Ultragenyx Pharmaceutical (RARE) 10-K 14 Feb 20 2019 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2019	Feb. 10, 2020	Jun. 30, 2019
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	FY
Trading Symbol	RARE
Entity Registrant Name	Ultragenyx Pharmaceutical Inc.
Entity Central Index Key	0001515673
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		57,888,859
Entity Public Float			$ 2.7
Security Exchange Name	NASDAQ
Title of 12(b) Security	Common Stock, $0.001 par value
Entity File Number	001-36276
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	27-2546083
Entity Address, Address Line One	60 Leveroni Court
Entity Address, City or Town	Novato
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94949
City Area Code	415
Local Phone Number	483-8800
Document Annual Report	true
Document Transition Report	false
Documents Incorporated by Reference	DOCUMENTS INCORPORATED BY REFERENCE Portions of the registrant’s definitive proxy statement relating to its 2020 Annual Meeting of Stockholders, to be held on or about June 26, 2020, are incorporated by reference into Part III of this Annual Report on Form 10-K where indicated. Such proxy statement will be filed with the U.S. Securities and Exchange Commission within 120 days after the end of the fiscal year to which this report relates.

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 433,584	$ 113,432
Short-term investments	321,646	346,274
Accounts receivable	32,844	12,740
Inventory	11,546	7,065
Prepaid expenses and other current assets	51,397	42,858
Total current assets	851,017	522,369
Property and equipment, net	44,348	20,046
Investment in Arcturus equity securities	27,752
Intangible assets, net	129,000	129,223
Goodwill	44,406	44,406
Right-of-use lease assets	30,328
Long-term investments	5,174
Other assets	3,471	3,514
Total assets	1,135,496	719,558
Current liabilities:
Accounts payable	12,871	12,275
Accrued liabilities	83,194	62,450
Short-term lease liabilities	7,235
Total current liabilities	103,300	74,725
Deferred tax liabilities	33,306	31,166
Long-term lease liabilities	29,757
Liability related to the sale of future royalties	315,369	0
Other liabilities		4,759
Total liabilities	481,732	110,650
Commitments and contingencies (Notes 9 and 16)
Stockholders’ equity:
Preferred stock, par value of $0.001 per share—25,000,000 shares authorized; nil outstanding as of December 31, 2019 and 2018
Common stock, par value of $0.001 per share—250,000,000 shares authorized; 57,838,220 and 50,860,588 shares issued and outstanding as of December 31, 2019 and 2018, respectively	58	51
Additional paid-in capital	2,086,863	1,639,773
Accumulated other comprehensive loss	(147)	(633)
Accumulated deficit	(1,433,010)	(1,030,283)
Total stockholders’ equity	653,764	608,908
Total liabilities and stockholders’ equity	$ 1,135,496	$ 719,558

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares	Dec. 31, 2019	Dec. 31, 2018
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	25,000,000	25,000,000
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	250,000,000	250,000,000
Common stock, shares issued	57,838,220	50,860,588
Common stock, shares outstanding	57,838,220	50,860,588

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenues:
Total revenues	$ 103,714	$ 51,495	$ 2,612
Operating expenses:
Cost of sales	9,008	1,146	1
Research and development	357,355	293,998	231,644
Selling, general and administrative	161,524	127,724	99,909
Total operating expenses	527,887	422,868	331,554
Loss from operations	(424,173)	(371,373)	(328,942)
Interest income	13,238	9,542	4,074
Gain from sale of priority review vouchers		170,322
Change in fair value of investment in Arcturus equity securities	13,413
Non-cash interest expense on liability related to the sale of future royalties	(1,135)
Other income (expense)	(787)	(5,588)	6,530
Loss before income taxes	(399,444)	(197,097)	(318,338)
Benefit from (provision for) income taxes	(3,283)	(514)	16,199
Net loss	$ (402,727)	$ (197,611)	$ (302,139)
Net loss per share, basic and diluted	$ (7.12)	$ (3.97)	$ (7.12)
Shares used in computing net loss per share, basic and diluted	56,576,885	49,775,223	42,453,135
Collaboration and License
Revenues:
Total revenues	$ 83,493	$ 41,693	$ 2,136
Product Sales
Revenues:
Total revenues	$ 20,221	$ 9,802	$ 476

CONSOLIDATED STATEMENTS OF COMP

CONSOLIDATED STATEMENTS OF COMPREHENSIVE LOSS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (402,727)	$ (197,611)	$ (302,139)
Other comprehensive income (loss):
Foreign currency translation adjustments	23	(303)	(10,110)
Transfer of currency translation adjustments balance to other income related to the liquidation of foreign subsidiaries		5,272	3,490
Unrealized gain on available-for-sale securities	463	78	35
Other comprehensive income (loss):	486	5,047	(6,585)
Total comprehensive loss	$ (402,241)	$ (192,564)	$ (308,724)

CONSOLIDATED STATEMENTS OF SHAR

CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Beginning balance at Dec. 31, 2016	$ 473,974	$ 41	$ 1,003,561	$ 905	$ (530,533)
Beginning balance, shares at Dec. 31, 2016		41,240,230
Issuance of common stock in connection with at-the-market offering, net of issuance costs	131,960	$ 2	131,958
Issuance of common stock in connection with at-the-market offering, net of issuance costs, shares		2,251,217
Fair value of vested stock options assumed from acquisition	8,979		8,979
Employee stock-based compensation	68,014		68,014
Issuance of common stock under equity plan awards, net of tax	9,251	$ 1	9,250
Issuance of common stock under equity plan awards, net of tax, shares		675,624
Other comprehensive income (loss)	(6,585)			(6,585)
Net loss	(302,139)				(302,139)
Ending balance at Dec. 31, 2017	383,454	$ 44	1,221,762	(5,680)	(832,672)
Ending balance, shares at Dec. 31, 2017		44,167,071
Issuance of common stock in connection with underwritten public offering, net of issuance costs	270,969	$ 5	270,964
Issuance of common stock in connection with underwritten public offering, net of issuance costs, shares		5,043,860
Issuance of common stock in connection with at-the-market offering, net of issuance costs	38,056	$ 1	38,055
Issuance of common stock in connection with at-the-market offering, net of issuance costs, shares		640,257
Employee stock-based compensation	81,165		81,165
Issuance of common stock under equity plan awards, net of tax	27,828	$ 1	27,827
Issuance of common stock under equity plan awards, net of tax, shares		1,009,400
Other comprehensive income (loss)	5,047			5,047
Net loss	(197,611)				(197,611)
Ending balance at Dec. 31, 2018	$ 608,908	$ 51	1,639,773	(633)	(1,030,283)
Ending balance, shares at Dec. 31, 2018	50,860,588	50,860,588
Issuance of common stock in connection with underwritten public offering, net of issuance costs	$ 330,415	$ 6	330,409
Issuance of common stock in connection with underwritten public offering, net of issuance costs, shares		5,833,333
Issuance of common stock in connection with at-the-market offering, net of issuance costs	24,828		24,828
Issuance of common stock in connection with at-the-market offering, net of issuance costs, shares		468,685
Employee stock-based compensation	82,025		82,025
Issuance of common stock under equity plan awards, net of tax	9,829	$ 1	9,828
Issuance of common stock under equity plan awards, net of tax, shares		675,614
Other comprehensive income (loss)	486			486
Net loss	(402,727)				(402,727)
Ending balance at Dec. 31, 2019	$ 653,764	$ 58	$ 2,086,863	$ (147)	$ (1,433,010)
Ending balance, shares at Dec. 31, 2019	57,838,220	57,838,220

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating activities:
Net loss	$ (402,727)	$ (197,611)	$ (302,139)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	81,995	80,107	68,014
Amortization of premium (discount) on investment securities, net	(6,214)	(2,641)	1,706
Depreciation and amortization	8,539	19,538	5,825
Foreign currency remeasurement (gain) loss	688	5,309	(7,018)
Change in fair value of investment in Arcturus equity securities	(13,413)
Non-cash interest expense on liability related to the sale of future royalties	1,135
Gain from sale of priority review vouchers		(170,322)
Other	557	(156)
Changes in operating assets and liabilities:
Accounts receivable	(20,104)	(7,583)	(5,172)
Inventory	(4,451)	(5,283)	(757)
Prepaid expenses and other assets	(8,216)	(14,285)	3,113
Right-of-use lease assets	(14,176)
Accounts payable, accrued, and other liabilities	13,312	2,361	(1,169)
Lease liabilities	15,552
Deferred tax liabilities	2,140		(16,246)
Net cash used in operating activities	(345,383)	(290,566)	(253,843)
Investing activities:
Purchase of property and equipment	(24,832)	(4,076)	(2,793)
Purchase of investments	(692,824)	(509,796)	(230,487)
Purchase of investment in Arcturus equity securities	(14,339)
Proceeds from sale of investments	42,718	7,655	157,934
Proceeds from maturities of investments	676,238	302,564	273,632
Proceeds from sale of priority review vouchers		170,322
Acquisition, net of cash acquired			(142,804)
Net cash provided by (used in) investing activities	(13,039)	(33,331)	55,482
Financing activities:
Proceeds from the sale of future royalties, net	314,234
Proceeds from the issuance of common stock in connection with underwritten public offerings, net	330,415	270,969
Proceeds from the issuance of common stock in connection with at-the-market offering, net	24,828	38,056	131,960
Proceeds from the issuance of common stock under equity plan awards, net	9,829	27,828	9,251
Repayment of note payable			(4,944)
Net cash provided by financing activities	679,306	336,853	136,267
Effect of exchange rate changes on cash	(165)	(472)	528
Net increase (decrease) in cash, cash equivalents, and restricted cash	320,719	12,484	(61,566)
Cash, cash equivalents, and restricted cash at beginning of year	115,525	103,041	164,607
Cash, cash equivalents, and restricted cash at end of year	436,244	115,525	103,041
Supplemental disclosures of non-cash investing and financing information:
Acquired lease liabilities arising from obtaining right-of-use lease assets	21,861
Stock-based compensation capitalized into ending inventory	1,206	1,058
Costs of property and equipment included in accounts payable and other liabilities	$ 10,367	$ 1,192	400
Fair value of vested stock options assumed in acquisition			$ 8,979

Organization and Basis of Prese

Organization and Basis of Presentation	12 Months Ended
	Dec. 31, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Organization and Basis of Presentation	1. Organization and Basis of Presentation Ultragenyx Pharmaceutical Inc. (the Company) is a biopharmaceutical company incorporated in California on April 22, 2010. The Company subsequently reincorporated in the state of Delaware in June 2011. The Company is focused on the identification, acquisition, development, and commercialization of novel products for the treatment of serious rare and ultra-rare genetic diseases. The Company has two approved therapies. Crysvita ® In addition to the approved treatments for XLH and MPS VII, the Company has four ongoing clinical development programs. Crysvita is being studied for the treatment of tumor induced osteomalacia (TIO), a rare disease that impairs bone mineralization. UX007 is being studied in patients severely affected by long-chain fatty acid oxidation disorders (LC-FAOD), a genetic disorder in which the body is unable to convert long chain fatty acids into energy. The Company has two gene therapy pipeline candidates: DTX301 is an adeno-associated virus 8 (AAV8) gene therapy product candidate in development for the treatment of patients with ornithine transcarbamylase (OTC) deficiency, the most common urea cycle disorder; and DTX401 is an AAV8 gene therapy product candidate for the treatment of patients with glycogen storage disease type Ia (GSDIa). The Company operates as one reportable segment. The Company has sustained operating losses and expects such annual losses to continue over the next several years. The Company’s ultimate success depends on the outcome of its research and development and commercialization activities, for which it expects to incur additional losses in the future. Management recognizes the need to raise additional capital to fully implement its business plan. Through December 31, 2019, the Company has relied primarily on the proceeds from equity offerings and its sale of future royalties to finance its operations. The Company intends to raise additional capital through the issuance of equity, borrowings, or strategic alliances with partner companies. However, if such financing is not available at adequate levels, the Company will need to reevaluate its operating plans.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Consolidation The consolidated financial statements include the accounts of Ultragenyx Pharmaceutical Inc. and our wholly-owned subsidiaries. All intercompany balances and transactions have been eliminated. Use of Estimates The accompanying consolidated financial statements have been prepared in accordance with U.S. generally accepted accounting principles (“GAAP”). The preparation of the consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclosure of contingent liabilities and the reported amounts of expenses in the consolidated financial statements and the accompanying notes. On an ongoing basis, management evaluates its estimates, including those related to clinical trial accruals, fair value of assets and liabilities, income taxes, and stock-based compensation. Management bases its estimates on historical experience and on various other market-specific and relevant assumptions that management believes to be reasonable under the circumstances. Actual results could differ from those estimates. Cash, Cash Equivalents, and Restricted Cash The Company considers all highly liquid investments with original maturities of three months or less from the date of purchase to be cash equivalents. Cash equivalents consist primarily of amounts invested in money market accounts. Restricted cash primarily consists of money market accounts used as collateral for the Company’s obligations under its facility leases. 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 amounts shown in the consolidated statement of cash flows (in thousands): December 31, 2019 2018 2017 Cash and cash equivalents $ 433,584 $ 113,432 $ 100,488 Restricted cash included in prepaid expenses and other current assets 161 271 461 Restricted cash included in other assets 2,499 1,822 2,092 Total cash, cash equivalents, and restricted cash shown in the statements of cash flows $ 436,244 $ 115,525 $ 103,041 Investments All investments have been classified as “available-for-sale” and are carried at estimated fair value as determined based upon quoted market prices or pricing models for similar securities. Management determines the appropriate classification of its investments at the time of purchase and reevaluates such designation as of each balance sheet date. Investments with a maturity of one year or less from the balance sheet date are reported as short-term investments and investments with a maturity of greater than one year from the balance sheet date are reported as long-term investments. Unrealized gains and losses are excluded from earnings and are reported as a component of comprehensive loss. Realized gains and losses and declines in fair value judged to be other than temporary, if any, on available-for-sale securities are included in other income (expense). The cost of securities sold is based on the specific-identification method. Interest on investments is included in interest income. Investment in Equity Securities In June 2019, the Company entered into an amendment to the Research Collaboration and License Agreement and an Equity Purchase Agreement with Arcturus Therapeutics Holdings Inc. (Arcturus). Pursuant to the Equity Purchase Agreement, the Company purchased 2,400,000 shares of common stock, or approximately 18.2% of Arcturus’s outstanding shares of common stock as of the closing date of the transaction and received an option to purchase an additional 600,000 shares of common stock. The investment is subject to the equity method of accounting as it was determined that the Company has significant influence over Arcturus, but does not control the significant activities of Arcturus. The Company elected to apply the fair value option to account for the equity investment in Arcturus. The decision to elect the fair value option is irrevocable and is determined on an instrument by instrument basis. The option to purchase additional stock is accounted for at fair value using the Black-Scholes option pricing method. The changes in fair value of the equity investment and option to purchase additional stock are included in the Consolidated Statements of Operations. See “Note 8. License and Research Agreements” for additional details on the Arcturus transaction. Concentration of Credit Risk and Other Risks and Uncertainties Financial instruments that potentially subject the Company to a concentration of credit risk consist of cash, cash equivalents, and investments. The Company’s cash, cash equivalents, and investments are held by financial institutions that management believes are of high credit quality. The Company’s investment policy limits investments to fixed income securities denominated and payable in U.S. dollars such as U.S. government obligations, money market instruments and funds, corporate bonds, commercial paper, and asset-backed securities and places restrictions on maturities and concentrations by type and issuer. Such deposits may, at times, exceed federally insured limits. The Company has not experienced any losses on its deposits of cash and cash equivalents and its accounts are monitored by management to mitigate risk. The Company is exposed to credit risk in the event of default by the financial institutions holding its cash and cash equivalents, corporate issuers, and other financial instruments, to the extent recorded in the balance sheets . The Company has not experienced any credit losses to date from credit risk concentration. Concentration of credit risk with respect to is primarily limited to collaboration partners, drug wholesalers, and retail pharmacy distributors. Credit is extended to our customers based on an evaluation of a customer’s financial condition, and collateral is not The Company is dependent on third-party manufacturers to supply products for research and development activities in its programs. In particular, the Company relies and expects to continue to rely on a small number of manufacturers to supply it with its requirements for the active pharmaceutical ingredients and formulated drugs related to these programs. These programs could be adversely affected by a significant interruption in the supply of active pharmaceutical ingredients and formulated drugs. Inventory The Company values inventory at the lower of cost and net realizable value and determines the cost of inventory using the average-cost method. The Company expenses costs associated with the manufacture of product candidates prior to regulatory approval. Inventories consist of currently approved products. The Company periodically reviews its inventories for excess amounts or obsolescence and writes down obsolete or otherwise unmarketable inventory to its estimated net realizable value. Management determines excess inventory based on expected future demand. Estimates related to future demand are sensitive to significant inputs and assumptions such as acceptance by patients and physicians and the availability of formulary coverage and adequate reimbursement from private third-party payers for the product. Property and Equipment Property and equipment are stated at cost, less accumulated depreciation and amortization. Depreciation and amortization is computed using the straight-line method over the estimated useful lives of the respective assets. Depreciation and amortization begins at the time the asset is placed in service. Maintenance and repairs are charged to operations as incurred. Upon sale or retirement of assets, the cost and related accumulated depreciation are removed from the balance sheet and the resulting gain or loss, if any, is reflected in operations. The useful lives of property and equipment are as follows: Research and development equipment 5 years Furniture and office equipment 5 years Computer equipment 3 years Software 3-5 years Leasehold improvements Shorter of lease term or estimated useful life Intangible Assets The Company recognizes an acquired intangible apart from goodwill whenever the intangible arises from contractual or other legal rights, or whenever it can be separated or divided from the acquired entity and sold, transferred, licensed, rented or exchanged, either individually or in combination with a related contract, asset or liability. The Company’s intangible assets consist of acquired in-process research and development (IPR&D) and an acquired contract asset. IPR&D assets represent capitalized incomplete research projects that the Company acquired through business combinations. The Company tests its definite and indefinite-lived intangible assets for impairment annually during the fourth quarter and more frequently if events or changes in circumstances indicate that it is more likely than not that the asset is impaired. If it is determined that the asset becomes impaired, the carrying value is written down to its fair value with the related impairment charge recognized in consolidated statements of operations in the period in which the impairment occurs. The Company has not recorded any impairments of intangible assets. Goodwill Goodwill represents the excess of purchase price over fair value of net assets acquired in a business combination and is not amortized. Goodwill is subject to impairment testing at least annually during the fourth quarter or when a triggering event occurs that could indicate a potential impairment. If it is determined that the goodwill becomes impaired, the carrying value is written down to its fair value with the related impairment charge recognized in consolidated statements of operations in the period in which the impairment occurs. The Company has not recorded any impairments of goodwill. Impairment of Long-Lived Assets The Company evaluates its long-lived assets, including property and equipment, for impairment whenever events or changes in circumstances indicate that the carrying value of these assets may not be recoverable. Recoverability of these assets is measured by comparison of the carrying amount of each asset to the future undiscounted cash flows expected to result from the use of the asset and its eventual disposition. If the asset is considered to be impaired, the amount of any impairment is measured as the difference between the carrying value and the fair value of the impaired asset. The Company has not recorded impairment of any long-lived assets. Accruals of Research and Development Costs The Company records accruals for estimated costs of research, preclinical and clinical studies and manufacturing development. These costs are a significant component of the Company’s research and development expenses. A substantial portion of the Company’s ongoing research and development activities are conducted by third-party service providers, including contract research organizations. The Company accrues the costs incurred under its agreements with these third parties based on actual work completed in accordance with agreements established with these third parties. The Company determines the actual costs through discussions with internal personnel and external service providers as to the progress or stage of completion of the services and the agreed-upon fee to be paid for such services. Revenue Recognition Collaboration and license revenue The Company has certain license and collaboration agreements that are within the scope of Accounting Standards Codification (ASC) 808, Collaborative Agreements provides guidance on the presentation and disclosure of collaborative arrangements. Generally, the classification of the transactions under the collaborative arrangements is determined based on the nature of contractual terms of the arrangement, along with the nature of the operations of the participants. When the collaborative partner is the principal in the sale transaction with the customer, the Company records its share of collaboration profit as collaboration revenue. Funding received related to research and development services and commercialization costs is generally classified as a reduction of research and development expenses and selling, general and administrative expenses, respectively, in the consolidated statement of operations, because the provision of such services for collaborative partners are not considered to be part of the Company’s ongoing major or central operations. The also receives royalty revenues under certain of the Company’s license or collaboration agreements in exchange for license of intellectual property. If the Company does not have any future performance obligations for these license or collaboration agreements, royalty revenue is recorded as the underlying sales occur. In to record collaboration revenue, the Company utilizes certain information from its , including revenue from the sale of the product, associated reserves on revenue, and costs incurred for development and sales activities. For the periods covered in the financial statements presented, there have been no material changes to prior period estimates of revenues and expenses. The terms of the Company’s collaboration agreements may contain multiple performance obligations, which may include licenses and research and development activities. The Company evaluates these agreements under ASC 606, Revenue from Contracts with Customers (ASC 606), to determine the distinct performance obligations. The Company analogizes to ASC 606 for the accounting for distinct performance obligations for which there is a customer relationship. Prior to recognizing revenue, the Company makes estimates of the transaction price, including variable consideration that is subject to a constraint. Amounts of variable consideration are included in the transaction price to the extent that it is probable that a significant reversal in the amount of cumulative revenue recognized will not occur and when the uncertainty associated with the variable consideration is subsequently resolved. Total consideration may include nonrefundable upfront license fees, payments for research and development activities, reimbursement of certain third-party costs, payments based upon the achievement of specified milestones, and royalty payments based on product sales derived from the collaboration. If there are multiple distinct performance obligations, the Company allocates the transaction price to each distinct performance obligation based on its relative standalone selling price. The standalone selling price is generally determined based on the prices charged to customers or using expected cost plus margin. The Company estimates the efforts needed to complete the performance obligation and recognizes revenue by measuring the progress towards complete satisfaction of the performance obligation using an input measure. Product sales The Company sells its approved products through a limited number of distributors. Under ASC 606, revenue from product sales is recognized at the point in time when the delivery is made and when title and risk of loss transfers to these distributors. The Company also recognizes revenue from sales of certain products on a “named patient” basis, which are allowed in certain countries prior to the commercial approval of the product. Prior to recognizing revenue, the Company makes estimates of the transaction price, including any variable consideration that is subject to a constraint. Amounts of variable consideration are included in the transaction price to the extent that it is probable that a significant reversal in the amount of cumulative revenue recognized will not occur and when the uncertainty associated with the variable consideration is subsequently resolved. Product sales are recorded net of estimated government-mandated rebates and chargebacks, estimated product returns, and other deductions. Provisions for returns and other adjustments are provided for in the period the related revenue is recorded, as estimated by management. periodically and adjusted as necessary. If actual results vary, the Company may need to adjust these estimates, which could have an effect on earnings in the period of the adjustment. Leases The Company adopted Accounting Standards Update (ASU) No. 2016-02, Leases (Topic 842) Leases (Topic 840) The Company determines if an arrangement includes a lease at inception. Right-of-use lease assets and lease liabilities are recognized based on the present value of the future minimum lease payments over the lease term at the commencement date. The right-of-use lease asset includes any lease payments made and excludes lease incentives. Incremental borrowing rate is used in determining the present value of future payments. The Company applies a portfolio approach to the property leases to apply an incremental borrowing rate to leases with similar lease terms. The lease terms may include options to extend or terminate the lease. The Company recognizes the options to extend the lease as part of the right-of-use lease assets and lease liabilities only if it is reasonably certain that the option would be exercised. Lease expense for minimum lease payments is recognized on a straight-line basis over the non-cancelable lease term. As a result of the adoption of the new guidance, the Company recorded a right-of-use lease asset of $16.2 million, a short-term lease liability of $4.5 million, and a long-term lease liability of $17.0 million and no cumulative effect adjustment was made to the retained earnings as of the adoption date. In addition, as of the adoption date, the Company derecognized a net deferred rent obligation of $5.2 million. See “Note 9. Leases” for further disclosure. Comprehensive Loss Comprehensive loss is the change in stockholders’ equity from transactions and other events and circumstances other than those resulting from investments by stockholders and distributions to stockholders. The Company’s other comprehensive loss is comprised of unrealized gains and losses on investments in available-for-sale securities and foreign currency translation adjustments. Research and Development Research and development costs are expensed as incurred and consist of salaries and benefits, stock-based compensation expense, lab supplies and facility costs, as well as fees paid to other nonemployees and entities that conduct certain research and development activities on the Company’s behalf. Amounts incurred in connection with license agreements are also included in research and development expense. Nonrefundable advance payments for goods or services to be received in the future for use in research and development activities are deferred. The deferred amounts are expensed as the related goods are delivered or the services are performed. Stock-Based Compensation Stock-based awards issued to employees, including stock options, restricted stock units (RSUs), and performance stock units (PSUs) are recorded at fair value as of the grant date and recognized as expense on a straight-line basis over the employee’s requisite service period (generally the vesting period). PSUs vest only if certain specified criteria are achieved and the employees’ continued service requirements are met; therefore, the expense recognition occurs when the likelihood of the PSUs being earned is deemed probable. Stock compensation expense on awards expected to vest are recognized net of estimated forfeitures. Income Taxes The Company uses the liability method of accounting for income taxes. Under this method, deferred tax assets and liabilities are determined based on the differences between the financial reporting and the tax bases of assets and liabilities and are measured using the enacted tax rates and laws that will be in effect when the differences are expected to reverse. The Company must then assess the likelihood that the resulting deferred tax assets will be realized. A valuation allowance is provided when it is more likely than not that some portion or all of a deferred tax asset will not be realized. Due to the Company’s lack of earnings history, the net deferred tax assets have been fully offset by a valuation allowance. In conjunction with Dimension acquisition, a deferred tax liability was recorded reflecting the tax impact of the difference between the book basis and tax basis of acquired IPR&D. Such deferred income tax liability is not used to offset deferred tax assets when analyzing the Company’s valuation allowance as the acquired IPR&D is considered to have an indefinite life until the Company completes or abandons development of the acquired IPR&D. The Company recognizes benefits of uncertain tax positions if it is more likely than not that such positions will be sustained upon examination based solely on their technical merits, as the largest amount of benefit that is more likely than not to be realized upon the ultimate settlement. The Company’s policy is to recognize interest and penalties related to the underpayment of income taxes as a component of income tax expense or benefit. To date, there have been no interest or penalties charged in relation to the unrecognized tax benefits. Foreign Currency Assets and liabilities of non-U.S. subsidiaries that operate in a local currency environment, where the local currency is the functional currency, are translated to U.S. dollars at exchange rates in effect at the balance sheet date, with the resulting translation adjustments directly recorded to a separate component of accumulated other comprehensive loss. Income and expense accounts are translated at average exchange rates for the period. Transactions which are not in the functional currency of the entity are remeasured into the functional currency and gains or losses resulting from the remeasurement recorded in other income (expense). Net Loss per Share Basic net loss per share is calculated by dividing the net loss by the weighted-average number of shares of common stock outstanding during the period, without consideration for common stock equivalents. Diluted net loss per share is the same as basic net loss per share, since the effects of potentially dilutive securities are antidilutive. In periods when we have incurred a net loss, options and warrants to purchase common stock are considered common stock equivalents, but have been excluded from the calculation of diluted net loss per share, as their effect is antidilutive. Business Combinations The Company applies the provisions of ASC 805, “Business Combinations” allocates Recent Accounting Pronouncements In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments — Credit Losses, (Topic 326): Measurement of Credit Losses on Financial Instruments,

Dimension Acquisition

Dimension Acquisition	12 Months Ended
	Dec. 31, 2019
Business Combinations [Abstract]
Dimension Acquisition	3. Dimension Acquisition On November 7 Cash payments $ 152,292 Fair value of vested stock options assumed 8,979 REGENX termination fee 2,850 Fair value of total consideration $ 164,121 The following table summarizes the fair values of assets acquired and liabilities assumed as of the date of acquisition (in thousands): Cash and cash equivalents $ 12,338 Short-term investments 9,737 Other current assets 11,155 Property and equipment 6,580 In-process research and development 129,000 Bayer collaboration agreement 13,526 Accounts payable and accrued liabilities (10,265 ) Notes payable (4,944 ) Deferred tax liabilities (47,412 ) Net identifiable net assets acquired 119,715 Goodwill 44,406 Net assets acquired $ 164,121 The transaction was accounted for as a business combination under the acquisition method of accounting as outlined in ASC 805, Business Combinations The Company recorded $47.4 million in non-current deferred tax liability resulting from the acquisition reflecting the tax impact of the difference between the book basis and tax basis of acquired IPR&D. Such deferred income tax liability is not used to offset deferred tax assets when analyzing the Company’s valuation allowance as the acquired IPR&D is considered to have an indefinite life until the Company completes or abandons development of the acquired IPR&D. Subsequent to the acquisition date, the deferred tax liability was reduced to $31.2 million due to the reduction of U.S. corporate tax rate from 34% to 21% in December 2017. As of December 31, 2019, the deferred tax liability was increased to $33.3 million due to changes in the estimated state tax apportionment. The goodwill balance is primarily attributed to the deferred tax liabilities arising from the temporary differences on IPR&D assets between book and tax basis as well as the relating to the assembled workforce and expanded market opportunities when integrating Dimension’s research with the Company. The goodwill balance is not deductible for U.S. income tax purposes. The assumed notes payable of $4.9 million, along with the outstanding interest was repaid in December 2017. In connection with the acquisition, the Company recognized transaction costs of $6.0 million as selling, general and administrative expense. There were no purchase price adjustments subsequent to the acquisition. Pro Forma Financial Information The Company's consolidated statement of operations from November 7, 2017 through December 31, 2017 includes Dimension total revenue of $2.1 million and a net loss of $7.5 million. If the acquisition had occurred on January 1, 2017, the supplemental unaudited pro forma financial results is $18.5 million in total revenues and $341.7 million in net loss for the year ended December 31, 2017. The unaudited pro forma financial information include pro forma adjustments that assume the acquisition occurred on January 1, 2017. These items include adjustments to remove the impact of transaction costs related to the acquisition of $9.6 million for the year ended December 31, 2017 and to record the amortization of definite-lived intangible assets of $1.8 million for the year ended December 31, 2017. Other adjustments include reduction of interest income, amounts related to severance of certain employees, acceleration of certain equity awards, and adjustments to conform to the Company’s accounting policies on revenue. These unaudited pro forma results are presented for informational purposes only and are not necessarily indicative of what the actual results of operations of the combined company would have been if the acquisition had occurred at the beginning of the period presented, nor are they indicative of future results of operations.

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Fair Value Measurements	4. Fair Value Measurements Financial assets and liabilities are recorded at fair value. The carrying amount of certain financial instruments, including cash and cash equivalents, accounts receivable, accounts payable and accrued liabilities approximate fair value due to their relatively short maturities. Assets and liabilities recorded at fair value on a recurring basis in the balance sheets are categorized based upon the level of judgment associated with the inputs used to measure their fair values. Fair value is defined as the exchange price that would be received for an asset or an exit price that would be paid to transfer a liability in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. The authoritative guidance on fair value measurements establishes a three-tier fair value hierarchy for disclosure of fair value measurements as follows: Level 1 —Inputs are unadjusted, quoted prices in active markets for identical assets or liabilities at the measurement date; Level 2 —Inputs are observable, unadjusted quoted prices in active markets for similar assets or liabilities, unadjusted quoted prices for identical or similar assets or liabilities in markets that are not active, or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the related assets or liabilities; and Level 3 —Unobservable inputs that are significant to the measurement of the fair value of the assets or liabilities that are supported by little or no market data. The Company’s financial instruments consist of Level 1, Level 2, and Level 3 assets. Where quoted prices are available in an active market, securities are classified as Level 1. Money market funds are classified as Level 1. Level 2 assets consist primarily of corporate bonds, asset backed securities, commercial paper and U.S. Government agency securities based upon quoted market prices for similar movements in active markets, quoted prices for identical or similar instruments in markets that are not active and model-based valuation techniques for which all significant inputs are observable in the market or can be corroborated by observable market data for substantially the full term of the assets. Where applicable these models project future cash flows and discount the future amounts to a present value using market-based observable inputs obtained from various third party data providers, including but not limited to, benchmark yields, interest rate curves, reported trades, broker/dealer quotes and reference data. The Company determines the fair value of the Arcturus common stock by using the quoted market price on December 31, 2019, which is a Level 1 fair value measurement. The change in fair value of the Arcturus common stock for the year ended December 31, 2019 was $12.2 million, which was recognized in the Consolidated Statements of Operations. T he The change in fair value of the option to purchase additional Arcturus common stock for the year ended , 2019 was $1.2 million, which was recognized in the Consolidated Statements of Operations. The following table sets forth the fair value of the Company’s financial assets and liabilities remeasured on a recurring basis based on the three-tier fair value hierarchy (in thousands): December 31, 2019 Level 1 Level 2 Level 3 Total Financial Assets: Money market funds $ 293,309 $ — $ — $ 293,309 Repurchase agreements — 100,000 — 100,000 Time deposits — 10,000 — 10,000 Corporate bonds — 77,026 — 77,026 Commercial paper — 80,119 — 80,119 Asset-backed securities — 30,406 — 30,406 U.S. Government Treasury and agency securities 96,329 53,979 — 150,308 Investment in Arcturus equity securities 26,088 — 1,664 27,752 Total $ 415,726 $ 351,530 $ 1,664 $ 768,920 December 31, 2018 Level 1 Level 2 Level 3 Total Financial Assets: Money market funds $ 48,999 $ — $ — $ 48,999 Repurchase agreements — 24,000 — 24,000 Time deposits — 10,000 — 10,000 Corporate bonds — 179,926 — 179,926 Commercial paper — 50,198 — 50,198 Asset-backed securities — 22,587 — 22,587 U.S. Government Treasury and agency securities — 99,034 — 99,034 Total $ 48,999 $ 385,745 $ — $ 434,744

Balance Sheet Components

Balance Sheet Components	12 Months Ended
	Dec. 31, 2019
Investments Debt And Equity Securities [Abstract]
Balance Sheet Components	5. Balance Sheet Components Cash Equivalents and Investments The fair values of cash equivalents and investments classified as available-for-sale securities consisted of the following (in thousands): December 31, 2019 Gross Unrealized Amortized Cost Gains Losses Estimated Fair Value Money market funds $ 293,309 $ — $ — $ 293,309 Repurchase agreements 100,000 — — 100,000 Time deposits 10,000 — — 10,000 Corporate bonds 77,022 17 (13 ) 77,026 Commercial paper 80,119 — — 80,119 Asset-backed securities 30,375 31 — 30,406 U.S. Government Treasury and agency securities 150,184 124 — 150,308 Total $ 741,009 $ 172 $ (13 ) $ 741,168 December 31, 2018 Gross Unrealized Amortized Cost Gains Losses Estimated Fair Value Money market funds $ 48,999 $ — $ — $ 48,999 Repurchase agreements 24,000 — — 24,000 Time deposits 10,000 — — 10,000 Corporate bonds 180,167 — (241 ) 179,926 Commercial paper 50,198 — — 50,198 Asset-backed securities 22,597 — (10 ) 22,587 U.S. Government Treasury and agency securities 99,087 2 (55 ) 99,034 Total $ 435,048 $ 2 $ (306 ) $ 434,744 At December 31, 2019, the remaining contractual maturities of available-for-sale securities were less than two years. There have been no significant realized gains or losses on available-for-sale securities for the periods presented. All marketable securities with unrealized losses at December 31, 2019 have been in a loss position for less than twelve months or the loss is not material and were temporary in nature. We do not intend to sell the investments that are in an unrealized loss position before recovery of their amortized cost basis. Inventory Inventory consists of the following (in thousands): December 31, 2019 2018 Work-in-process $ 8,191 $ 5,384 Finished goods 3,355 1,681 Total $ 11,546 $ 7,065 Property and Equipment, net Property and equipment, net consists of the following (in thousands): December 31, 2019 2018 Leasehold improvements $ 16,871 $ 15,705 Research and development equipment 17,881 9,856 Furniture and office equipment 3,496 3,379 Computer equipment and software 7,817 7,342 Construction-in-progress 24,271 1,970 Property and equipment, gross 70,336 38,252 Less accumulated depreciation (25,988 ) (18,206 ) Property and equipment, net $ 44,348 $ 20,046 Depreciation expense for the years ended December 31, 2019, 2018 and 2017 was $8.3 million, $7.2 million and $4.8 million respectively. Amortization of leasehold improvements and software is included in depreciation expense. Accrued Liabilities Accrued liabilities consists of the following (in thousands): December 31, 2019 2018 Research, clinical study, and manufacturing expenses $ 22,894 $ 16,912 Payroll and related expenses 41,324 36,443 Other 18,976 9,095 Total $ 83,194 $ 62,450

Intangible Assets, Net

Intangible Assets, Net	12 Months Ended
	Dec. 31, 2019
Intangible Assets Net Excluding Goodwill [Abstract]
Intangible Assets, Net	6. Intangible Assets, net In connection with the acquisition as described in Note 3 “Dimension Acquisition" the Company recognized IPR&D assets of $129.0 million and a contract asset of $13.5 million. The estimated fair value of these intangible assets was measured using Level 3 inputs as of the acquisition date. IPR&D assets represent the fair value of acquired programs to develop an AAV gene therapy for OTC deficiency and to develop an AAV gene therapy for glycogen storage disease type Ia. The fair value of IPR&D assets acquired was determined based on the discounted present value of each research project’s projected cash flows using an income approach, including the application of probability factors related to the likelihood of success of the program reaching final development and commercialization. Additionally, the projections consider the relevant market sizes and growth factors, estimated future cash flows from product sales resulting from completed products and in-process projects and timing and costs to complete the in-process projects. The rates utilized to discount the net cash flows to their present value are commensurate with the stage of development of the projects and uncertainties in the economic estimates used in the projections. IPR&D assets are considered to be indefinite-life until the completion or abandonment of the associated research and development efforts. The contract asset represents the fair value of the agreement with Bayer HealthCare LLC to research, develop, and commercialize AAV gene therapy products for treatment of hemophilia A. The fair value of the contract asset was determined based on the discounted present value of the estimated net future income and was amortized to research and development expense over the research term which was completed in 2019. The Company recorded research and development expense of $0.2 million, $12.3 million, and $1.0 million for the years ended December 31, 2019, 2018, and 2017, respectively, related to the amortization of the asset. The Company tests the intangible assets for impairment annually during its fourth quarter. No impairment charges have been recognized on intangible assets.

Revenue

Revenue	12 Months Ended
	Dec. 31, 2019
Revenue From Contract With Customer [Abstract]
Revenue	7. Revenue The following table disaggregates total revenues from external customers by collaboration and license revenue and product sales (in thousands): Year Ended December 31, 2019 2018 2017 Collaboration and license revenue: KKC (Crysvita) $ 82,989 $ 18,226 $ 9 Bayer 504 23,467 2,127 Total collaboration and license revenue 83,493 41,693 2,136 Product sales: Crysvita 4,286 644 — Mepsevii 12,634 7,903 476 UX007 3,301 1,255 — Total product sales 20,221 9,802 476 Total revenues $ 103,714 $ 51,495 $ 2,612 The following table disaggregates total revenues based on geographic location (in thousands): Year Ended December 31, 2019 2018 2017 United States $ 86,442 $ 45,339 $ 2,289 Europe 12,085 5,293 323 All other 5,187 863 — Total revenues $ 103,714 $ 51,495 $ 2,612 The following table presents the activity and ending balances for sales-related accruals and allowances (in thousands): December 31, 2019 2018 2017 Balance of product sales reserve at beginning of year $ 1,240 $ 41 $ — Provisions 3,846 2,466 41 Payments and adjustments (3,268 ) (1,267 ) — Balance of product sales reserve at end of year $ 1,818 $ 1,240 $ 41 The following table presents changes in the contract assets (liabilities) for the years ended December 31, 2019 and 2018 (in thousands): December 31, 2019 2018 Balance of contract assets (liabilities) at beginning of year $ 2,979 $ (5,986 ) Additions 504 24,055 Deductions (3,483 ) (15,090 ) Balance of contract assets at end of year $ — $ 2,979 The Company’s largest accounts receivable balance was from a collaboration partner and was 87% and 88% of the total accounts receivable balance as of December 31, 2019 and 2018, respectively

License and Research Agreements

License and Research Agreements	12 Months Ended
	Dec. 31, 2019
Research Grant Agreement [Abstract]
License and Research Agreements	8. License and Research Agreements Kyowa Kirin Co., Ltd. Collaboration and License Agreement In August 2013, the Company entered into a collaboration and license agreement with Kyowa Kirin Co., Ltd. (KKC or formerly Kyowa Hakko Kirin Co., Ltd. or KHK). Under the terms of this collaboration and license agreement, as amended, the Company and KKC collaborate on the development and commercialization of Crysvita in the field of orphan diseases in the United States and Canada, or the profit-share territory, and in the European Union, United Kingdom, and Switzerland, or the European territory, and the Company has the right to develop and commercialize such products in the field of orphan diseases in Mexico and Central and South America, or Latin America. Development Activities In the field of orphan diseases, and except for ongoing studies being conducted by KKC, the Company is the lead party for development activities in the profit-share territory and in the European territory until the applicable transition date; the Company is also the lead party for core development activities conducted in Japan and Korea, for which the core development plan is limited to clinical trials mutually agreed to by the Company and KKC. The Company shares the costs for development activities in the profit-share territory and the European territory conducted pursuant to the development plan before the applicable transition date equally with KKC. KKC is responsible for 100% of the costs for development activities in Japan and Korea. In April 2023, which is the transition date for the profit-share territory, and on the applicable transition date for the European territory, KKC will become the lead party and be responsible for the costs of the development activities. However, the Company will continue to share the costs of the studies commenced prior to the applicable transition date equally with KKC. Crysvita was approved in the European Union and United Kingdom in February 2018 and was approved by the FDA in April 2018. The collaboration and license agreements are within the scope of ASC 808, which provides guidance on the presentation and disclosure of collaborative arrangements. Collaboration revenue related to sales in profit-share territory The Company and KKC share commercial responsibilities and profits in the profit-share territory until April 2023. Under the collaboration agreement, KKC manufactures and supplies Crysvita for commercial use in the profit-share territory and charges the Company the transfer price of 35% of net sales through December 31, 2022, and 30% thereafter. The remaining profit or loss after supply costs from commercializing products in the profit-share territory are shared between the Company and KKC on a 50/50 basis until April 2023. Thereafter, the Company will be entitled to receive a tiered double-digit revenue share in the mid-to-high 20% range. As KKC is the principal in the sale transaction with the customer, the Company recognizes a pro-rata share of collaboration revenue, net of transfer pricing, in the period the sale occurs. The Company concluded that its portion of KKC’s sales in the profit-share territory is analogous to a royalty and therefore recorded its share as collaboration revenue, similar to a royalty. Royalty revenue related to sales in European territory KKC has the commercial responsibility for in the European territory. Prior to the Company’s sale of the royalty to Royalty Pharma, as described below, the Company received a royalty of up to 10% on net sales in the European territory, which was recognized as the underlying sales occur. The Company’s Year Ended December 31, 2019 2018 2017 Company's share of collaboration revenue in profit-share territory $ 74,869 $ 15,334 $ — Royalty revenue in European territory 8,120 2,892 9 Total $ 82,989 $ 18,226 $ 9 In December 2019, the Company entered into a Royalty Purchase Agreement with RPI Finance Trust (RPI), an affiliate of Royalty Pharma, and sold the future royalty payments related to Crysvita sales in the European territory to RPI. Going forward, the Company will record the royalty revenue in European territory as non-cash royalties. See “Note 11. Liability Related to the Sale of Future Royalties”. Product revenue related to sales in other territories The Company is responsible for commercializing in Latin America and Turkey. The Company is considered the principal in these territories as the Company controls the product before it is transferred to the customer. Accordingly, the Company records revenue on a gross basis related to the sale of once the product is delivered and the risk and title of the product is transferred to the distributor. For the years ended December 31, 2019 and 2018, the Company recorded product sales of $4.3 million and $0.6 million, respectively, net of estimated product returns and other deductions. There were no product sales recorded for Crysvita for the year ended December 31, 2017. KKC has the option to assume responsibility for commercialization efforts in Turkey from the Company, after a certain minimum period. Under KKC manufactures and supplies for sales in the above territories, and is based on 35% of the net sales through December 31, 2022 and 30% thereafter. The Company also pays to KKC a low single-digit royalty on net sales in Latin America Cost sharing payments Under the collaboration agreement, KKC and the Company share certain development and commercialization costs. Year Ended December 31, 2019 2018 2017 Research and development $ 27,309 $ 32,240 $ 31,165 Selling, general and administrative 21,828 14,228 4,466 Total $ 49,137 $ 46,468 $ 35,631 Collaboration receivable and payable The Company had accounts receivable from KKC in the amount of $28.5 million and $11.2 million from profit-share revenue and royalties and other receivables recorded in prepaid and other current assets of $17.8 million and $11.1 million and accrued liabilities of $0.9 million and $0.3 million from commercial and development activity reimbursements, as of December 31, 2019 and 2018, respectively. Saint Louis University License Agreement In November 2010, the Company entered into a license agreement with Saint Louis University (SLU). Under the terms of this license agreement, SLU granted the Company an exclusive worldwide license to make, have made, use, import, offer for sale, and sell therapeutics related to SLU’s beta-glucuronidase product for use in the treatment of human diseases. The Company made a milestone payment of $0.1 million upon approval of Mepsevii for treatment of MPS 7. The Company is required to pay to SLU a low single-digit royalty on net sales of the licensed products in any country or region, upon reaching a certain level of cumulative worldwide sales of the product. Baylor Research Institute License Agreement In September 2012, the Company entered into a license agreement with Baylor Research Institute (BRI). Under the terms of this license agreement, BRI exclusively licensed to the Company its territories for certain intellectual property related to triheptanoin (UX007). The Company may be obligated to make future payments of up to $5.3 million contingent upon attainment of various development milestones relating to the development of LC-FAOD and $7.5 million contingent upon attainment of various sales milestones. Additionally, the Company will pay to BRI a mid-single digit royalty on net sales of the licensed product in the licensed territories, if such product sales are ever achieved. REGENXBIO, Inc. The Company has a license agreement with REGENX, for an exclusive, sublicensable, worldwide commercial license under certain intellectual property for preclinical and clinical research and development, and commercialization of drug therapies using REGENX 's licensed patents for the treatment of hemophilia A, OTC deficiency, and GSD1a. The Company will pay an annual fee and certain milestone fees per disease indication, low to mid single-digit royalty percentages on net sales of licensed products, and milestone and sublicense fees owed by REGENX to its licensors, contingent upon the attainment of certain development activities as outlined in the agreement. The Company also has an option and license agreement with REGENX fee of $0.1 million Bayer HealthCare LLC The Company has an agreement with Bayer Healthcare LLC (Bayer) to research, develop and commercialize AAV gene therapy products for treatment of hemophilia A (DTX 201). Under this agreement, Bayer has been granted an exclusive license to develop and commercialize one or more novel gene therapies for hemophilia A. The agreement requires that Bayer use commercially reasonable efforts to conduct and fund a proof-of-concept (POC) clinical trial and any subsequent clinical trials and commercialization of gene therapy products for treatment of hemophilia A. Bayer will have worldwide rights to commercialize the potential future product. Bayer is responsible to fund certain research and development services performed by the Company in the performance of its obligations under the annual research plan and budget. Under the terms of the agreement with Bayer, the Company is eligible to receive development and commercialization milestone payments of up to $232.0 million, as well as, royalty payments ranging in the high single-digit to low double-digit percentages, not exceeding the mid-teens, of net sales of licensed products. The Company achieved the first milestone in December 2017, the second milestone in April 2018, and has received $15.0 million for such milestones to date. As of the acquisition date of Dimension on November 7, 2017, the Company valued the contract under ASC 805 and recorded an intangible asset of $13.5 million. The intangible asset was amortized to research and development expense over the research term and was completed in 2019. The Company recorded research and development expense of $0.2 million, $12.3 million, and $1.0 million for the years ended December 31, 2019, 2018, and 2017, respectively, for the amortization of the intangible asset. The Company evaluated the agreement under ASC 606 and recorded a contract liability as of November 7, 2017 of $2.5 million. It was determined that the performance obligations under the agreement include (i) research and development services to be provided over the research term, (ii) a development and commercialization license, and (iii) the Company’s participation in certain committees. It was determined that these performance obligations are not distinct in the context of the contract and therefore are a single performance obligation. The Company calculated the transaction price by including the unconstrained milestones along with the estimated payments for research and development services and recorded $0.5 million and $23.5 million, and $2.1 million as collaboration and license revenue for the years ended December 31, 2019, 2018, and 2017, respectively, by measuring the progress toward complete satisfaction of the performance obligation using an input measure. The performance obligation under the contract was substantially completed by end of 2019. As of December 31, 2019 and 2018, the Company had none and a $3.0 million contract asset associated with the performance obligation, respectively. University of Pennsylvania The Company has an agreement with University of Pennsylvania School of Medicine (Penn) to sponsor certain research related to liver and hemophilia gene therapy. In consideration for funding such research, Penn granted the Company an option to obtain a worldwide, non-exclusive or exclusive, royalty-bearing license, with the right to sublicense, under certain patent rights conceived, created or reduced to practice in the conduct of the research. The Company is required to reimburse Penn for filing, prosecuting and maintaining such patent rights unless and until the Company declines to exercise its option. Penn provides the Company with task-based, scientific reports of progress and results of the research, and granted the Company a royalty-free, nontransferable, non-exclusive right to copy and distribute any research reports furnished to the Company for any reasonable purpose, provided the results are not made publicly available until certain conditions are met, and the right to use, disclose and otherwise exploit the research results for any reasonable purpose, subject to similar restrictions on our public disclosure of the research results. Otherwise, the sponsored research agreement contains customary confidentiality provisions. The Company also has a research, collaboration, and license agreement with Penn, which provides the terms for the Company and Penn to collaborate with respect to the pre-clinical development of gene therapy products for the treatment of certain indications. Under the agreement, Penn granted the Company an exclusive, worldwide license to certain patent rights arising out of the research program, subject to certain retained rights, and a non-exclusive, worldwide license to certain Penn intellectual property, in each case to research, develop, make, have made, use, sell, offer for sale, commercialize and import licensed products in each indication for the term of the agreement. The Company will fund the cost of the research program in accordance with a mutually agreed-upon research budget and will be responsible for clinical development, manufacturing and commercialization of each indication. The Company may be obligated to make milestone payments of up to $5.0 million for each indication, if certain development milestones are achieved over time, as well as low to mid-single digit royalties on net sales of each licensed product. The Company may also be obligated to make milestone payments of up to $25.0 million per approved product if certain commercial milestones are achieved. Arcturus Research Collaboration and License Agreement In October 2015, the Company entered into a Research Collaboration and License Agreement with Arcturus Therapeutics, Inc. (Arcturus). The Company and Arcturus are collaborating on the research and development of therapies for select rare diseases. As consideration for entering into the arrangement, the Company paid Arcturus an upfront fee of $10.0 million. Arcturus has the primary responsibility for conducting certain research services, funded by the Company, and the Company will be responsible for development and commercialization costs. Pursuant to the agreement, the Company incurred $0.8 million, $1.9 million, and $4.3 million for the years ended December 31, 2019, 2018, and 2017, respectively, in research and development expense for the funding of certain research services received from Arcturus. As of December 31, 2019 and 2018, the Company has a balance of none and $0.5 million, respectively, in prepaid expenses and other current assets, and a balance of none and $0.4 million, respectively, in accrued liabilities related to Arcturus. In June 2019, the Company entered into an Equity Purchase Agreement and an amendment to the Research Collaboration and License Agreement to expand the field of use and increase the number of disease targets to include mRNA, DNA and siRNA therapeutics for up to 12 rare diseases. Pursuant to the agreements, the Company paid $ 6.0 million in cash upfront to Arcturus and purchased 2,400,000 shares of Arcturus’ common stock at a stated value of $ 10.00 per share, resulting in a total of $ 30.0 million of consideration paid at the close of the transaction. As a result, the Company received expanded license rights; the Arcturus common stock; an option to purchase an additional 600,000 shares of Arcturus’ common stock at $ 16.00 per share, which may be exercised up to two years after the agreement effective date, with certain restrictions; in addition to other changes as noted in the agreement. The period for the Company to exercise its option to purchase the additional stock may also be extended under certain circumstances as specified in the Equity Purchase Agreement. The Company is restricted from selling the 2,400,000 shares of common stock for a period of two years from the purchase date. The additional stock, if purchased, are also restricted from sale for a period of time as specified in the agreement. The Company also received the right to nominate one member to the Arcturus Board of Directors as well as one Board observer. Under the amended license agreement, certain early-stage milestone payments are reduced and the total potential milestone payments are increased due to the expanded number of targets. Arcturus is also entitled to reimbursement of related research expenses and royalties on commercial sales. Immediately after the purchase, the Company held 18.2% of Arcturus’ outstanding common stock, based on Arcturus’ outstanding common stock balance as of the transaction date. The recorded the common stock investment at $13.9 million on the transaction date, which was based on the quoted market price on the closing date. As a result of the equity ownership and the right to nominate a board member, it was determined that the Company has significant influence over Arcturus. The Company elected to apply the fair value option to account for the equity investment in Arcturus. The Company also accounts for the option to purchase additional shares of Arcturus common stock at fair value, which was recorded at $0.5 million on the transaction date based on the Black-Scholes option pricing method. The remaining $15.6 million of the total $30.0 million paid as consideration was attributed to the additional license rights obtained and was recorded as in-process research and development expense. The changes in the fair value of the Company’s investment in Arcturus securities were as follows (in thousands): Arcturus common stock Fair value of option to purchase additional shares of Arcturus common stock December 31, 2018 $ — $ — Acquisition of investment in Arcturus securities 13,872 467 Change in fair value 12,216 1,197 December 31, 2019 $ 26,088 $ 1,664 GeneTx In August 2019, the Company entered into a Program Agreement and a Unitholder Option Agreement with GeneTx Biotherapeutics, LLC (GeneTx) to collaborate on the development of GeneTx’s GTX-102, an antisense oligonucleotide (ASO) for the treatment of Angelman syndrome. Pursuant to the terms of the Unitholder Option Agreement, the Company made an upfront payment of $20.0 million for an exclusive option to acquire GeneTx. This option may be exercised any time prior to 30 days following notice of FDA acceptance of the IND for GTX-102 for an additional $50.0 million in payments. Alternatively, the Company may extend the option period by paying an option extension payment of $25.0 million. In the event the Company exercises the option extension, the Company has a right to acquire GeneTx for a payment of $125.0 million, at any time, until the earlier of 30 months from the first dosing of a patient in a planned Phase 1/2 study (subject to extensions) or 90 days after results are available from that study. This exclusive option to acquire GeneTx can be extended under certain circumstances, by up to four additional three-month periods, by paying an additional extension fee for each three-month period. During the exclusive option period, GeneTx is responsible for conducting the program based on the development plan agreed between the parties and, subject to the terms in the Program Agreement, has the decision-making authority on all matters in connection with the research, development, manufacturing and regulatory activities with respect to the Program. The Company will provide support, at its discretion, including strategic guidance and clinical expertise. The Company and GeneTx will collaborate on the submission of the IND and management of the Phase 1/2 study in patients with Angelman syndrome. If the Company acquires GeneTx, the Company will then be responsible for all development and commercialization activities from the date of acquisition. The Company would also be required to make payments upon achievement of certain development and commercial milestones, as well as royalties, depending upon the success of the program. Although GeneTx is a variable interest entity, the Company is not the primary beneficiary as it currently does not have the power to direct the activities that would most significantly impact the economic performance of GeneTx. Prior to product regulatory approval, all consideration paid to GeneTx represents rights to potential future benefits associated with GeneTx’s in-process research and development activities, which have not reached technological feasibility and have no alternative future use. Accordingly, for the year ended December 31, 2019, the Company recorded the $20.0 million payment as an in-process research and development expense. In February 2020, the Company paid $25.0 million upon acceptance of the IND to maintain its option to acquire GeneTx until the earlier of 30 months from the first dosing of a patient in the Phase 1/2 study (subject to extensions) or 90 days after results are available from that study.

Leases

Leases	12 Months Ended
	Dec. 31, 2019
Lessee Disclosure [Abstract]
Leases	9. Leases As described in “Note 2. Summary of Significant Accounting Policies”, the Company adopted Topic 842 Topic 840 The Company leases office space and research, testing and manufacturing laboratory space in various facilities in Novato and Brisbane, California, in Cambridge and Woburn, Massachusetts, and in certain foreign countries, under operating agreements expiring at various dates through 2028. Certain lease agreements include options for the Company to extend the lease for multiple renewal periods and also provide for annual minimum increases in rent, usually based on a consumer price index or annual minimum increases. None of these optional periods have been considered in the determination of the right-of-use lease asset or the lease liability for the leases as the Company did not consider it reasonably certain that it would exercise any such options. The Company recognizes lease expense on a straight-line basis over the non-cancelable term of its operating leases. The variable lease expense primarily consists of common area maintenance and other operating costs. The Company recognizes rent expense on a straight-line basis over the noncancelable term of its operating leases. Prior to adoption of Topic 842 Topic 840 The components of lease expense were as follows (in thousands): Year Ended December 31, 2019 Operating lease expense $ 9,163 Variable lease expense 2,779 Total $ 11,942 Cash paid for amounts included in the measurement of lease liabilities for the year ended December 31, 2019 was $9.0 million and was included in net cash provided by operating activities in the Consolidated Statements of Cash Flows. Future minimum lease payments under non-cancellable leases as of December 31, 2019 were as follows (in thousands): Year Ending December 31, Leases 2020 $ 9,786 2021 7,667 2022 7,521 2023 7,541 2024 5,571 Thereafter 7,582 Total future lease payments 45,668 Less: Amount representing interest (8,676 ) Present value of future lease payments 36,992 Less: Short-term lease liabilities (7,235 ) Long-term lease liabilities $ 29,757 Lease liabilities are based on the net present value of the remaining lease payments over the remaining lease term. As of December 31, 2019, the weighted-average remaining lease term was 5.64 years and the weighted-average discount rate used to determine the lease liability was 7.5%.

Gain from Sale of Priority Revi

Gain from Sale of Priority Review Vouchers	12 Months Ended
	Dec. 31, 2019
Goodwill And Intangible Assets Disclosure [Abstract]
Gain from Sale of Priority Review Vouchers	10. Gain from Sale of Priority Review Vouchers In January 2018, the Company completed the sale of a Rare Pediatric Disease Priority Review Voucher (PRV) it received in connection with the approval of Mepsevii for $130.0 million. In June 2018, the Company also completed the sale of the PRV it received in connection with the approval of Crysvita for $80.6 million, net, which was shared equally with KKC. As the PRVs did not have a carrying value, the gain recognized was equal to the net proceeds received. The Company recorded $170.3 million for its portion of the net proceeds for the year ended December 31, 2018 as a gain from the sale of the priority review vouchers.

Liability Related to the Sale o

Liability Related to the Sale of Future Royalties	12 Months Ended
	Dec. 31, 2019
Deferred Revenue Disclosure [Abstract]
Liability Related to the Sale of Future Royalties	11. Liability Related to the Sale of Future Royalties In December 2019, the Company entered into a Royalty Purchase Agreement with RPI Finance Trust (RPI), an affiliate of Royalty Pharma. Pursuant to the agreement, RPI paid $320.0 million to the Company in consideration for the right to receive royalty payments effective January 1, 2020, arising from the net sales of Crysvita in the European Union, the United Kingdom, As RPI’s rate of return is explicitly limited due to the cap on royalties they may receive, proceeds from the transaction was recorded as a liability (liability related to sale of future royalties on the Consolidated Balance Sheets). The Company will amortize $320.0 million, net of transaction cost of $5.8 million using the effective interest method over the estimated life of the arrangement. In order to determine the amortization of the liability, the Company is required to estimate the total amount of future royalty payments to be received by the Company and paid to RPI, subject to the capped amount, over the life of the arrangement. The aggregate future estimated royalty payments (subject to the capped amount), less the $314.2 million of net proceeds, will be recorded as non-cash interest expense over the life of the arrangement. Consequently, the Company estimates an imputed interest on the unamortized portion of the liability and records interest expense relating to the transaction. The Company will continue to record the royalty revenue rising from the net sales of Crysvita in the applicable European territories as non-cash royalty revenue in the Consolidated Statements of Operations over the term of the arrangement. The Company periodically assesses the expected royalty payments using a combination of historical results, internal projections and forecasts from external sources. To the extent such payments are greater or less than the Company’s initial estimates or the timing of such payments is materially different than its original estimates, the Company will prospectively adjust the amortization of the liability and the effective interest rate. Through December 31, 2019, the Company’s effective annual interest rate was approximately 10.1%. There are a number of factors that could materially affect the amount and timing of royalty payments from KKC in the applicable European territories, most of which are not within the Company’s control. Such factors include, but are not limited to, the success of KKC’s sales and promotion of Crysvita, changing standards of care, the introduction of competing products, approval of label expansion for adults, pricing for reimbursement in various European territories, manufacturing or other delays, intellectual property matters, adverse events that result in governmental health authority imposed restrictions on the use of Crysvita, significant changes in foreign exchange rates as the royalty payments are made in U.S. dollars (USD) while significant portions of the underlying European sales of Crysvita are made in currencies other than USD, and other events or circumstances that could result in reduced royalty payments from European sales of Crysvita, all of which would result in a reduction of non-cash royalty revenue and the non-cash interest expense over the life of the arrangement. Conversely, if sales of Crysvita in Europe are more than expected, the non-cash royalty revenue and the non-cash interest expense recorded by the Company would be greater over the term of the arrangement. The Company will record non-cash royalty revenues and non-cash interest expense within its Consolidated Statements of Operations over the term of the arrangement. The following table shows the activity within the liability account during the year ended December 31, 2019 (in thousands): December 31, 2019 Liability related to the sale of future royalties — beginning balance $ — Proceeds from sale of future royalties 314,234 Non-cash interest expense 1,135 Liability related to the sale of future royalties at end of year $ 315,369

Equity

Equity	12 Months Ended
	Dec. 31, 2019
Equity [Abstract]
Equity	12. Equity At-the-Market Offerings In July 2016, the Company entered into an At-The-Market (ATM) sales agreement with Cowen and Company, LLC (Cowen), whereby the Company sold $150.0 million in aggregate proceeds of common stock, through Cowen as our sales agent. During the year ended December 31, 2017, the Company sold 912,351 shares of common stock, resulting in net proceeds of approximately $67.6 million, after commissions and other offering costs. In July 2017, the Company entered into an additional ATM sales agreement with Cowen whereby the Company may sell up to $150.0 million in aggregate proceeds of common stock from time to time, through Cowen as its sales agent. During the years ended December 31, 2019, 2018, and 2017, the Company sold 468,685, 640,257, and 1,338,866 shares of common stock, respectively, resulting in net proceeds of approximately $24.8 million, $38.1 million, and $64.3 million, respectively, after commissions and other offering costs. Underwritten Public Offering In January 2018, the Company completed an underwritten public offering in which 5,043,860 shares of common stock were sold, which includes 657,895 shares purchased by the underwriters pursuant to an option granted to them in connection with the offering, at a public offering price of $57.00 per share. The total proceeds that the Company received from the offering were approximately $271.0 million, net of underwriting discounts and commissions. In February 2019, the Company completed an underwritten public offering in which 5,833,333 shares of common stock were sold, which included 760,869 shares purchased by the underwriters pursuant to an option granted to them in connection with the offering, at a public offering price of $60.00 per share. The total proceeds that the Company received from the offering were approximately $330.4 million, net of underwriting discounts and commissions. Common Stock Warrants As of December 31, 2019 and 2018, there was an aggregate of 149,700 of common stock warrants outstanding with exercise price of $3.01 and expiration dates in 2020 and 2021.

Stock-Based Awards

Stock-Based Awards	12 Months Ended
	Dec. 31, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Awards	13. Stock-Based Awards Equity Plan Awards In 2011, the Company adopted the 2011 Equity Incentive Plan (the 2011 Plan). The 2011 Plan provides for the granting of stock-based awards to employees, directors, and consultants under terms and provisions established by the board of directors. In 2014, the Company adopted the 2014 Incentive Plan (the 2014 Plan). The 2014 Plan had 2,250,000 shares of common stock available for future issuance at the time of its inception, which included 655,038 shares available under the 2011 Plan, which were transferred to the 2014 Plan upon adoption. No further grants subsequent to the IPO were made under the 2011 Plan. The 2014 Plan provides for automatic annual increases in shares available for grant, beginning on January 1, 2015 through January 1, 2024. Under the terms of the 2014 Plan, awards may be granted at an exercise price not less than fair market value. For employees holding more than 10% of the voting rights of all classes of stock, the exercise prices for awards must be at least 110% of fair market of the common stock on the grant date, as determined by the board of directors. The term of an award granted under the 2014 Plan may not exceed ten years. Typically, the vesting schedule for option grants to the employees provides that 1/4 1/48 1/4 As part of the acquisition of Dimension (discussed in “Note 3. Dimension Acquisition”), the Company assumed an equivalent 639,897 options to purchase shares of common stock of the Company from the equity plans of Dimension. No further grants subsequent to the acquisition are available under these equity plans. As of December 31, 2019, an aggregate of 10,521,371 shares of common stock have been authorized for issuance under the 2011 Plan, the 2014 Plan, and the assumed equity awards from the Dimension plans. Stock Option Activity The following table summarizes activity under the Company’s stock option plans, including the 2011 Plan, the 2014 Plan, the assumed equity awards from the Dimension plans and related information: Options Outstanding Number of Options Weighted- Average Exercise Weighted-Average Remaining Contractual Term Aggregate Intrinsic Value (In Outstanding — December 31, 2016 4,429,472 $ 61.85 8.22 $ 79,135 Options granted 1,328,860 71.99 Options assumed 639,897 27.97 Options exercised (478,470 ) 16.25 Options cancelled (520,349 ) 78.70 Outstanding — December 31, 2017 5,399,410 $ 62.75 7.40 $ 37,687 Options granted 1,479,451 55.54 Options exercised (713,263 ) 36.21 Options cancelled (812,474 ) 74.80 Outstanding — December 31, 2018 5,353,124 $ 62.46 7.22 $ 23,243 Options granted 1,762,075 63.03 Options exercised (235,678 ) 32.87 Options cancelled (766,457 ) 72.48 Outstanding — December 31, 2019 6,113,064 $ 62.51 7.00 $ 18,989 Vested and exercisable — December 31, 2019 3,429,579 $ 63.14 5.83 $ 18,514 Vested and expected to vest — December 31, 2019 5,847,778 $ 62.58 6.92 $ 18,945 The aggregate intrinsic values of options outstanding, vested and exercisable, and vested and expected to vest were calculated as the difference between the exercise price of the options and the fair value of the Company’s common stock. The total intrinsic value of options exercised during the years ended December 31, 2019, 2018, and 2017 was $6.5 million, $22.9 million and $20.4 million, respectively. Cash received from the exercise of options was $7.7 million, $25.8 million, and $7.8 million as of December 31, 2019, 2018, and 2017, respectively. The weighted-average estimated fair value of stock options granted was $37.15, $33.32 and $44.20 per share of the Company’s common stock during the years ended December 31, 2019, 2018, and 2017, respectively. The total estimated grant date fair value of options vested during the years ended December 31, 2019, 2018, and 2017 was $45.3 million, $51.7 million, and $49.1 million, respectively. Restricted Stock Units The following table summarizes activity under the Company’s Restricted Stock Units (RSUs) from the 2014 Plan and related information: RSUs Outstanding Number of Shares Weighted- Average Grant Date Fair Value Unvested — December 31, 2016 573,744 $ 71.45 RSUs granted 516,161 71.58 RSUs vested (156,021 ) 71.93 RSUs cancelled (112,324 ) 76.78 Unvested — December 31, 2017 821,560 $ 70.71 RSUs granted 555,905 56.01 RSUs vested (235,913 ) 71.01 RSUs cancelled (187,475 ) 65.37 Unvested — December 31, 2018 954,077 $ 63.12 RSUs granted 863,065 62.78 RSUs vested (313,682 ) 66.25 RSUs cancelled (205,310 ) 64.28 Unvested — December 31, 2019 1,298,150 $ 61.96 The fair value of the RSUs is determined on the grant date based on the fair value of the Company’s common stock. The fair value of the RSUs is recognized as expense ratably over the vesting period of one to four years. The total grant date fair value of the 313,682 shares vested during 2019 was approximately $20.8 million with an aggregate intrinsic value of the shares of $18.4 million. Performance Stock Units In December 2017, the Company began granting performance stock units (PSUs) to certain employees. The following table summarizes activity under the Company’s PSUs from the 2014 Plan and related information: PSUs Outstanding Number of Shares Weighted- Average Grant Date Fair Value Unvested — December 31, 2016 — $ — PSUs granted 508,850 48.03 Unvested — December 31, 2017 508,850 $ 48.03 PSUs granted 71,725 59.67 PSUs cancelled (97,375 ) 48.58 Unvested — December 31, 2018 483,200 $ 49.65 PSUs granted 61,500 67.31 PSUs vested (65,643 ) 48.03 PSUs cancelled (79,517 ) 50.01 Unvested — December 31, 2019 399,540 $ 52.56 The fair value of the PSUs is determined on the grant date based on the fair value of the Company’s common stock. PSUs are subject to vest only if certain specified criteria are achieved and the employees’ continued service with the Company after achievement of the specified criteria. For certain PSUs, the number of PSUs that may vest are also subject to the achievement of certain specified criteria. As of December 31, 2019, the specified criteria were deemed probable of achievement or already achieved. Stock-based compensation for these PSUs is recognized over the service period beginning in the period the Company determines it is probable that the performance criteria will be achieved. The total grant date fair value of the 65,643 shares vested during 2019 was approximately $3.2 million with an aggregate intrinsic value of the shares of $4.2 million. Employee Stock Purchase Plan In January 2014, the Company adopted the 2014 Employee Stock Purchase Plan (ESPP) and reserved a total of 600,000 shares of common stock for issuance under the ESPP. The ESPP provides for automatic annual increases in shares available for grant, beginning on January 1, 2015 through January 1, 2024. Eligible employees may purchase common stock at 85% of the lesser of the fair market value of common stock on the offering date or the purchase date with a six-month look-back feature. ESPP purchases are settled with common stock from the ESPP’s previously authorized and available pool of shares. During the year ended December 31, 2019, the Company issued 85,845 shares of common stock under the ESPP. As of December 31, 2019, an aggregate of 2,933,325 shares of common stock have been authorized for future issuance on the ESPP. Stock-Based Compensation Expense Total stock-based compensation recognized was as follows (in thousands): Year Ended December 31, 2019 2018 2017 Cost of sales $ 1,084 $ 146 $ — Research and development 44,205 45,572 38,212 Selling, general and administrative 36,706 34,389 29,802 Total stock-based compensation expense $ 81,995 $ 80,107 $ 68,014 Stock-based compensation of $1.4 million and $1.1 million was capitalized into inventory for the years ended December 31, 2019 and 2018, respectively. There was no stock-based compensation capitalized for the year ended December 31, 2017. Capitalized stock-based compensation is recognized as cost of sales when the related product is sold. As of December 31, 2019, the total unrecognized compensation expense related to unvested equity awards, net of estimated forfeitures, was $133.0 million, which the Company expects to recognize over an estimated weighted-average period of 2.40 years. In determining the estimated fair value of the stock options and ESPP, the Company uses the Black-Scholes option-pricing model and assumptions discussed below. Each of these inputs is subjective and generally requires significant judgment to determine. Expected Term —The Company’s expected term represents the period that the Company’s stock-based awards are expected to be outstanding and is determined using the simplified method (based on the mid-point between the vesting date and the end of the contractual term). Expected Volatility —The Company’s expected volatility is based on historical volatility over the look-back period corresponding to the expected term. Risk-Free Interest Rate —The risk-free interest rate is based on the U.S. Treasury zero coupon issues in effect at the time of grant for periods corresponding with the expected term of option. Expected Dividend —The Company has never paid dividends on its common stock and has no plans to pay dividends on its common stock. Therefore, the Company used an expected dividend yield of zero. The fair value of stock option awards granted was estimated at the date of grant using a Black-Scholes option-pricing model with the following weighted-average assumptions: Year Ended December 31, 2019 2018 2017 Expected term (years) 6.22 6.23 6.23 Expected volatility 61% 62% 65% Risk-free interest rate 2.4% 2.7% 2.1% Expected dividend rate 0.0% 0.0% 0.0%

Defined Contribution Plan

Defined Contribution Plan	12 Months Ended
	Dec. 31, 2019
Defined Contribution Pension And Other Postretirement Plans Disclosure [Abstract]
Defined Contribution Plan	14. Defined Contribution Plan The Company sponsors a retirement plan in which substantially all of its full-time employees in the United States and certain other foreign countries are eligible to participate. Eligible participants may contribute a percentage of their annual compensation to this plan, subject to statutory limitations. The Company recorded $3.6 million, $2.9 million, and $2.1 million as contribution expenses for the years ended December 31, 2019, 2018, and 2017, respectively.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Income Taxes	15. Income Taxes The components of the Company’s loss before income taxes were as follows (in thousands): Year Ended December 31, 2019 2018 2017 Domestic $ 399,709 $ 205,440 $ 250,917 Foreign (265 ) (8,343 ) 67,421 Total loss before income taxes $ 399,444 $ 197,097 $ 318,338 The components of the Company’s income tax provision were as follows (in thousands): Year Ended December 31, 2019 2018 2017 Current provision for income taxes: Federal $ — $ — $ — State 51 14 5 International 1,092 500 42 Total current tax provision 1,143 514 47 Deferred tax provision (benefit): Federal — — (16,243 ) State 2,140 — (3 ) International — — — Total deferred tax provision (benefit) 2,140 — (16,246 ) Total provision for (benefit from) income taxes $ 3,283 $ 514 $ (16,199 ) The Company has incurred net operating losses since inception. The Company has not reflected any benefit of such net operating loss carryforwards in the accompanying financial statements. The Company has established a full valuation allowance against its deferred tax assets due to the uncertainty surrounding the realization of such assets. The effective tax rate of our provision for income taxes differs from the federal statutory rate as follows: Year Ended December 31, 2019 2018 2017 Federal statutory income tax rate 21.0 % 21.0 % 34.0 % State income taxes, net of federal benefit (0.5 ) — — Federal tax credits 5.2 9.5 9.0 Other (0.2 ) (0.2 ) (0.9 ) Nondeductible permanent items (0.4 ) (0.8 ) — Stock-based compensation (1.0 ) (0.8 ) (0.6 ) Uncertain tax positions (1.0 ) (1.9 ) (1.8 ) Change in valuation allowance (23.6 ) (26.8 ) (32.5 ) Foreign rate differential (0.3 ) (0.3 ) (7.2 ) Change in federal tax rate — — 5.1 Provision for income taxes (0.8 ) % (0.3 ) % 5.1 % The tax effect of temporary differences that give rise to significant portions of the deferred tax assets is presented below (in thousands): Year Ended December 31, 2019 2018 Deferred tax assets: Loss carryforwards $ 185,892 $ 183,331 Tax credits 158,791 137,019 Stock options 40,676 29,925 Accruals and reserves 19,622 7,418 Fixed assets and intangibles 3,870 2,820 Liability related to sale of future royalties 81,424 — Other 4,183 1,189 Gross deferred tax assets 494,458 361,702 Valuation allowance (486,796 ) (361,702 ) Total deferred tax assets 7,662 — Deferred tax liabilities: In-process research and development (33,306 ) (31,166 ) Right-of-use lease assets (7,662 ) — Gross deferred tax liabilities (40,968 ) (31,166 ) Net deferred tax asset (liabilities) $ (33,306 ) $ (31,166 ) As of December 31, 2019 and 2018, the Company had $549.9 million and $558.3 million of federal net operating loss carryforwards available to reduce future taxable income that will begin to expire in 2032. As of December 31, 2019 and 2018, the Company had $520.2 million and $476.8 million of state net operating loss carryforwards available to reduce future taxable income that will begin to expire in 2031. As of December 31, 2019 and 2018, the Company had federal research tax credit carryforwards of $11.7 million and $7.5 million available to reduce future tax liabilities that will begin to expire in 2030. As of December 31, 2018 and 2017, the Company had state research credit carryforwards of $25.5 million and $17.3 million available to reduce future tax liabilities that will be carried forward indefinitely. As of December 31, 2019 and 2018, the Company had federal Orphan Drug Credits of $160.0 million and $143.5 million available to reduce future tax liabilities that will begin to expire in 2031. The Company’s ability to use net operating loss and tax credit carryforwards to reduce future taxable income and liabilities may be subject to annual limitations pursuant to Internal Revenue Code Sections 382 and 383 as a result of ownership changes in the past and future. As a result of ownership changes in 2012 and 2011, $3.6 $3.6 $0.2 On November 7, 2017, the Company acquired Dimension (see “Note 3. Dimension Acquisition”). The Company recorded a net $47.4 million deferred tax liability relating to the tax impact of future GAAP amortization or potential impairments associated with the identified intangible assets acquired, which are indefinitely lived assets and are not currently deductible for tax purposes. Due to the reduction of the US corporate tax rate to 21% in the period subsequent to the acquisition, the Company recorded a net decrease to the deferred tax liability of $16.2 million with a corresponding benefit from income taxes of $16.2 million for the year ended December 31, 2017. Due to a change in the state tax rates, the Company recorded a net increase to the deferred tax liability of $2.1 million with a corresponding deferred income tax expense of $2.1 million for the year ended December 31, 2019. The valuation allowance increased by $125.1 million and $57.5 million during the year ended December 31, 2019 and 2018, respectively. The Company recorded unrecognized tax benefits for uncertainties in income taxes. A reconciliation of the Company’s unrecognized tax benefits follows (in thousands): December 31, 2019 2018 2017 Balance at beginning of year $ 33,727 $ 28,377 $ 13,505 Additions based on tax positions related to current year 5,575 4,750 9,338 Additions for tax positions of prior years 652 600 5,534 Reductions for tax positions of prior years — — — Balance at end of year $ 39,954 $ 33,727 $ 28,377 The entire amount of the unrecognized tax benefits would not impact the Company’s effective tax rate if recognized. The Company has elected to include interest and penalties as a component of tax expense. During the years ended December 31, 2019 and 2018, the Company did not recognize accrued interest and penalties related to unrecognized tax benefits. The Company does not anticipate that the amount of existing unrecognized tax benefits will significantly increase or decrease during the next year. It is our intention to reinvest the earnings of our non-U.S. subsidiaries in their operations. As of December 31, 2019, the Company had not made a provision for any incremental foreign withholding taxes on approximately $2.1 million of the excess of the amount of net income for financial reporting over the tax basis of investments in foreign subsidiaries that are essentially permanent in duration. If these earnings were repatriated to the U.S., the deferred tax liability associated with these temporary differences would result in a nominal amount of withholding taxes. The Company files income tax returns in the U.S. federal, California, and other state tax jurisdictions. The federal and state income tax returns from inception to December 31, 2019 remain subject to examination.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2019
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	16. Commitments and Contingencies The Company has various manufacturing, clinical, research, and other contracts with vendors in the conduct of the normal course of its business. Other than as noted below, contracts are terminable, with varying provisions regarding termination. If a contract with a specific vendor were to be terminated, the Company would only be obligated for the products or services that the Company had received at the time the termination became effective. As of December 31, 2019, the aggregate payments under contractually binding manufacturing and service agreements are as follows (in thousands): Year Ending December 31, Manufacturing and Services 2020 7,854 2021 461 Thereafter — $ 8,315 See “Note 9. Leases” for lease commitments. Contingencies While there are no material legal proceedings the Company is aware of, the Company may become party to various claims and complaints arising in the ordinary course of business. Management does not believe that any ultimate liability resulting from any of these claims will have a material adverse effect on its results of operations, financial position, or liquidity. However, management cannot give any assurance regarding the ultimate outcome of these claims, and their resolution could be material to operating results for any particular period, depending upon the level of income for the period. Guarantees and Indemnifications The Company indemnifies each of its directors and officers for certain events or occurrences, subject to certain limits, while the director is or was serving at the Company’s request in such capacity, as permitted under Delaware law and in accordance with its certificate of incorporation and bylaws. The term of the indemnification period lasts as long as a director may be subject to any proceeding arising out of acts or omissions of such director in such capacity. The maximum amount of potential future indemnification is unlimited; however, the Company currently holds director liability insurance. This insurance allows the transfer of risk associated with the Company’s exposure and may enable it to recover a portion of any future amounts paid. The Company believes that the fair value of these indemnification obligations is minimal. Accordingly, it has not recognized any liabilities relating to these obligations for any period presented.

Net Loss per Share

Net Loss per Share	12 Months Ended
	Dec. 31, 2019
Earnings Per Share [Abstract]
Net Loss per Share	17. Net Loss per Share The following table sets forth the computation of the basic and diluted net loss per share during the years ended December 31, 2019, 2018, and 2017 (in thousands, except share and per share data): Year Ended December 31, 2019 2018 2017 Numerator: Net loss $ (402,727 ) $ (197,611 ) $ (302,139 ) Denominator: Weighted-average shares used to compute net loss per share, basic and diluted 56,576,885 49,775,223 42,453,135 Net loss per share, basic and diluted $ (7.12 ) $ (3.97 ) $ (7.12 ) The following weighted-average outstanding common stock equivalents were excluded from the computation of diluted net loss per share for the periods presented because including them would have been antidilutive: Year Ended December 31, 2019 2018 2017 Options to purchase common stock, RSUs, and PSUs 7,978,666 7,301,431 5,862,784 Employee stock purchase plan 3,953 3,345 2,728 Common stock warrants 149,700 149,700 149,700 8,132,319 7,454,476 6,015,212

Accumulated Other Comprehensive

Accumulated Other Comprehensive Loss	12 Months Ended
	Dec. 31, 2019
Other Comprehensive Income Loss Net Of Tax Period Increase Decrease [Abstract]
Accumulated Other Comprehensive Loss	18. Accumulated Other Comprehensive Loss Total accumulated other comprehensive loss consisted of the following (in thousands) : Year Ended December 31, 2019 2018 Cumulative foreign currency translation adjustment $ (306 ) $ (329 ) Unrealized gain (loss) on securities available-for-sale 159 (304 ) Total accumulated other comprehensive loss $ (147 ) $ (633 )

Quarterly Financial Data (unaud

Quarterly Financial Data (unaudited)	12 Months Ended
	Dec. 31, 2019
Quarterly Financial Data [Abstract]
Quarterly Financial Data (unaudited)	19. Quarterly Financial Data (unaudited) The following table presents certain unaudited quarterly financial information. This information has been prepared on the same basis as the audited financial statements and includes all adjustments (consisting only of normal recurring adjustments) necessary to present fairly the unaudited quarterly results of operations set forth herein (in thousands, except per share data): 2019 March 31, June 30, September 30, December 31, Revenue $ 18,172 $ 24,149 $ 25,800 $ 35,593 Operating expenses $ 117,386 $ 136,623 $ 143,833 $ 130,045 Net loss $ (96,756 ) $ (99,172 ) $ (112,994 ) $ (93,805 ) Net loss per share, basic and diluted $ (1.82 ) $ (1.72 ) $ (1.96 ) $ (1.62 ) 2018 March 31, June 30, September 30, December Revenue $ 10,677 $ 12,794 $ 11,763 $ 16,261 Operating expenses $ 107,164 $ 107,694 $ 101,409 $ 106,601 Net income (loss) $ 30,253 $ (52,728 ) $ (87,310 ) $ (87,826 ) Net income (loss) per share, basic $ 0.63 $ (1.06 ) $ (1.74 ) $ (1.73 ) Net income (loss) per share, diluted $ 0.62 $ (1.06 ) $ (1.74 ) $ (1.73 )

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Basis of Consolidation	Basis of Consolidation The consolidated financial statements include the accounts of Ultragenyx Pharmaceutical Inc. and our wholly-owned subsidiaries. All intercompany balances and transactions have been eliminated.
Use of Estimates	Use of Estimates The accompanying consolidated financial statements have been prepared in accordance with U.S. generally accepted accounting principles (“GAAP”). The preparation of the consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclosure of contingent liabilities and the reported amounts of expenses in the consolidated financial statements and the accompanying notes. On an ongoing basis, management evaluates its estimates, including those related to clinical trial accruals, fair value of assets and liabilities, income taxes, and stock-based compensation. Management bases its estimates on historical experience and on various other market-specific and relevant assumptions that management believes to be reasonable under the circumstances. Actual results could differ from those estimates.
Cash, Cash Equivalents, and Restricted Cash	Cash, Cash Equivalents, and Restricted Cash The Company considers all highly liquid investments with original maturities of three months or less from the date of purchase to be cash equivalents. Cash equivalents consist primarily of amounts invested in money market accounts. Restricted cash primarily consists of money market accounts used as collateral for the Company’s obligations under its facility leases. 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 amounts shown in the consolidated statement of cash flows (in thousands): December 31, 2019 2018 2017 Cash and cash equivalents $ 433,584 $ 113,432 $ 100,488 Restricted cash included in prepaid expenses and other current assets 161 271 461 Restricted cash included in other assets 2,499 1,822 2,092 Total cash, cash equivalents, and restricted cash shown in the statements of cash flows $ 436,244 $ 115,525 $ 103,041
Investments	Investments All investments have been classified as “available-for-sale” and are carried at estimated fair value as determined based upon quoted market prices or pricing models for similar securities. Management determines the appropriate classification of its investments at the time of purchase and reevaluates such designation as of each balance sheet date. Investments with a maturity of one year or less from the balance sheet date are reported as short-term investments and investments with a maturity of greater than one year from the balance sheet date are reported as long-term investments. Unrealized gains and losses are excluded from earnings and are reported as a component of comprehensive loss. Realized gains and losses and declines in fair value judged to be other than temporary, if any, on available-for-sale securities are included in other income (expense). The cost of securities sold is based on the specific-identification method. Interest on investments is included in interest income.
Investment in Equity Securities	Investment in Equity Securities In June 2019, the Company entered into an amendment to the Research Collaboration and License Agreement and an Equity Purchase Agreement with Arcturus Therapeutics Holdings Inc. (Arcturus). Pursuant to the Equity Purchase Agreement, the Company purchased 2,400,000 shares of common stock, or approximately 18.2% of Arcturus’s outstanding shares of common stock as of the closing date of the transaction and received an option to purchase an additional 600,000 shares of common stock. The investment is subject to the equity method of accounting as it was determined that the Company has significant influence over Arcturus, but does not control the significant activities of Arcturus. The Company elected to apply the fair value option to account for the equity investment in Arcturus. The decision to elect the fair value option is irrevocable and is determined on an instrument by instrument basis. The option to purchase additional stock is accounted for at fair value using the Black-Scholes option pricing method. The changes in fair value of the equity investment and option to purchase additional stock are included in the Consolidated Statements of Operations. See “Note 8. License and Research Agreements” for additional details on the Arcturus transaction.
Concentration of Credit Risk and Other Risks and Uncertainties	Concentration of Credit Risk and Other Risks and Uncertainties Financial instruments that potentially subject the Company to a concentration of credit risk consist of cash, cash equivalents, and investments. The Company’s cash, cash equivalents, and investments are held by financial institutions that management believes are of high credit quality. The Company’s investment policy limits investments to fixed income securities denominated and payable in U.S. dollars such as U.S. government obligations, money market instruments and funds, corporate bonds, commercial paper, and asset-backed securities and places restrictions on maturities and concentrations by type and issuer. Such deposits may, at times, exceed federally insured limits. The Company has not experienced any losses on its deposits of cash and cash equivalents and its accounts are monitored by management to mitigate risk. The Company is exposed to credit risk in the event of default by the financial institutions holding its cash and cash equivalents, corporate issuers, and other financial instruments, to the extent recorded in the balance sheets . The Company has not experienced any credit losses to date from credit risk concentration. Concentration of credit risk with respect to is primarily limited to collaboration partners, drug wholesalers, and retail pharmacy distributors. Credit is extended to our customers based on an evaluation of a customer’s financial condition, and collateral is not The Company is dependent on third-party manufacturers to supply products for research and development activities in its programs. In particular, the Company relies and expects to continue to rely on a small number of manufacturers to supply it with its requirements for the active pharmaceutical ingredients and formulated drugs related to these programs. These programs could be adversely affected by a significant interruption in the supply of active pharmaceutical ingredients and formulated drugs.
Inventory	Inventory The Company values inventory at the lower of cost and net realizable value and determines the cost of inventory using the average-cost method. The Company expenses costs associated with the manufacture of product candidates prior to regulatory approval. Inventories consist of currently approved products. The Company periodically reviews its inventories for excess amounts or obsolescence and writes down obsolete or otherwise unmarketable inventory to its estimated net realizable value. Management determines excess inventory based on expected future demand. Estimates related to future demand are sensitive to significant inputs and assumptions such as acceptance by patients and physicians and the availability of formulary coverage and adequate reimbursement from private third-party payers for the product.
Property and Equipment	Property and Equipment Property and equipment are stated at cost, less accumulated depreciation and amortization. Depreciation and amortization is computed using the straight-line method over the estimated useful lives of the respective assets. Depreciation and amortization begins at the time the asset is placed in service. Maintenance and repairs are charged to operations as incurred. Upon sale or retirement of assets, the cost and related accumulated depreciation are removed from the balance sheet and the resulting gain or loss, if any, is reflected in operations. The useful lives of property and equipment are as follows: Research and development equipment 5 years Furniture and office equipment 5 years Computer equipment 3 years Software 3-5 years Leasehold improvements Shorter of lease term or estimated useful life
Intangible Assets	Intangible Assets The Company recognizes an acquired intangible apart from goodwill whenever the intangible arises from contractual or other legal rights, or whenever it can be separated or divided from the acquired entity and sold, transferred, licensed, rented or exchanged, either individually or in combination with a related contract, asset or liability. The Company’s intangible assets consist of acquired in-process research and development (IPR&D) and an acquired contract asset. IPR&D assets represent capitalized incomplete research projects that the Company acquired through business combinations. The Company tests its definite and indefinite-lived intangible assets for impairment annually during the fourth quarter and more frequently if events or changes in circumstances indicate that it is more likely than not that the asset is impaired. If it is determined that the asset becomes impaired, the carrying value is written down to its fair value with the related impairment charge recognized in consolidated statements of operations in the period in which the impairment occurs. The Company has not recorded any impairments of intangible assets.
Goodwill	Goodwill Goodwill represents the excess of purchase price over fair value of net assets acquired in a business combination and is not amortized. Goodwill is subject to impairment testing at least annually during the fourth quarter or when a triggering event occurs that could indicate a potential impairment. If it is determined that the goodwill becomes impaired, the carrying value is written down to its fair value with the related impairment charge recognized in consolidated statements of operations in the period in which the impairment occurs. The Company has not recorded any impairments of goodwill.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets The Company evaluates its long-lived assets, including property and equipment, for impairment whenever events or changes in circumstances indicate that the carrying value of these assets may not be recoverable. Recoverability of these assets is measured by comparison of the carrying amount of each asset to the future undiscounted cash flows expected to result from the use of the asset and its eventual disposition. If the asset is considered to be impaired, the amount of any impairment is measured as the difference between the carrying value and the fair value of the impaired asset. The Company has not recorded impairment of any long-lived assets.
Accruals of Research and Development Costs	Accruals of Research and Development Costs The Company records accruals for estimated costs of research, preclinical and clinical studies and manufacturing development. These costs are a significant component of the Company’s research and development expenses. A substantial portion of the Company’s ongoing research and development activities are conducted by third-party service providers, including contract research organizations. The Company accrues the costs incurred under its agreements with these third parties based on actual work completed in accordance with agreements established with these third parties. The Company determines the actual costs through discussions with internal personnel and external service providers as to the progress or stage of completion of the services and the agreed-upon fee to be paid for such services.
Revenue Recognition	Revenue Recognition Collaboration and license revenue The Company has certain license and collaboration agreements that are within the scope of Accounting Standards Codification (ASC) 808, Collaborative Agreements provides guidance on the presentation and disclosure of collaborative arrangements. Generally, the classification of the transactions under the collaborative arrangements is determined based on the nature of contractual terms of the arrangement, along with the nature of the operations of the participants. When the collaborative partner is the principal in the sale transaction with the customer, the Company records its share of collaboration profit as collaboration revenue. Funding received related to research and development services and commercialization costs is generally classified as a reduction of research and development expenses and selling, general and administrative expenses, respectively, in the consolidated statement of operations, because the provision of such services for collaborative partners are not considered to be part of the Company’s ongoing major or central operations. The also receives royalty revenues under certain of the Company’s license or collaboration agreements in exchange for license of intellectual property. If the Company does not have any future performance obligations for these license or collaboration agreements, royalty revenue is recorded as the underlying sales occur. In to record collaboration revenue, the Company utilizes certain information from its , including revenue from the sale of the product, associated reserves on revenue, and costs incurred for development and sales activities. For the periods covered in the financial statements presented, there have been no material changes to prior period estimates of revenues and expenses. The terms of the Company’s collaboration agreements may contain multiple performance obligations, which may include licenses and research and development activities. The Company evaluates these agreements under ASC 606, Revenue from Contracts with Customers (ASC 606), to determine the distinct performance obligations. The Company analogizes to ASC 606 for the accounting for distinct performance obligations for which there is a customer relationship. Prior to recognizing revenue, the Company makes estimates of the transaction price, including variable consideration that is subject to a constraint. Amounts of variable consideration are included in the transaction price to the extent that it is probable that a significant reversal in the amount of cumulative revenue recognized will not occur and when the uncertainty associated with the variable consideration is subsequently resolved. Total consideration may include nonrefundable upfront license fees, payments for research and development activities, reimbursement of certain third-party costs, payments based upon the achievement of specified milestones, and royalty payments based on product sales derived from the collaboration. If there are multiple distinct performance obligations, the Company allocates the transaction price to each distinct performance obligation based on its relative standalone selling price. The standalone selling price is generally determined based on the prices charged to customers or using expected cost plus margin. The Company estimates the efforts needed to complete the performance obligation and recognizes revenue by measuring the progress towards complete satisfaction of the performance obligation using an input measure. Product sales The Company sells its approved products through a limited number of distributors. Under ASC 606, revenue from product sales is recognized at the point in time when the delivery is made and when title and risk of loss transfers to these distributors. The Company also recognizes revenue from sales of certain products on a “named patient” basis, which are allowed in certain countries prior to the commercial approval of the product. Prior to recognizing revenue, the Company makes estimates of the transaction price, including any variable consideration that is subject to a constraint. Amounts of variable consideration are included in the transaction price to the extent that it is probable that a significant reversal in the amount of cumulative revenue recognized will not occur and when the uncertainty associated with the variable consideration is subsequently resolved. Product sales are recorded net of estimated government-mandated rebates and chargebacks, estimated product returns, and other deductions. Provisions for returns and other adjustments are provided for in the period the related revenue is recorded, as estimated by management. periodically and adjusted as necessary. If actual results vary, the Company may need to adjust these estimates, which could have an effect on earnings in the period of the adjustment.
Leases	Leases The Company adopted Accounting Standards Update (ASU) No. 2016-02, Leases (Topic 842) Leases (Topic 840) The Company determines if an arrangement includes a lease at inception. Right-of-use lease assets and lease liabilities are recognized based on the present value of the future minimum lease payments over the lease term at the commencement date. The right-of-use lease asset includes any lease payments made and excludes lease incentives. Incremental borrowing rate is used in determining the present value of future payments. The Company applies a portfolio approach to the property leases to apply an incremental borrowing rate to leases with similar lease terms. The lease terms may include options to extend or terminate the lease. The Company recognizes the options to extend the lease as part of the right-of-use lease assets and lease liabilities only if it is reasonably certain that the option would be exercised. Lease expense for minimum lease payments is recognized on a straight-line basis over the non-cancelable lease term. As a result of the adoption of the new guidance, the Company recorded a right-of-use lease asset of $16.2 million, a short-term lease liability of $4.5 million, and a long-term lease liability of $17.0 million and no cumulative effect adjustment was made to the retained earnings as of the adoption date. In addition, as of the adoption date, the Company derecognized a net deferred rent obligation of $5.2 million. See “Note 9. Leases” for further disclosure.
Comprehensive Loss	Comprehensive Loss Comprehensive loss is the change in stockholders’ equity from transactions and other events and circumstances other than those resulting from investments by stockholders and distributions to stockholders. The Company’s other comprehensive loss is comprised of unrealized gains and losses on investments in available-for-sale securities and foreign currency translation adjustments.
Research and Development	Research and Development Research and development costs are expensed as incurred and consist of salaries and benefits, stock-based compensation expense, lab supplies and facility costs, as well as fees paid to other nonemployees and entities that conduct certain research and development activities on the Company’s behalf. Amounts incurred in connection with license agreements are also included in research and development expense. Nonrefundable advance payments for goods or services to be received in the future for use in research and development activities are deferred. The deferred amounts are expensed as the related goods are delivered or the services are performed.
Stock-Based Compensation	Stock-Based Compensation Stock-based awards issued to employees, including stock options, restricted stock units (RSUs), and performance stock units (PSUs) are recorded at fair value as of the grant date and recognized as expense on a straight-line basis over the employee’s requisite service period (generally the vesting period). PSUs vest only if certain specified criteria are achieved and the employees’ continued service requirements are met; therefore, the expense recognition occurs when the likelihood of the PSUs being earned is deemed probable. Stock compensation expense on awards expected to vest are recognized net of estimated forfeitures.
Income Taxes	Income Taxes The Company uses the liability method of accounting for income taxes. Under this method, deferred tax assets and liabilities are determined based on the differences between the financial reporting and the tax bases of assets and liabilities and are measured using the enacted tax rates and laws that will be in effect when the differences are expected to reverse. The Company must then assess the likelihood that the resulting deferred tax assets will be realized. A valuation allowance is provided when it is more likely than not that some portion or all of a deferred tax asset will not be realized. Due to the Company’s lack of earnings history, the net deferred tax assets have been fully offset by a valuation allowance. In conjunction with Dimension acquisition, a deferred tax liability was recorded reflecting the tax impact of the difference between the book basis and tax basis of acquired IPR&D. Such deferred income tax liability is not used to offset deferred tax assets when analyzing the Company’s valuation allowance as the acquired IPR&D is considered to have an indefinite life until the Company completes or abandons development of the acquired IPR&D. The Company recognizes benefits of uncertain tax positions if it is more likely than not that such positions will be sustained upon examination based solely on their technical merits, as the largest amount of benefit that is more likely than not to be realized upon the ultimate settlement. The Company’s policy is to recognize interest and penalties related to the underpayment of income taxes as a component of income tax expense or benefit. To date, there have been no interest or penalties charged in relation to the unrecognized tax benefits.
Foreign Currency	Foreign Currency Assets and liabilities of non-U.S. subsidiaries that operate in a local currency environment, where the local currency is the functional currency, are translated to U.S. dollars at exchange rates in effect at the balance sheet date, with the resulting translation adjustments directly recorded to a separate component of accumulated other comprehensive loss. Income and expense accounts are translated at average exchange rates for the period. Transactions which are not in the functional currency of the entity are remeasured into the functional currency and gains or losses resulting from the remeasurement recorded in other income (expense).
Net Loss per Share	Net Loss per Share Basic net loss per share is calculated by dividing the net loss by the weighted-average number of shares of common stock outstanding during the period, without consideration for common stock equivalents. Diluted net loss per share is the same as basic net loss per share, since the effects of potentially dilutive securities are antidilutive. In periods when we have incurred a net loss, options and warrants to purchase common stock are considered common stock equivalents, but have been excluded from the calculation of diluted net loss per share, as their effect is antidilutive.
Business Combinations	Business Combinations The Company applies the provisions of ASC 805, “Business Combinations” allocates
Recent Accounting Pronouncements	Recent Accounting Pronouncements In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments — Credit Losses, (Topic 326): Measurement of Credit Losses on Financial Instruments,

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Summary of Reconciliation of Cash, Cash Equivalents and Restricted Cash	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 amounts shown in the consolidated statement of cash flows (in thousands): December 31, 2019 2018 2017 Cash and cash equivalents $ 433,584 $ 113,432 $ 100,488 Restricted cash included in prepaid expenses and other current assets 161 271 461 Restricted cash included in other assets 2,499 1,822 2,092 Total cash, cash equivalents, and restricted cash shown in the statements of cash flows $ 436,244 $ 115,525 $ 103,041
Summary of Useful Lives of Property and Equipment	The useful lives of property and equipment are as follows: Research and development equipment 5 years Furniture and office equipment 5 years Computer equipment 3 years Software 3-5 years Leasehold improvements Shorter of lease term or estimated useful life

Dimension Acquisition (Tables)

Dimension Acquisition (Tables)	12 Months Ended
	Dec. 31, 2019
Business Combinations [Abstract]
Summary of Fair Value of Consideration Transferred	The acquisition date fair value of the consideration transferred for Dimension was approximately $164.1 million, which consisted of the following (in thousands): Cash payments $ 152,292 Fair value of vested stock options assumed 8,979 REGENX termination fee 2,850 Fair value of total consideration $ 164,121
Summary of Fair Values of Assets Acquired and Liabilities Assumed	The following table summarizes the fair values of assets acquired and liabilities assumed as of the date of acquisition (in thousands): Cash and cash equivalents $ 12,338 Short-term investments 9,737 Other current assets 11,155 Property and equipment 6,580 In-process research and development 129,000 Bayer collaboration agreement 13,526 Accounts payable and accrued liabilities (10,265 ) Notes payable (4,944 ) Deferred tax liabilities (47,412 ) Net identifiable net assets acquired 119,715 Goodwill 44,406 Net assets acquired $ 164,121

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Summary of Financial Assets and Liabilities Measured on Recurring Basis	The following table sets forth the fair value of the Company’s financial assets and liabilities remeasured on a recurring basis based on the three-tier fair value hierarchy (in thousands): December 31, 2019 Level 1 Level 2 Level 3 Total Financial Assets: Money market funds $ 293,309 $ — $ — $ 293,309 Repurchase agreements — 100,000 — 100,000 Time deposits — 10,000 — 10,000 Corporate bonds — 77,026 — 77,026 Commercial paper — 80,119 — 80,119 Asset-backed securities — 30,406 — 30,406 U.S. Government Treasury and agency securities 96,329 53,979 — 150,308 Investment in Arcturus equity securities 26,088 — 1,664 27,752 Total $ 415,726 $ 351,530 $ 1,664 $ 768,920 December 31, 2018 Level 1 Level 2 Level 3 Total Financial Assets: Money market funds $ 48,999 $ — $ — $ 48,999 Repurchase agreements — 24,000 — 24,000 Time deposits — 10,000 — 10,000 Corporate bonds — 179,926 — 179,926 Commercial paper — 50,198 — 50,198 Asset-backed securities — 22,587 — 22,587 U.S. Government Treasury and agency securities — 99,034 — 99,034 Total $ 48,999 $ 385,745 $ — $ 434,744

Balance Sheet Components (Table

Balance Sheet Components (Tables)	12 Months Ended
	Dec. 31, 2019
Investments Debt And Equity Securities [Abstract]
Summary of Cash Equivalents and Investments Classified as Available For Sale Securities	The fair values of cash equivalents and investments classified as available-for-sale securities consisted of the following (in thousands): December 31, 2019 Gross Unrealized Amortized Cost Gains Losses Estimated Fair Value Money market funds $ 293,309 $ — $ — $ 293,309 Repurchase agreements 100,000 — — 100,000 Time deposits 10,000 — — 10,000 Corporate bonds 77,022 17 (13 ) 77,026 Commercial paper 80,119 — — 80,119 Asset-backed securities 30,375 31 — 30,406 U.S. Government Treasury and agency securities 150,184 124 — 150,308 Total $ 741,009 $ 172 $ (13 ) $ 741,168 December 31, 2018 Gross Unrealized Amortized Cost Gains Losses Estimated Fair Value Money market funds $ 48,999 $ — $ — $ 48,999 Repurchase agreements 24,000 — — 24,000 Time deposits 10,000 — — 10,000 Corporate bonds 180,167 — (241 ) 179,926 Commercial paper 50,198 — — 50,198 Asset-backed securities 22,597 — (10 ) 22,587 U.S. Government Treasury and agency securities 99,087 2 (55 ) 99,034 Total $ 435,048 $ 2 $ (306 ) $ 434,744
Summary of Inventory	Inventory consists of the following (in thousands): December 31, 2019 2018 Work-in-process $ 8,191 $ 5,384 Finished goods 3,355 1,681 Total $ 11,546 $ 7,065
Summary of Property and Equipment	Property and equipment, net consists of the following (in thousands): December 31, 2019 2018 Leasehold improvements $ 16,871 $ 15,705 Research and development equipment 17,881 9,856 Furniture and office equipment 3,496 3,379 Computer equipment and software 7,817 7,342 Construction-in-progress 24,271 1,970 Property and equipment, gross 70,336 38,252 Less accumulated depreciation (25,988 ) (18,206 ) Property and equipment, net $ 44,348 $ 20,046
Accrued Liabilities	Accrued liabilities consists of the following (in thousands): December 31, 2019 2018 Research, clinical study, and manufacturing expenses $ 22,894 $ 16,912 Payroll and related expenses 41,324 36,443 Other 18,976 9,095 Total $ 83,194 $ 62,450

Revenue (Tables)

Revenue (Tables)	12 Months Ended
	Dec. 31, 2019
Revenue From Contract With Customer [Abstract]
Summary of Disaggregation of Total Revenues	The following table disaggregates total revenues from external customers by collaboration and license revenue and product sales (in thousands): Year Ended December 31, 2019 2018 2017 Collaboration and license revenue: KKC (Crysvita) $ 82,989 $ 18,226 $ 9 Bayer 504 23,467 2,127 Total collaboration and license revenue 83,493 41,693 2,136 Product sales: Crysvita 4,286 644 — Mepsevii 12,634 7,903 476 UX007 3,301 1,255 — Total product sales 20,221 9,802 476 Total revenues $ 103,714 $ 51,495 $ 2,612 The following table disaggregates total revenues based on geographic location (in thousands): Year Ended December 31, 2019 2018 2017 United States $ 86,442 $ 45,339 $ 2,289 Europe 12,085 5,293 323 All other 5,187 863 — Total revenues $ 103,714 $ 51,495 $ 2,612
Schedule of Sales-Related Accruals and Allowances	The following table presents the activity and ending balances for sales-related accruals and allowances (in thousands): December 31, 2019 2018 2017 Balance of product sales reserve at beginning of year $ 1,240 $ 41 $ — Provisions 3,846 2,466 41 Payments and adjustments (3,268 ) (1,267 ) — Balance of product sales reserve at end of year $ 1,818 $ 1,240 $ 41
Summary of Changes in Contract Assets (Liabilities)	The following table presents changes in the contract assets (liabilities) for the years ended December 31, 2019 and 2018 (in thousands): December 31, 2019 2018 Balance of contract assets (liabilities) at beginning of year $ 2,979 $ (5,986 ) Additions 504 24,055 Deductions (3,483 ) (15,090 ) Balance of contract assets at end of year $ — $ 2,979

License and Research Agreemen_2

License and Research Agreements (Tables)	12 Months Ended
	Dec. 31, 2019
Research Grant Agreement [Abstract]
Share of Collaboration and Royalty Revenue Related to Crysvita	The Company’s Year Ended December 31, 2019 2018 2017 Company's share of collaboration revenue in profit-share territory $ 74,869 $ 15,334 $ — Royalty revenue in European territory 8,120 2,892 9 Total $ 82,989 $ 18,226 $ 9
Schedule of Cost Sharing Payments	Under the collaboration agreement, KKC and the Company share certain development and commercialization costs. Year Ended December 31, 2019 2018 2017 Research and development $ 27,309 $ 32,240 $ 31,165 Selling, general and administrative 21,828 14,228 4,466 Total $ 49,137 $ 46,468 $ 35,631
Schedule of Changes in Fair Value of Investment in Arcturus Securities	The changes in the fair value of the Company’s investment in Arcturus securities were as follows (in thousands): Arcturus common stock Fair value of option to purchase additional shares of Arcturus common stock December 31, 2018 $ — $ — Acquisition of investment in Arcturus securities 13,872 467 Change in fair value 12,216 1,197 December 31, 2019 $ 26,088 $ 1,664

Leases (Tables)

Leases (Tables)	12 Months Ended
	Dec. 31, 2019
Lessee Disclosure [Abstract]
Summary of Components of Lease Expense	The components of lease expense were as follows (in thousands): Year Ended December 31, 2019 Operating lease expense $ 9,163 Variable lease expense 2,779 Total $ 11,942
Schedule of Future Minimum Lease Payments under Non-cancellable Leases	Future minimum lease payments under non-cancellable leases as of December 31, 2019 were as follows (in thousands): Year Ending December 31, Leases 2020 $ 9,786 2021 7,667 2022 7,521 2023 7,541 2024 5,571 Thereafter 7,582 Total future lease payments 45,668 Less: Amount representing interest (8,676 ) Present value of future lease payments 36,992 Less: Short-term lease liabilities (7,235 ) Long-term lease liabilities $ 29,757

Liability Related to the Sale_2

Liability Related to the Sale of Future Royalties (Tables)	12 Months Ended
	Dec. 31, 2019
Deferred Revenue Disclosure [Abstract]
Schedule of Activity Within Liability Related to Sale of Future Royalties	The following table shows the activity within the liability account during the year ended December 31, 2019 (in thousands): December 31, 2019 Liability related to the sale of future royalties — beginning balance $ — Proceeds from sale of future royalties 314,234 Non-cash interest expense 1,135 Liability related to the sale of future royalties at end of year $ 315,369

Stock-Based Awards (Tables)

Stock-Based Awards (Tables)	12 Months Ended
	Dec. 31, 2019
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Summary of Activity Under Stock Option Plans Including 2011 Plan and 2014 Plan	Stock Option Activity The following table summarizes activity under the Company’s stock option plans, including the 2011 Plan, the 2014 Plan, the assumed equity awards from the Dimension plans and related information: Options Outstanding Number of Options Weighted- Average Exercise Weighted-Average Remaining Contractual Term Aggregate Intrinsic Value (In Outstanding — December 31, 2016 4,429,472 $ 61.85 8.22 $ 79,135 Options granted 1,328,860 71.99 Options assumed 639,897 27.97 Options exercised (478,470 ) 16.25 Options cancelled (520,349 ) 78.70 Outstanding — December 31, 2017 5,399,410 $ 62.75 7.40 $ 37,687 Options granted 1,479,451 55.54 Options exercised (713,263 ) 36.21 Options cancelled (812,474 ) 74.80 Outstanding — December 31, 2018 5,353,124 $ 62.46 7.22 $ 23,243 Options granted 1,762,075 63.03 Options exercised (235,678 ) 32.87 Options cancelled (766,457 ) 72.48 Outstanding — December 31, 2019 6,113,064 $ 62.51 7.00 $ 18,989 Vested and exercisable — December 31, 2019 3,429,579 $ 63.14 5.83 $ 18,514 Vested and expected to vest — December 31, 2019 5,847,778 $ 62.58 6.92 $ 18,945
Summary of Stock-Based Compensation Expense	Total stock-based compensation recognized was as follows (in thousands): Year Ended December 31, 2019 2018 2017 Cost of sales $ 1,084 $ 146 $ — Research and development 44,205 45,572 38,212 Selling, general and administrative 36,706 34,389 29,802 Total stock-based compensation expense $ 81,995 $ 80,107 $ 68,014
Fair Value of Stock Option Awards Granted Estimated Using Black-Scholes Option-Pricing Model	The fair value of stock option awards granted was estimated at the date of grant using a Black-Scholes option-pricing model with the following weighted-average assumptions: Year Ended December 31, 2019 2018 2017 Expected term (years) 6.22 6.23 6.23 Expected volatility 61% 62% 65% Risk-free interest rate 2.4% 2.7% 2.1% Expected dividend rate 0.0% 0.0% 0.0%
Restricted Stock Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Summary of Activity Under Stock Units from 2014 Plan	The following table summarizes activity under the Company’s Restricted Stock Units (RSUs) from the 2014 Plan and related information: RSUs Outstanding Number of Shares Weighted- Average Grant Date Fair Value Unvested — December 31, 2016 573,744 $ 71.45 RSUs granted 516,161 71.58 RSUs vested (156,021 ) 71.93 RSUs cancelled (112,324 ) 76.78 Unvested — December 31, 2017 821,560 $ 70.71 RSUs granted 555,905 56.01 RSUs vested (235,913 ) 71.01 RSUs cancelled (187,475 ) 65.37 Unvested — December 31, 2018 954,077 $ 63.12 RSUs granted 863,065 62.78 RSUs vested (313,682 ) 66.25 RSUs cancelled (205,310 ) 64.28 Unvested — December 31, 2019 1,298,150 $ 61.96
Performance Stock Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Summary of Activity Under Stock Units from 2014 Plan	The following table summarizes activity under the Company’s PSUs from the 2014 Plan and related information: PSUs Outstanding Number of Shares Weighted- Average Grant Date Fair Value Unvested — December 31, 2016 — $ — PSUs granted 508,850 48.03 Unvested — December 31, 2017 508,850 $ 48.03 PSUs granted 71,725 59.67 PSUs cancelled (97,375 ) 48.58 Unvested — December 31, 2018 483,200 $ 49.65 PSUs granted 61,500 67.31 PSUs vested (65,643 ) 48.03 PSUs cancelled (79,517 ) 50.01 Unvested — December 31, 2019 399,540 $ 52.56

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Components of Company's Loss Before Income Taxes	The components of the Company’s loss before income taxes were as follows (in thousands): Year Ended December 31, 2019 2018 2017 Domestic $ 399,709 $ 205,440 $ 250,917 Foreign (265 ) (8,343 ) 67,421 Total loss before income taxes $ 399,444 $ 197,097 $ 318,338
Components of Company's Income Tax Provision	The components of the Company’s income tax provision were as follows (in thousands): Year Ended December 31, 2019 2018 2017 Current provision for income taxes: Federal $ — $ — $ — State 51 14 5 International 1,092 500 42 Total current tax provision 1,143 514 47 Deferred tax provision (benefit): Federal — — (16,243 ) State 2,140 — (3 ) International — — — Total deferred tax provision (benefit) 2,140 — (16,246 ) Total provision for (benefit from) income taxes $ 3,283 $ 514 $ (16,199 )
Effective Tax Rate of Provision for Income Taxes from Federal Statutory Rate	The effective tax rate of our provision for income taxes differs from the federal statutory rate as follows: Year Ended December 31, 2019 2018 2017 Federal statutory income tax rate 21.0 % 21.0 % 34.0 % State income taxes, net of federal benefit (0.5 ) — — Federal tax credits 5.2 9.5 9.0 Other (0.2 ) (0.2 ) (0.9 ) Nondeductible permanent items (0.4 ) (0.8 ) — Stock-based compensation (1.0 ) (0.8 ) (0.6 ) Uncertain tax positions (1.0 ) (1.9 ) (1.8 ) Change in valuation allowance (23.6 ) (26.8 ) (32.5 ) Foreign rate differential (0.3 ) (0.3 ) (7.2 ) Change in federal tax rate — — 5.1 Provision for income taxes (0.8 ) % (0.3 ) % 5.1 %
Schedule of Tax Effect of Temporary Differences to Significant Portions of Deferred Tax Assets	The tax effect of temporary differences that give rise to significant portions of the deferred tax assets is presented below (in thousands): Year Ended December 31, 2019 2018 Deferred tax assets: Loss carryforwards $ 185,892 $ 183,331 Tax credits 158,791 137,019 Stock options 40,676 29,925 Accruals and reserves 19,622 7,418 Fixed assets and intangibles 3,870 2,820 Liability related to sale of future royalties 81,424 — Other 4,183 1,189 Gross deferred tax assets 494,458 361,702 Valuation allowance (486,796 ) (361,702 ) Total deferred tax assets 7,662 — Deferred tax liabilities: In-process research and development (33,306 ) (31,166 ) Right-of-use lease assets (7,662 ) — Gross deferred tax liabilities (40,968 ) (31,166 ) Net deferred tax asset (liabilities) $ (33,306 ) $ (31,166 )
Unrecognized Tax Benefits	A reconciliation of the Company’s unrecognized tax benefits follows (in thousands): December 31, 2019 2018 2017 Balance at beginning of year $ 33,727 $ 28,377 $ 13,505 Additions based on tax positions related to current year 5,575 4,750 9,338 Additions for tax positions of prior years 652 600 5,534 Reductions for tax positions of prior years — — — Balance at end of year $ 39,954 $ 33,727 $ 28,377

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2019
Commitments And Contingencies Disclosure [Abstract]
Summary of Aggregate Payments under Contractually Binding Manufacturing and Service Agreements	As of December 31, 2019, the aggregate payments under contractually binding manufacturing and service agreements are as follows (in thousands): Year Ending December 31, Manufacturing and Services 2020 7,854 2021 461 Thereafter — $ 8,315

Net Loss per Share (Tables)

Net Loss per Share (Tables)	12 Months Ended
	Dec. 31, 2019
Earnings Per Share [Abstract]
Computation of Basic and Diluted Net Loss per Share	The following table sets forth the computation of the basic and diluted net loss per share during the years ended December 31, 2019, 2018, and 2017 (in thousands, except share and per share data): Year Ended December 31, 2019 2018 2017 Numerator: Net loss $ (402,727 ) $ (197,611 ) $ (302,139 ) Denominator: Weighted-average shares used to compute net loss per share, basic and diluted 56,576,885 49,775,223 42,453,135 Net loss per share, basic and diluted $ (7.12 ) $ (3.97 ) $ (7.12 )
Outstanding Shares of Common Stock Equivalents Excluded from Computation of Diluted Net Loss per Share	The following weighted-average outstanding common stock equivalents were excluded from the computation of diluted net loss per share for the periods presented because including them would have been antidilutive: Year Ended December 31, 2019 2018 2017 Options to purchase common stock, RSUs, and PSUs 7,978,666 7,301,431 5,862,784 Employee stock purchase plan 3,953 3,345 2,728 Common stock warrants 149,700 149,700 149,700 8,132,319 7,454,476 6,015,212

Accumulated Other Comprehensi_2

Accumulated Other Comprehensive Loss (Tables)	12 Months Ended
	Dec. 31, 2019
Other Comprehensive Income Loss Net Of Tax Period Increase Decrease [Abstract]
Schedule of Total Accumulated Other Comprehensive Loss	Total accumulated other comprehensive loss consisted of the following (in thousands) : Year Ended December 31, 2019 2018 Cumulative foreign currency translation adjustment $ (306 ) $ (329 ) Unrealized gain (loss) on securities available-for-sale 159 (304 ) Total accumulated other comprehensive loss $ (147 ) $ (633 )

Quarterly Financial Data (una_2

Quarterly Financial Data (unaudited) (Tables)	12 Months Ended
	Dec. 31, 2019
Quarterly Financial Data [Abstract]
Summary of Unaudited Quarterly Financial Information	The following table presents certain unaudited quarterly financial information. This information has been prepared on the same basis as the audited financial statements and includes all adjustments (consisting only of normal recurring adjustments) necessary to present fairly the unaudited quarterly results of operations set forth herein (in thousands, except per share data): 2019 March 31, June 30, September 30, December 31, Revenue $ 18,172 $ 24,149 $ 25,800 $ 35,593 Operating expenses $ 117,386 $ 136,623 $ 143,833 $ 130,045 Net loss $ (96,756 ) $ (99,172 ) $ (112,994 ) $ (93,805 ) Net loss per share, basic and diluted $ (1.82 ) $ (1.72 ) $ (1.96 ) $ (1.62 ) 2018 March 31, June 30, September 30, December Revenue $ 10,677 $ 12,794 $ 11,763 $ 16,261 Operating expenses $ 107,164 $ 107,694 $ 101,409 $ 106,601 Net income (loss) $ 30,253 $ (52,728 ) $ (87,310 ) $ (87,826 ) Net income (loss) per share, basic $ 0.63 $ (1.06 ) $ (1.74 ) $ (1.73 ) Net income (loss) per share, diluted $ 0.62 $ (1.06 ) $ (1.74 ) $ (1.73 )

Organization and Basis of Pre_2

Organization and Basis of Presentation - Additional Information (Details)	12 Months Ended
	Dec. 31, 2019Segment
United States of America
Organization And Nature Of Business [Line Items]
Number of reportable segments	1

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Summary of Reconciliation of Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Cash and cash equivalents	$ 433,584	$ 113,432	$ 100,488
Restricted cash included in prepaid expenses and other current assets	$ 161	$ 271	$ 461
Restricted Cash and Cash Equivalents, Current, Asset, Statement of Financial Position [Extensible List]	us-gaap:PrepaidExpensesAndOtherCurrentAssetsMember	us-gaap:PrepaidExpensesAndOtherCurrentAssetsMember	us-gaap:PrepaidExpensesAndOtherCurrentAssetsMember
Restricted cash included in other assets	$ 2,499	$ 1,822	$ 2,092
Restricted Cash and Cash Equivalents, Noncurrent, Asset, Statement of Financial Position [Extensible List]	us-gaap:OtherNoncurrentAssetsMember	us-gaap:OtherNoncurrentAssetsMember	us-gaap:OtherNoncurrentAssetsMember
Total cash, cash equivalents, and restricted cash shown in the statements of cash flows	$ 436,244	$ 115,525	$ 103,041	$ 164,607

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Additional Information (Details) - USD ($)	1 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
Summary Of Significant Accounting Policies [Line Items]
Common stock, shares issued		57,838,220		50,860,588
Allowance for doubtful accounts		$ 0
Right-to-use lease asset		30,328,000
Short-term lease liabilities		7,235,000
Long-term lease liability		29,757,000
Unrecognized tax benefits related to interest or penalties		$ 0
Accounting Standards Update 2016-02
Summary Of Significant Accounting Policies [Line Items]
Right-to-use lease asset			$ 16,200,000
Short-term lease liabilities			4,500,000
Long-term lease liability			17,000,000
Adjustment to retained earnings			0
Deferred rent derecognized, net			$ 5,200,000
Arcturus Research Collaboration and License Agreement
Summary Of Significant Accounting Policies [Line Items]
Common stock, shares issued	2,400,000
Percentage of outstanding common shares	18.20%
Option to purchase additional common stock, shares	600,000

Summary of Significant Accoun_6

Summary of Significant Accounting Policies - Summary of Useful Lives of Property and Equipment (Details)	12 Months Ended
	Dec. 31, 2019
Research and Development Equipment
Property Plant And Equipment [Line Items]
Property plant and equipment useful life	5 years
Furniture and Office Equipment
Property Plant And Equipment [Line Items]
Property plant and equipment useful life	5 years
Computer Equipment
Property Plant And Equipment [Line Items]
Property plant and equipment useful life	3 years
Software | Minimum
Property Plant And Equipment [Line Items]
Property plant and equipment useful life	3 years
Software | Maximum
Property Plant And Equipment [Line Items]
Property plant and equipment useful life	5 years
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property plant and equipment useful life	Shorter of lease term or estimated useful life

Dimension Acquisition - Additio

Dimension Acquisition - Additional Information (Details) - USD ($)	Nov. 07, 2017	Dec. 31, 2017	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Increase (decrease) in deferred tax liability											$ 2,100,000		$ (16,200,000)
Federal statutory income tax rate											21.00%	21.00%	34.00%
Total revenues			$ 35,593,000	$ 25,800,000	$ 24,149,000	$ 18,172,000	$ 16,261,000	$ 11,763,000	$ 12,794,000	$ 10,677,000	$ 103,714,000	$ 51,495,000	$ 2,612,000
Net loss			(93,805,000)	$ (112,994,000)	$ (99,172,000)	$ (96,756,000)	$ (87,826,000)	$ (87,310,000)	$ (52,728,000)	$ 30,253,000	(402,727,000)	$ (197,611,000)	(302,139,000)
Proforma total revenues													18,500,000
Proforma net loss													341,700,000
Dimension
Business Acquisition [Line Items]
Business acquisition date	Nov. 7, 2017
Business acquisition, purchase price per share	$ 6
Business acquisition, cash	$ 152,292,000
REGENX termination fee paid	2,850,000
Business acquisition, assumed equity awards value	15,400,000
Business acquisition, acceleration of certain awards recognized as expense in post-combination financial statements	2,200,000
Business acquisition, being recognized as expense over employee's remaining service period	4,200,000
Business acquisition, consideration transferred	164,121,000
Non-current deferred tax liability	47,412,000
Increase (decrease) in deferred tax liability											$ 33,300,000		$ (31,200,000)
Federal statutory income tax rate											21.00%		34.00%
Notes payable	4,944,000
Transaction costs	6,000,000
Notes payable			$ 0								$ 0
Total revenues		$ 2,100,000
Net loss		$ 7,500,000
Transaction costs related to acquisition													$ 9,600,000
Amortization of definite-lived intangible assets													$ 1,800,000
Stock Options | Dimension
Business Acquisition [Line Items]
Business acquisition, assumed equity awards value	$ 8,979,000

Dimension Acquisition - Summary

Dimension Acquisition - Summary of Fair Value of Consideration Transferred (Details) - Dimension $ in Thousands	Nov. 07, 2017USD ($)
Business Acquisition [Line Items]
Cash payments	$ 152,292
Fair value of vested stock options assumed	15,400
REGENX termination fee	2,850
Fair value of total consideration	164,121
Stock Options
Business Acquisition [Line Items]
Fair value of vested stock options assumed	$ 8,979

Dimension Acquisition - Summa_2

Dimension Acquisition - Summary of Fair Values of Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Nov. 07, 2017
Business Acquisition [Line Items]
Goodwill	$ 44,406	$ 44,406
Dimension
Business Acquisition [Line Items]
Cash and cash equivalents			$ 12,338
Short-term investments			9,737
Other current assets			11,155
Property and equipment			6,580
In-process research and development			129,000
Bayer collaboration agreement			13,526
Accounts payable and accrued liabilities			(10,265)
Notes payable			(4,944)
Deferred tax liabilities			(47,412)
Net identifiable net assets acquired			119,715
Goodwill			44,406
Net assets acquired			$ 164,121

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Details) - Arcturus Research Collaboration and License Agreement $ in Thousands	12 Months Ended
	Dec. 31, 2019USD ($)
Schedule Of Available For Sale Securities [Line Items]
Change in fair value of common stock	$ 12,216
Change in fair value of additional common stock	$ 1,197

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Financial Assets and Liabilities Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value	$ 768,920	$ 434,744
Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value	415,726	48,999
Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value	351,530	385,745
Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value	1,664
Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value	293,309	48,999
Money Market Funds | Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value	293,309	48,999
Asset-backed Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value	30,406	22,587
Asset-backed Securities | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value	30,406	22,587
Repurchase Agreements
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value	100,000	24,000
Repurchase Agreements | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value	100,000	24,000
Time Deposits
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value	10,000	10,000
Time Deposits | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value	10,000	10,000
Corporate Bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value	77,026	179,926
Corporate Bonds | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value	77,026	179,926
Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value	80,119	50,198
Commercial Paper | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value	80,119	50,198
U.S. Government Treasury and Agency Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value	150,308	99,034
U.S. Government Treasury and Agency Securities | Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value	96,329
U.S. Government Treasury and Agency Securities | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value	53,979	$ 99,034
Investment in Arcturus Equity Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value	27,752
Investment in Arcturus Equity Securities | Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value	26,088
Investment in Arcturus Equity Securities | Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value	$ 1,664

Balance Sheet Components - Summ

Balance Sheet Components - Summary of Cash Equivalents and Investments Classified as Available For Sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 741,009	$ 435,048
Gross Unrealized Gains	172	2
Gross Unrealized Losses	(13)	(306)
Estimated Fair Value	741,168	434,744
Money Market Funds
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	293,309	48,999
Estimated Fair Value	293,309	48,999
Asset-backed Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	30,375	22,597
Gross Unrealized Gains	31
Gross Unrealized Losses		(10)
Estimated Fair Value	30,406	22,587
Repurchase Agreements
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	100,000	24,000
Estimated Fair Value	100,000	24,000
Time Deposits
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	10,000	10,000
Estimated Fair Value	10,000	10,000
Corporate Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	77,022	180,167
Gross Unrealized Gains	17
Gross Unrealized Losses	(13)	(241)
Estimated Fair Value	77,026	179,926
Commercial Paper
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	80,119	50,198
Estimated Fair Value	80,119	50,198
U.S. Government Treasury and Agency Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	150,184	99,087
Gross Unrealized Gains	124	2
Gross Unrealized Losses		(55)
Estimated Fair Value	$ 150,308	$ 99,034

Balance Sheet Components - Addi

Balance Sheet Components - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Significant realized gains or losses on available-for-sale securities	$ 0
Depreciation expense	$ 8,300,000	$ 7,200,000	$ 4,800,000
Maximum
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale securities remaining contractual maturities	2 years

Balance Sheet Components - Su_2

Balance Sheet Components - Summary of Inventory (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Investments Debt And Equity Securities [Abstract]
Work-in-process	$ 8,191	$ 5,384
Finished goods	3,355	1,681
Total	$ 11,546	$ 7,065

Balance Sheet Components - Su_3

Balance Sheet Components - Summary of Property and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 70,336	$ 38,252
Less accumulated depreciation	(25,988)	(18,206)
Property and equipment, net	44,348	20,046
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	16,871	15,705
Research and Development Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	17,881	9,856
Furniture and Office Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	3,496	3,379
Computer Equipment and Software
Property Plant And Equipment [Line Items]
Property and equipment, gross	7,817	7,342
Construction-in-progress
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 24,271	$ 1,970

Balance Sheet Components - Accr

Balance Sheet Components - Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Payables And Accruals [Abstract]
Research, clinical study, and manufacturing expenses	$ 22,894	$ 16,912
Payroll and related expenses	41,324	36,443
Other	18,976	9,095
Total	$ 83,194	$ 62,450

Intangible Assets, Net - Additi

Intangible Assets, Net - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Nov. 07, 2017
Finite Lived Intangible Assets [Line Items]
Research and development expense related to amortization of assets	$ 357,355	$ 293,998	$ 231,644
Intangible asset impairment charges	0
Dimension
Finite Lived Intangible Assets [Line Items]
Business combination recognized intangible assets				$ 129,000
Research and development expense related to amortization of assets	$ 200	$ 12,300	$ 1,000
Dimension | IPR&D Assets
Finite Lived Intangible Assets [Line Items]
Business combination recognized intangible assets				129,000
Dimension | Contract Asset
Finite Lived Intangible Assets [Line Items]
Business combination recognized intangible assets				$ 13,500

Revenue - Summary of Disaggrega

Revenue - Summary of Disaggregation of Total Revenues from External Customers (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Total revenues
Total revenues	$ 35,593	$ 25,800	$ 24,149	$ 18,172	$ 16,261	$ 11,763	$ 12,794	$ 10,677	$ 103,714	$ 51,495	$ 2,612
Bayer HealthCare LLC
Total revenues
Total revenues									500	23,500	2,100
Collaboration and License
Total revenues
Total revenues									83,493	41,693	2,136
Collaboration and License | Kyowa Kirin Collaboration
Total revenues
Total revenues									82,989	18,226	9
Collaboration and License | Bayer HealthCare LLC
Total revenues
Total revenues									504	23,467	2,127
Product Sales
Total revenues
Total revenues									20,221	9,802	476
Product Sales | Crysvita
Total revenues
Total revenues									4,286	644
Product Sales | Mepsevii
Total revenues
Total revenues									12,634	7,903	$ 476
Product Sales | UX007
Total revenues
Total revenues									$ 3,301	$ 1,255

Revenue - Summary of Disaggre_2

Revenue - Summary of Disaggregation of Total Revenues (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation Of Revenue [Line Items]
Total revenues	$ 35,593	$ 25,800	$ 24,149	$ 18,172	$ 16,261	$ 11,763	$ 12,794	$ 10,677	$ 103,714	$ 51,495	$ 2,612
United States
Disaggregation Of Revenue [Line Items]
Total revenues									86,442	45,339	2,289
Europe
Disaggregation Of Revenue [Line Items]
Total revenues									12,085	5,293	$ 323
All other
Disaggregation Of Revenue [Line Items]
Total revenues									$ 5,187	$ 863

Revenue - Schedule of Sales-Rel

Revenue - Schedule of Sales-Related Accruals and Allowances (Details) - Product Sales - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation Of Revenue [Line Items]
Balance of product sales reserve at beginning of year	$ 1,240	$ 41
Provisions	3,846	2,466	$ 41
Payments and adjustments	(3,268)	(1,267)
Balance of product sales reserve at end of year	$ 1,818	$ 1,240	$ 41

Revenue - Summary of Changes in

Revenue - Summary of Changes in Contract Assets (Liabilities) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Disaggregation Of Revenue [Abstract]
Contract assets	$ 2,979
Contract liabilities		$ (5,986)
Additions	504	24,055
Deductions	$ (3,483)	(15,090)
Contract assets		$ 2,979

Revenue - Additional Informatio

Revenue - Additional Information (Details)	Dec. 31, 2019	Dec. 31, 2018
Disaggregation Of Revenue [Abstract]
Percentage of gross accounts receivable balance	87.00%	88.00%

License and Research Agreemen_3

License and Research Agreements - Additional Information (Details)	Jan. 01, 2023	Nov. 07, 2017USD ($)	Feb. 29, 2020USD ($)	Aug. 31, 2019USD ($)	Jun. 30, 2019USD ($)DiseaseTarget$ / sharesshares	Oct. 31, 2018USD ($)	Oct. 31, 2015USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2019USD ($)$ / sharesshares	Sep. 30, 2019USD ($)	Jun. 30, 2019USD ($)$ / sharesshares	Mar. 31, 2019USD ($)	Dec. 31, 2018USD ($)$ / sharesshares	Sep. 30, 2018USD ($)	Jun. 30, 2018USD ($)	Mar. 31, 2018USD ($)	Dec. 31, 2019USD ($)DiseaseIndicationQuarter$ / sharesshares	Dec. 31, 2018USD ($)$ / sharesshares	Dec. 31, 2017USD ($)	Dec. 31, 2022
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration and license revenue									$ 35,593,000	$ 25,800,000	$ 24,149,000	$ 18,172,000	$ 16,261,000	$ 11,763,000	$ 12,794,000	$ 10,677,000	$ 103,714,000	$ 51,495,000	$ 2,612,000
Research and development																	357,355,000	293,998,000	231,644,000
Contract asset													2,979,000					2,979,000
Prepaid expenses and other current assets									51,397,000				42,858,000				51,397,000	42,858,000
Accrued liabilities									$ 83,194,000				$ 62,450,000				$ 83,194,000	$ 62,450,000
Common stock purchased | shares									57,838,220				50,860,588				57,838,220	50,860,588
Common stock. per share | $ / shares									$ 0.001				$ 0.001				$ 0.001	$ 0.001
Common stock investment value									$ 58,000				$ 51,000				$ 58,000	$ 51,000
Dimension
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration and license revenue								$ 2,100,000
Business acquisition date		Nov. 7, 2017
Business combination recognized intangible assets		$ 129,000,000
Research and development																	$ 200,000	12,300,000	1,000,000
Amortization completion year																	2019
Consideration paid		164,121,000
Business acquisition, consideration transferred		164,121,000
Dimension | Contract Asset
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Business combination recognized intangible assets		13,500,000
Bayer HealthCare LLC
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration and license revenue																	$ 500,000	23,500,000	2,100,000
Contract liability		$ 2,500,000
Contract asset									0				3,000,000				0	3,000,000
Product Sales
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration and license revenue																	20,221,000	9,802,000	476,000
Kyowa Kirin Collaboration | License Agreement | Prepaid and Other Current Assets
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
License agreement other receivables									17,800,000								17,800,000
Kyowa Kirin Collaboration | License Agreement | Product Sales
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration and license revenue																	4,300,000	600,000
Kyowa Kirin Collaboration | License Agreement | Profit Share Revenue
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
License agreement accounts receivable									28,500,000				11,200				28,500,000	11,200
Kyowa Kirin Collaboration | License Agreement | Commercial and Development Activity Reimbursements
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
License agreement other receivables													11,100,000					11,100,000
License agreement accrued liabilities									900,000				300,000				$ 900,000	300,000
Kyowa Kirin Collaboration | License Agreement | Scenario Forecast
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Profit loss sharing percentage on net sales	30.00%																			35.00%
Kyowa Kirin Collaboration | Profit Share Territory | License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Remaining profit or loss share percentage on commercializing products																	50.00%
Tiered double-digit revenue share percentage entitled to receive																	20.00%
Kyowa Kirin Collaboration | European Territory | License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Percentage of royalty on net sales receives																	10.00%
Crysvita | Product Sales
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration and license revenue																	$ 4,286,000	644,000
Crysvita | License Agreement | Product Sales
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration and license revenue																			0
Saint Louis University License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payments paid																	100,000
Baylor Research Institute License Agreement | Development Milestones
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Future contingent milestone payments									5,300,000								5,300,000
Baylor Research Institute License Agreement | Sales Milestones
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Future contingent milestone payments									7,500,000								$ 7,500,000
Option and License Agreement | REGENXBIO, Inc.
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Option and license agreement description																	The Company also has an option and license agreement with REGENX under which the Company has an exclusive, sublicensable, worldwide license to make, have made, use, import, sell, and offer for sale licensed products with respect to three disease indications, subject to certain exclusions and has an option for another disease indication. In October 2018, the Company exercised its remaining option with REGENX for the additional disease indication and paid a $1.0 million fee for the exercise of the option.
Number of disease indications for exclusive license | DiseaseIndication																	3
Fee paid for exercise of option						$ 1,000,000
Annual maintenance fee for each option exercised																	$ 100,000
Option and License Agreement | REGENXBIO, Inc. | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Future contingent milestone payments									9,000,000								9,000,000
Research, Develop and Commercialize AAV Gene Therapy Products | Bayer HealthCare LLC
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payments received																	15,000,000
Research, Develop and Commercialize AAV Gene Therapy Products | Bayer HealthCare LLC | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Future contingent milestone payments									232,000,000								232,000,000
Research Collaboration and License Agreement Development Milestones | University of Pennsylvania School of Medicine (Penn) | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Future contingent milestone payments									5,000,000								5,000,000
Research Collaboration and License Agreement Commercial Milestones | University of Pennsylvania School of Medicine (Penn) | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Future contingent milestone payments									25,000,000								25,000,000
Arcturus Research Collaboration and License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Research and development					$ 6,000,000		$ 10,000,000										800,000	1,900,000	$ 4,300,000
Prepaid expenses and other current assets									0				500,000				0	500,000
Accrued liabilities									$ 0				$ 400,000				0	$ 400,000
Common stock purchased | shares					2,400,000						2,400,000
Common stock. per share | $ / shares					$ 10						$ 10
Consideration paid					$ 30,000,000
Additional option to purchase common stock | shares					600,000
Additional option to purchase common stock value per share | $ / shares					$ 16						$ 16
Number of shares restricted for sale | shares					2,400,000						2,400,000
Number of years restricted for selling common stock					2 years
Option exercising period					2 years
Percentage of outstanding common shares					18.20%						18.20%
Common stock investment value					$ 13,900,000						$ 13,900,000
Fair value of additional option to purchase common stock					500,000						$ 500,000
Remaining consideration paid					15,600,000
Business acquisition, consideration transferred					$ 30,000,000
Arcturus Research Collaboration and License Agreement | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of disease targets | DiseaseTarget					12
Program Agreement and Unitholder Option Agreement | GeneTx Biotherapeutics LLC
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Research and development																	$ 20,000,000
Consideration paid				$ 125,000,000
Business acquisition, consideration transferred				125,000,000
Upfront payment for exclusive option to acquisition				20,000,000
Business acquisition option exercise period, description																	This option may be exercised any time prior to 30 days following notice of FDA acceptance of the IND for GTX-102 for an additional $50.0 million in payments.
Additional payment for exclusive option				50,000,000
Option extension Premium				$ 25,000,000
Exercise of option extension, right to acquire condition																	In the event the Company exercises the option extension, the Company has a right to acquire GeneTx for a payment of $125.0 million, at any time, until the earlier of 30 months from the first dosing of a patient in a planned Phase 1/2 study (subject to extensions) or 90 days after results are available from that study.
Program Agreement and Unitholder Option Agreement | GeneTx Biotherapeutics LLC | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Extension period available for right to acquisition | Quarter																	4
Program Agreement and Unitholder Option Agreement | Scenario Forecast | GeneTx Biotherapeutics LLC
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Option extension Premium			$ 25,000,000

License and Research Agreemen_4

License and Research Agreements - Share of Collaboration and Royalty Revenue Related to Crysvita (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation Of Revenue [Line Items]
Collaboration and license revenue	$ 35,593	$ 25,800	$ 24,149	$ 18,172	$ 16,261	$ 11,763	$ 12,794	$ 10,677	$ 103,714	$ 51,495	$ 2,612
Collaboration and Royalty | Kyowa Kirin Collaboration
Disaggregation Of Revenue [Line Items]
Collaboration and license revenue									82,989	18,226	9
Collaboration and Royalty | Kyowa Kirin Collaboration | Profit Share Territory
Disaggregation Of Revenue [Line Items]
Collaboration and license revenue									74,869	15,334
Collaboration and Royalty | Kyowa Kirin Collaboration | Royalty Revenue in European Territory
Disaggregation Of Revenue [Line Items]
Collaboration and license revenue									$ 8,120	$ 2,892	$ 9

License and Research Agreemen_5

License and Research Agreements - Schedule of Cost Sharing Payments (Details) - Kyowa Kirin Collaboration - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cost Sharing Payments [Line Items]
Research and development	$ 27,309	$ 32,240	$ 31,165
Selling, general and administrative	21,828	14,228	4,466
Total	$ 49,137	$ 46,468	$ 35,631

License and Research Agreemen_6

License and Research Agreements - Schedule of Changes in Fair Value of Investment in Arcturus Securities (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Arcturus common stock, Beginning balance	$ 434,744
Arcturus common stock, Ending balance	768,920
Arcturus Research Collaboration and License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Arcturus common stock, Acquisition of investment in Arcturus securities	13,872
Arcturus common stock, Change in fair value	12,216
Arcturus common stock, Ending balance	26,088
Fair value of option to purchase additional shares of Arcturus common stock, Acquisition of investment in Arcturus securities	467
Fair value of option to purchase additional shares of Arcturus common stock, Change in fair value	1,197
Fair value of option to purchase additional shares of Arcturus common stock, Ending balance	$ 1,664

Leases - Additional Information

Leases - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Lessee Disclosure [Abstract]
Operating lease extension details	multiple renewal periods
Operating lease, existence of option to extend	true
Operating lease expiry term	2028
Rent expense		$ 6.4	$ 4.5
Cash paid for measurement of lease liabilities	$ 9
Weighted average remaining lease term	5 years 7 months 20 days
Weighted average discount rate	7.50%

Leases - Summary of Components

Leases - Summary of Components of Lease Expense (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019USD ($)
Lease Cost [Abstract]
Operating lease expense	$ 9,163
Variable lease expense	2,779
Total	$ 11,942

Leases - Summary of Future Mini

Leases - Summary of Future Minimum Lease Payments under Non-cancellable Leases (Details) $ in Thousands	Dec. 31, 2019USD ($)
Lessee Disclosure [Abstract]
2020	$ 9,786
2021	7,667
2022	7,521
2023	7,541
2024	5,571
Thereafter	7,582
Total future lease payments	45,668
Less: Amount representing interest	(8,676)
Present value of future lease payments	36,992
Less: Short-term lease liabilities	(7,235)
Long-term lease liabilities	$ 29,757

Gain from Sale of Priority Re_2

Gain from Sale of Priority Review Vouchers - Additional Information (Details) - USD ($) $ in Thousands	1 Months Ended		12 Months Ended
	Jun. 30, 2018	Jan. 31, 2018	Dec. 31, 2018
Finite Lived Intangible Assets [Line Items]
Sale of PRV			$ 170,322
Gain from sale of priority review vouchers			$ 170,322
Mepsevii
Finite Lived Intangible Assets [Line Items]
Sale of PRV		$ 130,000
Crysvita
Finite Lived Intangible Assets [Line Items]
Sale of PRV	$ 80,600

Liability Related to the Sale_3

Liability Related to the Sale of Future Royalties - Additional Information (Details)	1 Months Ended	12 Months Ended
	Dec. 31, 2019USD ($)	Dec. 31, 2019USD ($)
Liability Related To Sale Of Future Royalties [Line Items]
Proceeds from the sale of future royalties, net		$ 314,234,000
Royalty Purchase Agreement | RPI Finance Trust (RPI)
Liability Related To Sale Of Future Royalties [Line Items]
Proceeds from sale of future royalties	$ 320,000,000
Royalty payment termination description		The agreement with RPI will automatically terminate, and the payment of royalties to RPI will cease, in the event aggregate royalty payments received by RPI are equal to or greater than $608.0 million prior to December 31, 2030, or in the event aggregate royalty payments received by RPI are less than $608.0 million prior to December 31, 2030, when aggregate royalty payments received by RPI are equal to $800.0 million.
Royalties transaction costs net	5,800,000
Proceeds from the sale of future royalties, net	$ 314,200,000
Effective annual interest rate	10.10%	10.10%
Royalty Purchase Agreement | Minimum | RPI Finance Trust (RPI)
Liability Related To Sale Of Future Royalties [Line Items]
Royalty agreement termination threshold amount	$ 608,000,000	$ 608,000,000
Royalty Purchase Agreement | Maximum | RPI Finance Trust (RPI)
Liability Related To Sale Of Future Royalties [Line Items]
Royalty agreement termination threshold amount	$ 800,000,000	$ 800,000,000

Liability Related to the Sale_4

Liability Related to the Sale of Future Royalties - Schedule of Activity Within Liability Related to Sale of Future Royalties (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019USD ($)
Deferred Revenue Disclosure [Abstract]
Liability related to the sale of future royalties — beginning balance	$ 0
Proceeds from sale of future royalties	314,234
Non-cash interest expense	1,135
Liability related to the sale of future royalties at end of year	$ 315,369

Equity - Additional Information

Equity - Additional Information (Details) - USD ($)	1 Months Ended				12 Months Ended
	Feb. 28, 2019	Jan. 31, 2018	Jul. 31, 2017	Jul. 31, 2016	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Stockholders Equity [Line Items]
Net proceeds from sale of common stock					$ 330,415,000	$ 270,969,000
Common Stock Warrants Expiring on 2020
Stockholders Equity [Line Items]
Common stock warrants outstanding					149,700	149,700
Common stock warrants, exercise price					$ 3.01	$ 3.01
Common stock warrants, expiration period					2020	2020
Common Stock Warrants Expiring on 2021
Stockholders Equity [Line Items]
Common stock warrants outstanding					149,700	149,700
Common stock warrants, exercise price					$ 3.01	$ 3.01
Common stock warrants, expiration period					2021	2021
ATM Sales Agreement
Stockholders Equity [Line Items]
Option to sell common stock for cash				$ 150,000,000
Common stock shares sold							912,351
Net proceeds from sale of common stock							$ 67,600,000
Additional ATM Sales Agreement
Stockholders Equity [Line Items]
Common stock shares sold					468,685	640,257	1,338,866
Net proceeds from sale of common stock					$ 24,800,000	$ 38,100,000	$ 64,300,000
Additional ATM Sales Agreement | Maximum
Stockholders Equity [Line Items]
Option to sell common stock for cash			$ 150,000,000
Underwritten Public Offering
Stockholders Equity [Line Items]
Common stock shares sold	5,833,333	5,043,860
Net proceeds from sale of common stock	$ 330,400,000	$ 271,000,000
Shares purchased by underwriters	760,869	657,895
Public offering price	$ 60	$ 57

Stock-Based Awards - Additional

Stock-Based Awards - Additional Information (Details) - USD ($)	Nov. 07, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Feb. 05, 2014	Jan. 31, 2014
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total intrinsic value of options exercised		$ 6,500,000	$ 22,900,000	$ 20,400,000
Cash received from exercise of options		$ 7,700,000	$ 25,800,000	$ 7,800,000
Weighted-average estimated fair value of stock option		$ 37.15	$ 33.32	$ 44.20
Total estimated fair value of options vested		$ 45,300,000	$ 51,700,000	$ 49,100,000
Total unrecognized compensation cost		$ 133,000,000
Weighted-average period to recognize cost		2 years 4 months 24 days
Stock-based compensation capitalized to inventory		$ 1,400,000	$ 1,100,000	$ 0
Dimension
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options assumed to purchase shares of common stock from acquisition	639,897
Restricted Stock Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares vested during the period		313,682	235,913	156,021
Total fair value of shares vested		$ 20,800,000
Aggregate intrinsic value		$ 18,400,000
Performance Stock Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares vested during the period		65,643
Total fair value of shares vested		$ 3,200,000
Aggregate intrinsic value		$ 4,200,000
Maximum | Restricted Stock Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock based awards vesting period		4 years
Minimum | Restricted Stock Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock based awards vesting period		1 year
2014 Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share of common stock authorized for future issuance					2,250,000
Automatic increases in shares available for grant effective date		Jan. 1, 2015
Shares available for grant, ending date		Jan. 1, 2024
2014 Incentive Plan | Employee Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Minimum percentage of voting rights of all classes of stock		10.00%
Percentage of statutory stock awards		110.00%
2014 Incentive Plan | Maximum | Employee Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Award expiration period		10 years
2011 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share of common stock authorized for future issuance					655,038
2011 Equity Incentive Plan | Restricted Stock Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Option vesting rights, percentage		25.00%
Stock based awards vesting period		4 years
2011 Equity Incentive Plan | First Anniversary | Employee Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Option vesting rights, percentage		25.00%
2011 Equity Incentive Plan | Shares Vesting Monthly After First Anniversary | Employee Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Option vesting rights, percentage		2.08%
2011 Equity Incentive Plan and 2014 Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share of common stock authorized for future issuance		10,521,371
2014 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share of common stock authorized for future issuance		2,933,325
Automatic increases in shares available for grant effective date		Jan. 1, 2015
Shares available for grant, ending date		Jan. 1, 2024
Percentage of statutory stock awards		85.00%
Share of common stock available for future issuance						600,000
Common stock shares issued		85,845

Stock-Based Awards - Summary of

Stock-Based Awards - Summary of Activity Under Stock Option Plans Including 2011 Plan and 2014 Plan (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2023
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Options, Outstanding, Beginning Balance	5,353,124	5,399,410	4,429,472
Number of Options, Options granted	1,762,075	1,479,451	1,328,860
Number of Options, Options assumed			639,897
Number of Options, Options exercised	(235,678)	(713,263)	(478,470)
Number of Options, Options cancelled	(766,457)	(812,474)	(520,349)
Number of Options, Outstanding, Ending Balance	6,113,064	5,353,124	5,399,410	4,429,472
Number of Options, Vested and exercisable — December 31, 2019	3,429,579
Number of Options, Vested and expected to vest — December 31, 2019	5,847,778
Weighted Average Exercise Price, Outstanding, Beginning Balance	$ 62.46	$ 62.75	$ 61.85
Weighted Average Exercise Price, Options granted	63.03	55.54	71.99
Weighted Average Exercise Price, Options assumed			27.97
Weighted Average Exercise Price, Options exercised	32.87	36.21	16.25
Weighted Average Exercise Price, Options cancelled	72.48	74.80	78.70
Weighted Average Exercise Price, Outstanding, Ending Balance	62.51	$ 62.46	$ 62.75	$ 61.85
Weighted Average Exercise Price, Vested and exercisable — December 31, 2019	63.14
Weighted Average Exercise Price, Vested and expected to vest — December 31, 2019	$ 62.58
Weighted Average Remaining Contractual Term (Years), Outstanding	7 years	7 years 2 months 19 days	7 years 4 months 24 days	8 years 2 months 19 days
Weighted Average Remaining Contractual Term (Years), Vested and exercisable — December 31, 2019	5 years 9 months 29 days
Weighted Average Remaining Contractual Term (Years), Vested and expected to vest — December 31, 2019	6 years 11 months 1 day
Aggregate Intrinsic Value, Outstanding		$ 23,243	$ 37,687	$ 79,135	$ 18,989
Aggregate Intrinsic Value, Vested and exercisable — December 31, 2019					18,514
Aggregate Intrinsic Value, Vested and expected to vest — December 31, 2019					$ 18,945

Stock-Based Awards - Summary _2

Stock-Based Awards - Summary of Activity Under Restricted Stock Units (RSUs) from 2014 Plan (Details) - Restricted Stock Units - $ / shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Outstanding, Number of Shares, Unvested, Beginning Balance	954,077	821,560	573,744
Number of Shares, granted	863,065	555,905	516,161
Number of Shares, vested	(313,682)	(235,913)	(156,021)
Number of Shares, cancelled	(205,310)	(187,475)	(112,324)
Outstanding, Number of Shares, Unvested, Ending Balance	1,298,150	954,077	821,560
Outstanding, Weighted-Average Grant Date Fair Value, Unvested, Beginning Balance	$ 63.12	$ 70.71	$ 71.45
Weighted-Average Grant Date Fair Value, granted	62.78	56.01	71.58
Weighted-Average Grant Date Fair Value, vested	66.25	71.01	71.93
Weighted-Average Grant Date Fair Value, cancelled	64.28	65.37	76.78
Outstanding, Weighted-Average Grant Date Fair Value, Unvested, Ending Balance	$ 61.96	$ 63.12	$ 70.71

Stock-Based Awards - Summary _3

Stock-Based Awards - Summary of Activity Under PSUs from 2014 Plan (Details) - Performance Stock Units - $ / shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Outstanding, Number of Shares, Unvested, Beginning Balance	483,200	508,850
Number of Shares, granted	61,500	71,725	508,850
Number of Shares, vested	(65,643)
Number of Shares, cancelled	(79,517)	(97,375)
Outstanding, Number of Shares, Unvested, Ending Balance	399,540	483,200	508,850
Outstanding, Weighted-Average Grant Date Fair Value, Unvested, Beginning Balance	$ 49.65	$ 48.03
Weighted-Average Grant Date Fair Value, granted	67.31	59.67	$ 48.03
Weighted-Average Grant Date Fair Value, vested	48.03
Weighted-Average Grant Date Fair Value, cancelled	50.01	48.58
Outstanding, Weighted-Average Grant Date Fair Value, Unvested, Ending Balance	$ 52.56	$ 49.65	$ 48.03

Stock-Based Awards - Summary _4

Stock-Based Awards - Summary of Stock-Based Compensation Recognized (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 81,995	$ 80,107	$ 68,014
Cost of sales
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	1,084	146
Research and development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	44,205	45,572	38,212
Selling, general and administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 36,706	$ 34,389	$ 29,802

Stock-Based Awards - Fair Value

Stock-Based Awards - Fair Value of Stock Option Awards Granted Estimated Using Black-Scholes Option-Pricing Model (Details) - Stock Options	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (years)	6 years 2 months 19 days	6 years 2 months 23 days	6 years 2 months 23 days
Expected volatility	61.00%	62.00%	65.00%
Risk-free interest rate	2.40%	2.70%	2.10%
Expected dividend rate	0.00%	0.00%	0.00%

Defined Contribution Plan - Add

Defined Contribution Plan - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Contribution Pension And Other Postretirement Plans Disclosure [Abstract]
Contribution expenses	$ 3.6	$ 2.9	$ 2.1

Income Taxes - Components of Co

Income Taxes - Components of Company's Loss Before Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Domestic	$ 399,709	$ 205,440	$ 250,917
Foreign	(265)	(8,343)	67,421
Total loss before income taxes	$ 399,444	$ 197,097	$ 318,338

Income Taxes - Components of _2

Income Taxes - Components of Company's Income Tax Provision (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Current provision for income taxes:
State	$ 51	$ 14	$ 5
International	1,092	500	42
Total current tax provision	1,143	514	47
Deferred tax provision (benefit):
Federal			(16,243)
State	2,140		(3)
Total deferred tax provision (benefit)	2,140		(16,246)
Total provision for (benefit from) income taxes	$ 3,283	$ 514	$ (16,199)

Income Taxes - Effective Tax Ra

Income Taxes - Effective Tax Rate of Provision for Income Taxes from Federal Statutory Rate (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Effective Income Tax Rate Continuing Operations Tax Rate Reconciliation [Abstract]
Federal statutory income tax rate	21.00%	21.00%	34.00%
State income taxes, net of federal benefit	(0.50%)
Federal tax credits	5.20%	9.50%	9.00%
Other	(0.20%)	(0.20%)	(0.90%)
Nondeductible permanent items	(0.40%)	(0.80%)
Stock-based compensation	(1.00%)	(0.80%)	(0.60%)
Uncertain tax positions	(1.00%)	(1.90%)	(1.80%)
Change in valuation allowance	(23.60%)	(26.80%)	(32.50%)
Foreign rate differential	(0.30%)	(0.30%)	(7.20%)
Change in federal tax rate			5.10%
Provision for income taxes	(0.80%)	(0.30%)	5.10%

Income Taxes - Schedule of Tax

Income Taxes - Schedule of Tax Effect of Temporary Differences to Significant Portions of Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2012	Dec. 31, 2011
Deferred tax assets:
Loss carryforwards	$ 185,892	$ 183,331	$ 200	$ 200
Tax credits	158,791	137,019
Stock options	40,676	29,925
Accruals and reserves	19,622	7,418
Fixed assets and intangibles	3,870	2,820
Liability related to sale of future royalties	81,424
Other	4,183	1,189
Gross deferred tax assets	494,458	361,702
Valuation allowance	(486,796)	(361,702)
Total deferred tax assets	7,662
Deferred tax liabilities:
Gross deferred tax liabilities	(40,968)	(31,166)
Net deferred tax asset (liabilities)	(33,306)	(31,166)
Right-of-use Lease Assets
Deferred tax liabilities:
Gross deferred tax liabilities	(7,662)
In-process Research and Development
Deferred tax liabilities:
Gross deferred tax liabilities	$ (33,306)	$ (31,166)

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Nov. 07, 2017	Dec. 31, 2012	Dec. 31, 2011
Tax Credit Carryforward [Line Items]
Deferred tax assets net operating loss carryforwards	$ 185,892,000	$ 183,331,000			$ 200,000	$ 200,000
Federal statutory income tax rate	21.00%	21.00%	34.00%
Deferred tax liability increase (decrease), net	$ 2,100,000		$ (16,200,000)
Benefit from (provision for) income taxes	(3,283,000)	$ (514,000)	16,199,000
Deferred income tax expense	2,140,000		$ (16,246,000)
Valuation allowance increased	125,100,000	57,500,000
Unrecognized tax benefits, income tax penalties and interest accrued	0	0
Undistributed earnings of foreign subsidiaries	$ 2,100,000
Dimension
Tax Credit Carryforward [Line Items]
Deferred tax liability				$ 47,412,000
Federal statutory income tax rate	21.00%		34.00%
Deferred tax liability increase (decrease), net	$ 33,300,000		$ (31,200,000)
Federal
Tax Credit Carryforward [Line Items]
Deferred tax assets operating loss carryforwards subject to expiration	$ 549,900,000	558,300,000
Operating loss carryforwards expiration year	2032
Tax credit carryforwards	$ 11,700,000	7,500,000
Tax credit carryforwards expiration year	2030
Deferred tax assets net operating loss carryforwards					3,600,000	3,600,000
Federal | Orphan Drug Credits [Member]
Tax Credit Carryforward [Line Items]
Deferred tax assets operating loss carryforwards subject to expiration	$ 160,000,000	143,500,000
Operating loss carryforwards expiration year	2031
State
Tax Credit Carryforward [Line Items]
Deferred tax assets operating loss carryforwards subject to expiration	$ 520,200,000	476,800,000
Operating loss carryforwards expiration year	2031
Tax credit carryforwards	$ 25,500,000	$ 17,300,000
Deferred tax assets net operating loss carryforwards					$ 3,600,000	$ 3,600,000

Income Taxes - Unrecognized Tax

Income Taxes - Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Balance at beginning of year	$ 33,727	$ 28,377	$ 13,505
Additions based on tax positions related to current year	5,575	4,750	9,338
Additions for tax positions of prior years	652	600	5,534
Balance at end of year	$ 39,954	$ 33,727	$ 28,377

Commitments and Contingencies -

Commitments and Contingencies - Summary of Aggregate Payments under Contractually Binding Manufacturing and Service Agreements (Details) $ in Thousands	Dec. 31, 2019USD ($)
Commitments And Contingencies Disclosure [Abstract]
Manufacturing and Services, 2020	$ 7,854
Manufacturing and Services, 2021	461
Manufacturing and Services, Total	$ 8,315

Net Loss per Share - Computatio

Net Loss per Share - Computation of Basic and Diluted Net Loss per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Numerator:
Net loss	$ (93,805)	$ (112,994)	$ (99,172)	$ (96,756)	$ (87,826)	$ (87,310)	$ (52,728)	$ 30,253	$ (402,727)	$ (197,611)	$ (302,139)
Denominator:
Weighted-average shares used to compute net loss per share, basic and diluted									56,576,885	49,775,223	42,453,135
Net loss per share, basic and diluted	$ (1.62)	$ (1.96)	$ (1.72)	$ (1.82)					$ (7.12)	$ (3.97)	$ (7.12)

Net Loss per Share - Outstandin

Net Loss per Share - Outstanding Shares of Common Stock Equivalents Excluded from Computation of Diluted Net Loss per Share (Details) - shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from computation of diluted net loss per share	8,132,319	7,454,476	6,015,212
Options to Purchase Common Stock, RSUs, and PSUs
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from computation of diluted net loss per share	7,978,666	7,301,431	5,862,784
Common Stock Warrants
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from computation of diluted net loss per share	149,700	149,700	149,700
Employee Stock Purchase Plan
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from computation of diluted net loss per share	3,953	3,345	2,728

Accumulated Other Comprehensi_3

Accumulated Other Comprehensive Loss - Schedule of Total Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Accumulated Other Comprehensive Income Loss Net Of Tax [Abstract]
Cumulative foreign currency translation adjustment	$ (306)	$ (329)
Unrealized gain (loss) on securities available-for-sale	159	(304)
Total accumulated other comprehensive loss	$ (147)	$ (633)

Quarterly Financial Data (una_3

Quarterly Financial Data (unaudited) - Summary of Unaudited Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Selected Quarterly Financial Information [Abstract]
Revenue	$ 35,593	$ 25,800	$ 24,149	$ 18,172	$ 16,261	$ 11,763	$ 12,794	$ 10,677	$ 103,714	$ 51,495	$ 2,612
Operating expenses	130,045	143,833	136,623	117,386	106,601	101,409	107,694	107,164	527,887	422,868	331,554
Net income (loss)	$ (93,805)	$ (112,994)	$ (99,172)	$ (96,756)	$ (87,826)	$ (87,310)	$ (52,728)	$ 30,253	$ (402,727)	$ (197,611)	$ (302,139)
Net loss per share, basic and diluted	$ (1.62)	$ (1.96)	$ (1.72)	$ (1.82)					$ (7.12)	$ (3.97)	$ (7.12)
Net income (loss) per share, basic					$ (1.73)	$ (1.74)	$ (1.06)	$ 0.63
Net income (loss) per share, diluted					$ (1.73)	$ (1.74)	$ (1.06)	$ 0.62