities
Assets:
Fair value of assets		39,513	145,991
U.S. government agency bonds
Assets:
Fair value of assets		53,582	11,991
Money market
Assets:
Fair value of assets		4,427	4,768
Level 2
Assets:
Fair value of assets		324,055	314,270
Liabilities:
Contingent consideration payable		0	0
Deferred compensation plan liability		4,078	4,419
Fair value of liabilities		4,078	4,419
Level 2 \| Commercial paper
Assets:
Fair value of assets		217,095	73,710
Level 2 \| Asset-backed securities
Assets:
Fair value of assets		9,438	77,810
Level 2 \| Corporate debt securities
Assets:
Fair value of assets		39,513	145,991
Level 2 \| U.S. government agency bonds
Assets:
Fair value of assets		53,582	11,991
Level 2 \| Money market
Assets:
Fair value of assets		4,427	4,768
Level 3
Liabilities:
Contingent consideration payable		25,825	22,681
Deferred compensation plan liability		0	0
Fair value of liabilities		$ 25,825	$ 22,681
Level 3 \| ATB-200 Pompe Program \| Clinical and Regulatory Approval Milestones \| Probability Weighted Discounted Cash Flow \| Callidus \| Discount Rate
Liabilities:
Contingent consideration, measurement input		0.075

Assets and Liabilities Measured at Fair Value - Level 3 Roll Forward (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of the period	$ 22,681	$ 19,700
Changes in fair value during the period, included in the Consolidated Statements of Operations	3,144	3,297	$ 3,300
Adjustment for contingent consideration paid in stock	0	(316)
Balance, end of the period	$ 25,825	$ 22,681	$ 19,700

Debt - Summary of Long Term Debt (Details) $ in Thousands	1 Months Ended	12 Months Ended
Debt - Summary of Long Term Debt (Details) $ in Thousands	Dec. 31, 2016 day	Dec. 31, 2020 USD ($) day	Dec. 31, 2019 USD ($)
Debt Instruments
Long-term Debt, Total		$ 389,254	$ 149,505
Convertible Notes \| Senior Secured Term Loan due 2026
Debt Instruments
Principal		400,000	0
Less: debt discount		(7,434)	0
Less: deferred financing		(5,592)	0
Long-term Debt, Total		386,974	0
Convertible Notes \| Senior Secured Term Loan Due2023
Debt Instruments
Principal		0	150,000
Less: debt discount		0	(2,315)
Less: deferred financing		0	(311)
Long-term Debt, Total		0	147,374
Convertible Notes \| Convertible Senior Notes 2016, Due 2023
Debt Instruments
Principal		2,825	2,825
Less: debt discount		(518)	(659)
Less: deferred financing		(27)	(35)
Long-term Debt, Total		$ 2,280	$ 2,131
Threshold percentage	130.00%	130.00%
Threshold trading days		20 days
Threshold consecutive trading days \| day	30	30

Debt - Narrative (Details)	1 Months Ended			6 Months Ended	12 Months Ended
Debt - Narrative (Details)	Jul. 31, 2020 USD ($) day	Sep. 30, 2018 USD ($)	Dec. 31, 2016 USD ($) day $ / shares shares	Jun. 30, 2019 USD ($) $ / shares shares	Dec. 31, 2020 USD ($) day $ / shares	Dec. 31, 2019 USD ($) $ / shares	Dec. 31, 2018 USD ($)	Jun. 30, 2018 $ / shares
Convertible Debt
Loss on extinguishment of debt					$ 7,276,000	$ 0	$ 0
Proceeds from long-term debt, net of issuance costs					$ 385,929,000	$ 0	146,596,000
Exchange agreement, aggregate principal amount				$ 247,200,000
Exchange agreement, authorized (in shares) \| shares				44,000,000.0
Common stock, par value (in dollars per share) \| $ / shares				$ 0.01	$ 0.01	$ 0.01		$ 0.01
Accrued and unpaid interest on exchange agreement				$ 1,300,000
Gain (loss) on extinguishment of debt, additional paid-in capital						$ 215,000,000.0
Gain (loss) on extinguishment of debt, fair value of common shares issued						400,000
Loss on exchange of convertible notes					$ 0	40,624,000	0
Proceeds from capped call confirmations, exchange of convertible debt				$ 19,900,000	$ 0	$ (19,875,000)	$ 0
2016 Convertible Notes \| Convertible Notes
Convertible Debt
Aggregate principal amount			$ 250,000,000
Proceeds from long-term debt, net of issuance costs			243,000,000.0
Proceeds for capped call confirmations			$ 13,500,000
Shares issued per increment of convertible debt (in shares) \| shares			40,800,000
Debt conversion ratio (in shares)			0.1633987
Conversion price (in dollars per share) \| $ / shares			$ 6.12
Threshold trading days					20 days
Threshold consecutive trading days \| day			30		30
Threshold percentage			130.00%		130.00%
Period following 5 consecutive trading day period, trading price less than 98% of product of sales price			5 years
Period following issuance of stock rights options or warrants with price per share less than last reported sales price			60 days
Period for measurement of sales price			10 days
Excess of value per share for triggering event, percent			10.00%
2016 Convertible Notes \| Convertible Notes \| Private Placement Purchase Agreement \| Over-allotment Option
Convertible Debt
Aggregate principal amount			$ 25,000,000
2016 Convertible Notes \| Convertible Notes \| Minimum
Convertible Debt
Threshold trading days			20 days
Secured Debt \| Senior Secured Term Loan due 2026
Convertible Debt
Maximum borrowing capacity	$ 400,000,000
Number of payments \| day	9
Periodic payment	$ 44,400,000
Proceeds from secured lines of credit	385,900,000
Early repayment of senior debt	156,300,000
Interest expense, debt	1,100,000
Early settlement premiums	5,200,000
Loss on extinguishment of debt	$ 7,300,000
Secured Debt \| Senior Secured Term Loan due 2026 \| LIBOR
Convertible Debt
Variable rate	1.00%
Basis spread on variable rate	6.50%
Secured Debt \| Senior Secured Term Loan due 2026 \| Senior Loans
Convertible Debt
Maximum borrowing capacity	$ 400,000,000
Proceeds from secured lines of credit	385,900,000
Minimum liquidity covenant	$ 75,000,000
Debt instrument term	6 years
Secured Debt \| Senior Secured Term Loan due 2026 \| Senior Loans \| Minimum
Convertible Debt
Consolidated revenue covenant, amount	$ 140,000,000
Secured Debt \| Senior Secured Term Loan due 2026 \| Senior Loans \| Maximum
Convertible Debt
Consolidated revenue covenant, amount	$ 225,000,000
Secured Debt \| Senior Secured Term Loan due 2026 \| Senior Loans \| LIBOR \| Minimum
Convertible Debt
Variable rate	1.00%
Secured Debt \| Senior Secured Term Loan due 2026 \| Senior Loans \| LIBOR \| Maximum
Convertible Debt
Basis spread on variable rate	6.50%
Line of Credit
Convertible Debt
Maximum borrowing capacity		$ 150,000,000
Debt instrument term		5 years
Periodic payment, percentage		12.50%
Proceeds from issuance of debt		$ 146,600,000
Line of Credit \| LIBOR
Convertible Debt
Basis spread on variable rate		7.50%

Debt - Interest Expense (Details) - USD ($) $ in Thousands	12 Months Ended
Debt - Interest Expense (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Convertible Notes \| 2016 Convertible Notes
Interest expense
Contractual interest expense	$ 20,933	$ 16,483
Amortization of debt discount	1,267	2,381
Amortization of deferred financing	$ 527	$ 165
Effective interest rate of the liability component	10.60%	10.60%
Senior Notes
Interest expense
Effective interest rate of the liability component	8.40%	10.20%

Leases (Details) - USD ($) $ in Thousands	12 Months Ended
Leases (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Leases [Abstract]
Operating lease cost	$ 11,100	$ 10,800
Cash paid for amounts included in operating lease liabilities	9,600	4,900
Right-of-use assets obtained in exchange for new operating lease liabilities	1,700	21,800
Tenant improvements paid through lease incentive	470	19,388	$ 0
Operating lease ROU asset	23,296	33,315
Current portion of the operating lease liabilities	6,872	7,189
Non-current portion of the operating lease liabilities	45,604	53,531
Operating lease liability	$ 52,476	$ 60,720
Weighted-average remaining lease terms (years)	19 years 9 months 18 days	18 years 2 months 12 days
Weighted-average discount rate	13.00%	13.10%
Operating Lease
2021	$ 8,549	$ 9,738
2022	8,494	10,787
2023	8,691	10,459
2024	8,404	10,809
2025	8,371	10,582
Thereafter	153,241	163,266
Total lease payments	195,750	215,641
Lessee, Operating Lease, Liability, Tenant Incentives	(28,939)	(28,939)
Lessee, Operating Lease, Liability, Undiscounted Excess Amount	(114,335)	(125,982)
Total operating lease liability	$ 52,476	$ 60,720
Rent expense, including fees for utilities and common area maintenances			$ 5,700

Income Taxes - Statutory Rate (Details) - USD ($)		12 Months Ended
Income Taxes - Statutory Rate (Details) - USD ($)	Dec. 22, 2017	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income (loss) before income taxes:
United States		$ (365,332,000)	$ (393,955,000)	$ (309,183,000)
Foreign		91,078,000	38,045,000	(39,906,000)
Loss before income tax		(274,254,000)	(355,910,000)	(349,089,000)
Current
Federal		0	0	0
State		0	0	6,000
Foreign		4,163,000	2,877,000	(100,000)
Deferred
Federal		(1,410,000)	(984,000)	0
State		(155,000)	(1,415,000)	0
Foreign		0	0	0
Income tax expense (benefit)		$ 2,598,000	$ 478,000	$ (94,000)
Reconciliation of the statutory tax rates and the effective tax rates
Statutory rate	(21.00%)	(21.00%)	(21.00%)	(21.00%)
State taxes, net of federal benefit		0.00%	0.00%	(4.00%)
Nondeductible IPR&D		0.00%	0.00%	6.00%
Contingent consideration		0.00%	0.00%	1.00%
Tax credits		(7.00%)	(16.00%)	(10.00%)
Impact of foreign operations		4.00%	9.00%	2.00%
Nondeductible executive compensation		2.00%	1.00%	0.00%
Nondeductible debt conversion		0.00%	(1.00%)	0.00%
Other		0.00%	(1.00%)	0.00%
Valuation allowance		23.00%	29.00%	26.00%
Net		1.00%	0.00%	0.00%
Deferred federal, state and local, tax expense (benefit)		$ (1,600,000)
Accrual for interest and penalties		0
Accrual for uncertain tax positions		$ 0

Income Taxes - Deferred and Valuation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets
Intellectual property	$ 68,209	$ 46,521
Amortization/depreciation	489	2,329
Research tax credit	182,651	141,669
Net operating loss carry forwards	305,876	286,850
Deferred reimbursements	1,912	2,230
Non-cash stock issue	17,553	19,058
Interest carry forward limitation	5,655	2,224
Lease liability	9,235	6,983
Other	10,855	13,921
Gross deferred tax assets	602,435	521,785
Deferred tax liabilities
Business acquisition	(4,937)	(5,051)
Royalty payable	(68,209)	(46,521)
Convertible notes	(94)	(653)
Advanced R&D payments	(1,776)	(2,611)
Right of use asset	(3,420)	(5,643)
Total net deferred tax assets	523,999	461,306
Less: valuation allowance	(528,895)	(466,357)
Net deferred tax liability	(4,896)	$ (5,051)
Increase in the valuation allowance	$ 62,500

Income Taxes - Loss Carryforwards (Details) $ in Millions	Dec. 31, 2020 USD ($)
Research and experimentation tax credit carryforward
Operating Loss Carryforwards [Line Items]
Tax credit carryforward amount	$ 30.4
Orphan drug tax credit carryforwards
Operating Loss Carryforwards [Line Items]
Tax credit carryforward amount	141.8
Federal
Operating Loss Carryforwards [Line Items]
Amount of net operating loss carry forwards	1,156.0
State
Operating Loss Carryforwards [Line Items]
Amount of net operating loss carry forwards	$ 889.4

Collaborative Agreements (Details) $ in Thousands	1 Months Ended		3 Months Ended								12 Months Ended
Collaborative Agreements (Details) $ in Thousands	Oct. 31, 2018 USD ($)	Nov. 30, 2013 USD ($) market	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Collaborative Agreements
Revenue recognized			$ 70,571	$ 67,437	$ 62,353	$ 60,525	$ 55,293	$ 48,768	$ 44,130	$ 34,046	$ 260,886	$ 182,237	$ 91,245
Deferred reimbursements			7,406				8,906				7,406	8,906
Accrued expenses and other current liabilities			96,841				$ 99,901				96,841	$ 99,901
Collaborative Arrangements
Collaborative Agreements
Conditional aggregate payments per indication	$ 88,000
Discovery program fee	$ 10,000
Term of agreement	5 years
Payments for collaborative agreement	$ 7,000
Collaborative Arrangements \| GSK
Collaborative Agreements
Payments for collaborative agreement											1,300
Revised Agreement \| GSK
Collaborative Agreements
Potential milestone payments upon achievement of post-approval and sales-based milestones		$ 40,000
Number of major markets outside the U.S. from whom parties to contractual arrangement is eligible to receive post-approval and sales-based milestones \| market		8
Revenue recognized											21,700
Deferred reimbursements			8,900								8,900
Accrued expenses and other current liabilities			$ 6,600								$ 6,600

Basic and Diluted Net Loss per Common Share (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Numerator:
Net loss attributable to common stockholders	$ (71,401)	$ (64,011)	$ (52,492)	$ (88,948)	$ (89,729)	$ (61,809)	$ (84,551)	$ (120,299)	$ (276,852)	$ (356,388)	$ (348,995)
Denominator:
Weighted-average common shares outstanding - basic and diluted (in shares)									258,867,380	240,421,001	185,790,021
Antidilutive securities
Antidilutive securities excluded from computation of diluted earnings per share (in shares)									24,129,000	25,533,000	63,029,000
Options to purchase common stock
Antidilutive securities
Antidilutive securities excluded from computation of diluted earnings per share (in shares)									14,032,000	16,724,000	15,810,000
Convertible notes
Antidilutive securities
Antidilutive securities excluded from computation of diluted earnings per share (in shares)									462,000	462,000	40,850,000
Outstanding warrants, convertible to common stock
Antidilutive securities
Antidilutive securities excluded from computation of diluted earnings per share (in shares)									2,555,000	2,555,000	2,657,000
Unvested restricted stock units
Antidilutive securities
Antidilutive securities excluded from computation of diluted earnings per share (in shares)									7,080,000	5,792,000	3,712,000

Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Selected Quarterly Financial Data
Net product sales	$ 70,571	$ 67,437	$ 62,353	$ 60,525	$ 55,293	$ 48,768	$ 44,130	$ 34,046	$ 260,886	$ 182,237	$ 91,245
Net loss	$ (71,401)	$ (64,011)	$ (52,492)	$ (88,948)	$ (89,729)	$ (61,809)	$ (84,551)	$ (120,299)	$ (276,852)	$ (356,388)	$ (348,995)
Basic and diluted net loss per common share (in dollars per share)	$ (0.27)	$ (0.25)	$ (0.20)	$ (0.35)	$ (0.35)	$ (0.24)	$ (0.36)	$ (0.56)	$ (1.07)	$ (1.48)	$ (1.88)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

Label	Element	Value
Accounting Standards Update [Extensible List]	us-gaap_AccountingStandardsUpdateExtensibleList	us-gaap:AccountingStandardsUpdate201802Member

- Definition

Indicates amendment to accounting standards.

+ References

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+ Details