Ultragenyx Pharmaceutical (RARE) 10-K 21 Feb 18 2017 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2017	Feb. 15, 2018	Jun. 30, 2017
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	FY
Trading Symbol	RARE
Entity Registrant Name	Ultragenyx Pharmaceutical Inc.
Entity Central Index Key	1,515,673
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		49,605,852
Entity Public Float			$ 2.6

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 100,488	$ 161,120
Short-term investments	134,005	219,028
Accounts receivable	5,172
Inventory	757
Restricted cash	461	1,411
Prepaid expenses and other current assets	28,700	20,136
Total current assets	269,583	401,695
Property and equipment, net	21,837	17,055
Restricted cash	2,092	2,076
Long-term investments	9,975	117,963
Intangible assets, net	141,545
Goodwill	44,406
Other assets	1,315	1,837
Total assets	490,753	540,626
Current liabilities:
Accounts payable	8,886	5,364
Accrued liabilities	61,427	54,554
Deferred rent—current portion	701	341
Total current liabilities	71,014	60,259
Deferred tax liabilities	31,166
Other liabilities	5,119	6,393
Total liabilities	107,299	66,652
Commitments and contingencies (Note 12)
Stockholders’ equity:
Preferred stock, par value of $0.001 per share—25,000,000 shares authorized; nil outstanding as of December 31, 2017 and 2016
Common stock, par value of $0.001 per share—250,000,000 shares authorized; 44,167,071 and 41,240,230 shares issued and outstanding as of December 31, 2017 and 2016, respectively	44	41
Additional paid-in capital	1,221,762	1,003,561
Accumulated other comprehensive income (loss)	(5,680)	905
Accumulated deficit	(832,672)	(530,533)
Total stockholders’ equity	383,454	473,974
Total liabilities and stockholders’ equity	$ 490,753	$ 540,626

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
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	44,167,071	41,240,230
Common stock, shares outstanding	44,167,071	41,240,230

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues:
Collaboration and license	$ 2,136
Product sales	476	$ 133
Total revenues	2,612	133
Operating expenses:
Cost of sales	1
Research and development	231,644	183,204	$ 114,737
Selling, general and administrative	99,909	64,936	33,001
Total operating expenses	331,554	248,140	147,738
Loss from operations	(328,942)	(248,007)	(147,738)
Interest income	4,074	3,789	2,320
Other income (expense)	6,530	(1,621)	(200)
Loss before income taxes	(318,338)	(245,839)	(145,618)
Benefit from (provision for) income taxes	16,199	(35)
Net loss	$ (302,139)	$ (245,874)	$ (145,618)
Net loss per share, basic and diluted	$ (7.12)	$ (6.21)	$ (3.96)
Shares used in computing net loss per share, basic and diluted	42,453,135	39,586,908	36,782,603

CONSOLIDATED STATEMENTS OF COMP

CONSOLIDATED STATEMENTS OF COMPREHENSIVE LOSS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (302,139)	$ (245,874)	$ (145,618)
Other comprehensive income (loss):
Foreign currency translation adjustments	(10,110)	1,322
Transfer of currency translation adjustments balance to other income for the liquidation of a foreign entity	3,490
Unrealized gain (loss) on available-for-sale securities	35	451	(694)
Other comprehensive income (loss):	(6,585)	1,773	(694)
Total comprehensive loss	$ (308,724)	$ (244,101)	$ (146,312)

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, 2014	$ 184,945	$ 32	$ 324,128	$ (174)	$ (139,041)
Beginning balance, shares at Dec. 31, 2014		31,934,682
Issuance of common stock in connection with underwritten public offering, net of issuance costs	461,136	$ 6	461,130
Issuance of common stock in connection with underwritten public offering, net of issuance costs, shares		5,980,000
Employee stock-based compensation	24,884		24,884
Issuance of common stock under equity plan awards, net of tax	6,437	$ 1	6,436
Issuance of common stock under equity plan awards, net of tax, shares		967,712
Other comprehensive income (loss)	(694)			(694)
Net loss	(145,618)				(145,618)
Ending balance at Dec. 31, 2015	531,090	$ 39	816,578	(868)	(284,659)
Ending balance, shares at Dec. 31, 2015		38,882,394
Issuance of common stock in connection with at-the-market offering, net of issuance costs	79,486	$ 1	79,485
Issuance of common stock in connection with at-the-market offering, net of issuance costs, shares		1,159,415
Issuance of common stock in connection with collaboration agreement, net of issuance costs	52,272	$ 1	52,271
Issuance of common stock in connection with collaboration agreement, net of issuance costs, shares		727,120
Put option grant in connection with collaboration agreement	(916)		(916)
Employee stock-based compensation	48,309		48,309
Issuance of common stock under equity plan awards, net of tax	7,834		7,834
Issuance of common stock under equity plan awards, net of tax, shares		471,301
Other comprehensive income (loss)	1,773			1,773
Net loss	(245,874)				(245,874)
Ending balance at Dec. 31, 2016	$ 473,974	$ 41	1,003,561	905	(530,533)
Ending balance, shares at Dec. 31, 2016	41,240,230	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	44,167,071

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating activities:
Net loss	$ (302,139)	$ (245,874)	$ (145,618)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	68,014	48,309	24,884
Amortization of premium on investment securities, net	1,706	4,842	5,637
Depreciation and amortization	5,825	3,424	1,384
Foreign currency remeasurement (gain) loss	(7,018)	1,322
Non-cash license fee from collaboration arrangement		700
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(3,338)	(7,147)	(7,131)
Other assets	522	(1,242)	179
Accounts payable	3,459	2,502	(1,982)
Accrued liabilities and other liabilities	(4,628)	32,189	16,670
Deferred tax liabilities	(16,246)
Net cash used in operating activities	(253,843)	(160,975)	(105,977)
Investing activities:
Acquisition, net of cash acquired	(142,804)
Purchase of property and equipment	(2,793)	(10,188)	(4,955)
Purchase of investments	(230,487)	(442,490)	(624,226)
Proceeds from sale of investments	157,934	140,556	89,321
Proceeds from maturities of investments	273,632	403,239	249,050
Decrease (increase) in restricted cash	934	(1,202)	(1,541)
Net cash provided by (used in) investing activities	56,416	89,915	(292,351)
Financing activities:
Proceeds from the issuance of common stock in connection with underwritten public offerings, net			461,136
Proceeds from the issuance of common stock in connection with at-the-market offering, net	131,960	79,486
Proceeds from the issuance of common stock in connection with collaboration agreement, net		51,356
Proceeds from the issuance of common stock under equity plan awards, net of tax	9,251	7,834	6,437
Repayment of note payable	(4,944)
Net cash provided by financing activities	136,267	138,676	467,573
Effect of exchange rate changes on cash	528	(65)
Net increase (decrease) in cash and cash equivalents	(60,632)	67,551	69,245
Cash and cash equivalents at beginning of year	161,120	93,569	24,324
Cash and cash equivalents at end of year	100,488	161,120	93,569
Supplemental disclosures of non-cash investing and financing information:
Fair value of vested stock options assumed in acquisition	8,979
Costs of property and equipment included in accounts payable and accrued liabilities	$ 400	147	$ 769
Tenant improvement allowance		$ 3,467

Organization and Basis of Prese

Organization and Basis of Presentation	12 Months Ended
	Dec. 31, 2017
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 and was 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 rare and ultra-rare diseases, with a focus on serious, debilitating genetic diseases. On November 15, 2017, the U.S. Food and Drug Administration (FDA), approved the Company’s first product, Mepsevii (vestronidase alfa), the first medicine approved for the treatment of children and adults with MPS VII, also known as Sly syndrome. Mepsevii is an intravenous enzyme replacement therapy for the treatment of MPS VII, a rare lysosomal storage disease that often leads to multi-organ dysfunction, pervasive skeletal disease, and death. The Company is conducting Phase 2 and Phase 3 studies of burosumab (KRN23 or UX023), an antibody targeting fibroblast growth factor 23 (FGF23), in pediatric and adult patients with X-linked hypophosphatemia (XLH) and a Phase 2 study in tumor induced osteomalacia (TIO), both rare diseases that impair bone mineralization; a Phase 3 study for UX007 in patients with glucose transporter type-1 deficiency syndrome (Glut1 DS), a brain energy deficiency, who are experiencing movement disorders; a Phase 2 clinical study of UX007 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 and a Phase 1/2 open label clinical study of DTX 301, a n adeno-associated virus (AAV8) gene therapy product candidate designed for the treatment of patients with deficiency, the most common urea cycle disorder. In the course of its research activities, 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 activities. Management expects to incur additional losses in the future to conduct product research and development and recognizes the need to raise additional capital to fully implement its business plan. Through December 31, 2017, the Company has relied primarily on the proceeds from equity offerings 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, 2017
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 and Cash Equivalents 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. 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. 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, 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 bond issuers and other financial instruments to the extent recorded in the balance sheets . Concentrations on credit risk with respect to accounts receivable are limited due to the Company’s limited number of customers. 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. Inventories consist of currently approved product. 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. Restricted Cash Restricted cash primarily consists of money market accounts as collateral for its obligations under its facility leases of the Company’s corporate headquarters in Novato, California and for its facilities in Brisbane, California and Woburn and Cambridge, Massachusetts. 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 the 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. The Company also reviews the useful lives of its assets periodically to determine whether events and circumstances warrant a revision to the remaining useful life. Changes in the useful life are adjusted prospectively by revising the remaining period over which the asset is amortized. The Company has not recorded any impairments nor made any adjustments to the useful lives of its assets since inception. 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. The Company has not recorded any impairments since inception. 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 since inception. 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 In May 2014, the Financial Revenue from Contracts with Customers (ASC 606) Product sales The Company sells Mepsevii The Company also recognizes revenue from sales of Mepsevii on a “named patient” basis, which are allowed in certain countries prior to the commercial approval of the product in the territory. Collaboration and license revenue The Company has certain license and collaboration agreements that are within the scope of ASC 808, Collaborative Agreements The Company 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 under these license or collaborations agreements, royalty revenue is recorded as the underlying sales occur. The Company also recognizes collaboration and license revenue under certain of the Company’s license or collaboration agreements that are within the scope of ASC 606. The terms of these agreements may contain multiple performance obligations, which may include licenses and research and development activities. The Company evaluates these agreements under ASC 606 to determine the distinct performance obligations. 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. Variable 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. Revenue is recognized by measuring the progress toward complete satisfaction of the performance obligations using an input measure. Leases The Company enters into lease agreements for its office and laboratory facilities. These leases are classified as operating leases. Rent expense is recognized on a straight-line basis over the term of the lease and, accordingly, the Company records the difference between cash rent payments and the recognition of rent expense as a deferred rent liability. Incentives granted under the Company’s facilities leases, including allowances to fund leasehold improvements, are deferred and are recognized as adjustments to rental expense on a straight-line basis over the term of the lease. 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 and capitalized. The capitalized 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. Noncash stock compensation expense on awards expected to vest are recognized net of estimated forfeitures. In March 2016, the FASB issued ASU 2016-09, Compensation — Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting 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 Topic 805, “Business Combinations” (Topic 805) allocates Recent Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) In October 2016, the FASB issued ASU 2016-16, Income Taxes - Intra-Entity Transfers of Assets Other Than Inventory In November 2016, the FASB issued ASU 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash In January 2017, the FASB issued ASU 2017-04, Intangibles – Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment

Acquisition

Acquisition	12 Months Ended
	Dec. 31, 2017
Business Combinations [Abstract]
Acquisition	3. 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. The goodwill balance is primarily attributed to the deferred tax liabilities arising on 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 any 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. 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. The following supplemental unaudited pro forma information presents the financial results as if the acquisition had occurred on January 1, 2016 (in thousands): For the year ended December 31, 2017 2016 Total revenues $ 18,528 $ 12,684 Net loss 341,737 306,596 The unaudited pro forma financial information include pro forma adjustments that assume the acquisition occurred on January 1, 2016. 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 and $11.1 million for the years ended December 31, 2017 and 2016, respectively. 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, 2017
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 and Level 2 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 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, 2017 Level 1 Level 2 Level 3 Total Financial Assets: Money market funds $ 79,670 $ — $ — $ 79,670 Corporate bonds — 39,240 — 39,240 U.S. Government Treasury and agency securities — 104,739 — 104,739 Total financial assets $ 79,670 $ 143,979 $ — $ 223,649 December 31, 2016 Level 1 Level 2 Level 3 Total Financial Assets: Money market funds $ 123,536 $ — $ — $ 123,536 Corporate bonds — 207,726 — 207,726 Commercial paper — 11,970 — 11,970 Asset-backed securities — 27,713 — 27,713 U.S. Government Treasury and agency securities — 111,931 — 111,931 Total financial assets $ 123,536 $ 359,340 $ — $ 482,876

Balance Sheet Components

Balance Sheet Components	12 Months Ended
	Dec. 31, 2017
Investments Debt And Equity Securities [Abstract]
Balance Sheet Components	5. Balance Sheet Components Cash Equivalents and Investments The fair values of cash equivalents, short-term investments, and long-term investments classified as available-for-sale securities, consisted of the following (in thousands): December 31, 2017 Gross Unrealized Amortized Cost Gains Losses Estimated Fair Value Money market funds $ 79,670 $ — $ — $ 79,670 Corporate bonds 39,330 — (90 ) 39,240 U.S. Government Treasury and agency securities 105,029 — (290 ) 104,739 Total $ 224,029 $ — $ (380 ) $ 223,649 December 31, 2016 Gross Unrealized Amortized Cost Gains Losses Estimated Fair Value Money market funds $ 123,536 $ — $ — $ 123,536 Corporate bonds 207,909 14 (197 ) 207,726 Commercial paper 11,970 — — 11,970 Asset-backed securities 27,712 3 (2 ) 27,713 U.S. Government Treasury and agency securities 112,166 10 (245 ) 111,931 Total $ 483,293 $ 27 $ (444 ) $ 482,876 At December 31, 2017, 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, 2017 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, 2017 2016 Work-in-process $ 737 $ — Finished goods 20 — Total inventory $ 757 $ — Property and Equipment, net Property and equipment, net consists of the following (in thousands): December 31, 2017 2016 Research and development equipment $ 7,696 $ 2,154 Furniture and office equipment 2,873 2,595 Computer equipment 2,563 2,013 Software 4,182 3,931 Leasehold improvements 15,085 12,486 Construction-in-progress 429 37 Property and equipment, gross 32,828 23,216 Less accumulated depreciation (10,991 ) (6,161 ) Property and equipment, net $ 21,837 $ 17,055 Depreciation expense for the years ended December 31, 2017, 2016 and 2015 was $4.8 million, $3.4 million and $1.4 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, 2017 2016 Research and clinical study expenses $ 17,141 $ 18,593 Payroll and related expenses 26,527 17,226 Repayment liability under collaboration agreement 3,681 13,650 Contract liability 5,986 — Other 8,092 5,085 Total accrued liabilities $ 61,427 $ 54,554

Intangible Assets, Net

Intangible Assets, Net	12 Months Ended
	Dec. 31, 2017
Intangible Assets Net Excluding Goodwill [Abstract]
Intangible Assets, Net	6. Intangible Assets, net In connection with the acquisition as described in Note 3 “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-live 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 would be amortized to research and development expense over the research term which is expected to be complete in 2019. The Company recorded research and development expense of $1.0 million for the period from November 7, 2017 through December 31, 2017 related to the amortization of the asset. As of December 31, 2017, the estimated future amortization expense associated with the contract asset is $10.5 million in 2018 and $2.0 million in 2019.

License and Research Agreements

License and Research Agreements	12 Months Ended
	Dec. 31, 2017
Research Grant Agreement [Abstract]
License and Research Agreements	7. License and Research Agreements 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, if such product sales are ever achieved. St. Jude Children’s Research Hospital License Agreement In September 2012, the Company entered into a license agreement with St. Jude Children’s Research Hospital (St. Jude). Under the terms of this license agreement, St. Jude granted the Company an exclusive license under certain know-how to research, develop, make, use, offer to sell, import, and otherwise commercialize and exploit St. Jude’s protective protein, cathepsin, a protein product to treat, prevent, and/or diagnose galactosialidosis and other monogenetic diseases. The Company will pay to St. Jude a royalty of less than 1% on net sales of the licensed products in the licensed territories, if such product sales are ever achieved. 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. The Company may make future payments of up to $10.5 million contingent upon attainment of various development milestones 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. Kyowa Hakko Kirin Collaboration and License Agreement In August 2013, the Company entered into a collaboration and license agreement with Kyowa Hakko Kirin Co., Ltd. (KHK). Under the terms of this collaboration and license agreement, as amended, the Company and KHK will collaborate on the development and commercialization of certain products containing burosumab in the field of orphan diseases in the United States and Canada, or the profit share territory, and in the European Union and Switzerland, or the European territory, and the Company will have the right to develop and commercialize such products in the field of orphan diseases in Mexico and Central and South America, or Latin America. In the field of orphan diseases, and except for ongoing studies being conducted by KHK, the Company will be the lead party for development activities in the profit share territory and in the European territory until the applicable transition date; the Company will also be 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 KHK. The Company will share 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 KHK, and KHK shall be responsible for 100% of the costs for development activities in Japan and Korea. On the applicable transition date in the profit share territory and the European territory, KHK 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 KHK. The Company has the primary responsibility for conducting certain research and development services. If burosumab is approved, the Company and KHK will share commercial responsibilities and profits in the profit share territory until the applicable transition date and KHK will commercialize burosumab in the European territory. The Company will develop and commercialize burosumab in Latin America. KHK will manufacture and supply burosumab for clinical use globally and will manufacture and supply burosumab for commercial use in the profit share territory and Latin America. The remaining profit or loss from commercializing products in the profit-share territory, until the applicable transition date, will be shared between the Company and KHK on a 50/50 basis. Thereafter, the Company will be entitled to receive a tiered double-digit revenue share in the mid-to-high 20% range in the profit share territory. Net proceeds from the anticipated sale of the priority review voucher, if received, will be shared equally with KHK. In May 2017, the Company signed an agreement with a wholly-owned subsidiary of KHK pursuant to which the Company was granted the right to commercialize burosumab in Turkey. KHK’s subsidiary has the option to assume responsibility for commercialization efforts from the Company, after a certain minimum period . The Company is accounting for the agreement as a collaboration arrangement as defined in ASC 808, Collaborative Agreements. As of December 31, 2017 and 2016, the Company had receivables in the amount of $10.3 million and $8.6 million, respectively, for this collaboration arrangement. 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. Takeda License and Collaboration and Purchase Agreements In June 2016, the Company executed a collaboration and license agreement with Takeda Pharmaceutical Company Limited (Takeda). Pursuant to the agreement, which became effective in July 2016, the Company obtained an exclusive license for a pre-clinical compound from Takeda in a pre-determined field of use. The Company is responsible for the development costs for the pre-clinical compound and the identified option product pursuant to an initial development plan. Because the license to the pre-clinical compound has no alternative future use, the estimated fair value of $0.7 million was recorded as a research and development expense upon acquisition. Any products resulting from the pre-clinical compound or the identified option product is referred to in this report as the “licensed product.” The Company discontinued the development efforts on the pre-clinical compound in the pre-determined field of use and the identified option product. As part of the agreement, the Company and Takeda established a five-year research collaboration whereby the parties may mutually agree to add additional option products candidates to the collaboration, in which case the Company will bear the cost of the development activities, with certain exceptions. In July 2016, the Company consummated a common stock purchase agreement, executed in conjunction with the collaboration and license agreement, whereby Takeda purchased 374,590 shares of the Company’s common stock for $40.0 million in cash. The fair market value of the common stock issued to Takeda was $27.3 million, based on the closing stock price of $72.95 on the date of issuance, resulting in a $12.7 million premium paid to the Company. The Company also received a put option to require Takeda to purchase an additional $25.0 million in shares of the Company’s common stock which was exercised in October 2016, whereby Takeda purchased 352,530 shares of the Company’s common stock for $25.0 million in cash. Takeda is subject to a five-year standstill (subject to customary exceptions or release). The Company estimated the fair value of the put options to be $0.9 million and recorded the put options in additional paid-in capital. The Company also granted Takeda an exclusive option for Asian rights, for a limited period, to any licensed products and any additional products resulting from the collaboration, as well as an option to exclusively license one of the Company’s products for development and commercialization in Japan. If Takeda exercises any of its option rights to license a product pursuant to the agreement, Takeda will pay for the development costs within the licensed territory, will share in a portion of the global development costs, and will make a milestone payment upon regulatory approval. Takeda will also owe royalties on net sales in the licensed territory for any licensed product, depending on the development stage when the product is licensed as well as sales levels. The royalties related to the option to license the Company’s product, as well as the additional product are subject to future good faith negotiations at the time that the option is exercised. The research and license agreement and the stock purchase agreement are being accounted for as one arrangement because they were entered into at the same time with interrelated financial terms. The Company analogized to Topic 606 The Company is responsible for the costs under the initial development plan. A significant portion of this work is performed by Takeda which has an estimated cost of approximately $10.0 million to $11.2 million and is subject to changes as development activities are adjusted and cost estimates are refined. The Company concluded that the payment to Takeda is not in return for a distinct service that Takeda transfers to the Company, therefore, the payment made to Takeda is accounted for as a reduction in the transaction price. As of December 31, 2017, the Company concluded that $3.1 million of the estimated transaction price should not be constrained because it is probable that a significant reversal in the amount to be recognized will not occur. The unconstrained transaction price was allocated to the distinct performance obligations on a relative standalone selling price basis. The Company recorded $2.5 million for the year ended December 31, 2017, respectively, as a reduction of research and development expenses by measuring the progress toward complete satisfaction of the individual performance obligation using an input measure. The Company concluded that the remaining transaction price should continue to be constrained as of December 31, 2017. The Company will continue to re-evaluate the application of the constraint to the transaction price at each reporting period end date. Costs incurred by the Company associated with co-development activities performed under this collaboration are included in research and development expense in the accompanying consolidated statements of operations. As of December 31, 2017 and 2016, the Company had a repayment liability in the amount of $3.7 million and $14.3 million, respectively, and a contract liability in the amount of $0.6 million as of December 31, 2017 and none as of December 31, 2016. 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 will 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 will also make milestone payments per approved product if certain commercial milestones are 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, hemophilia B, 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 1 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 Company is responsible for the development of DTX201 under the agreement through a proof-of-concept (POC) clinical trial, in accordance with the mutually agreed upon research budget. Upon the successful demonstration of clinical POC, the agreement requires that Bayer use commercially reasonable efforts to manage and fund 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. In December 2017, the first milestone was achieved and the Company invoiced Bayer $5.0 million for the milestone. As of the acquisition date of November 7, 2017, the Company valued the contract under ASC 805 Business Combinations The Company also evaluated the agreement under ASC 606 Revenue from Contracts with Customers,

Equity

Equity	12 Months Ended
	Dec. 31, 2017
Equity [Abstract]
Equity	8. 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 years ended December 31, 2017 and 2016, the Company sold 912,351 and 1,159,415 shares of common stock, resulting in net proceeds of approximately $67.6 million and $79.5 million, respectively, 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 year ended December 31, 2017, the Company sold an additional 1,338,866 shares of common stock, resulting in net proceeds of approximately $64.3 million, after commissions and other offering costs. Common Stock Warrants As of December 31, 2017 and 2016, there was an aggregate of 149,700 .

Stock-Based Awards

Stock-Based Awards	12 Months Ended
	Dec. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Awards	9. 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 of the grant vests upon the first anniversary of the date of grant, with the remainder of the shares vesting monthly thereafter at a rate of 1/48 of the total shares subject to the option. The vesting schedule for RSU grants provides that 1/4 of the grant vests upon the annual anniversary of the date of grant over the period of four years. As part of the acquisition of Dimension (discussed in Note 3 “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, 2017, an aggregate of 9,732,069 shares of common stock have been authorized for issuance under all equity award 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, 2014 2,739,475 $ 16.15 8.43 $ 79,840 Options granted 2,013,350 90.60 Options exercised (795,825 ) 8.65 Options cancelled (130,037 ) 31.49 Outstanding — December 31, 2015 3,826,963 $ 56.36 8.58 $ 217,386 Options granted 1,435,995 67.00 Options exercised (425,922 ) 21.21 Options cancelled (407,564 ) 70.87 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 Vested and exercisable — December 31, 2017 2,709,510 $ 53.53 6.22 $ 34,558 Vested and expected to vest — December 31, 2017 5,216,110 $ 62.50 7.35 $ 37,528 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, 2017, 2016, and 2015 was $20.4 million, $22.2 million and $59.0 million, respectively. Cash received from the exercise of options was $7.8 The weighted-average estimated fair value of stock options granted was $44.20, $40.49 and $54.90 per share of the Company’s common stock during the years ended December 31, 2017, 2016, and 2015, respectively. The total estimated fair value of options vested during the years ended December 31, 2017, 2016, and 2015 was $49.1 million, $43.8 million, and $10.7 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, 2014 31,000 $ 53.23 RSUs granted 187,260 89.67 RSUs released (18,209 ) 54.72 RSUs cancelled (2,900 ) 84.89 Unvested — December 31, 2015 197,151 $ 87.24 RSUs granted 477,816 66.83 RSUs released (52,273 ) 82.59 RSUs cancelled (48,950 ) 78.04 Unvested — December 31, 2016 573,744 $ 71.45 RSUs granted 516,161 71.58 RSUs released (156,021 ) 71.93 RSUs cancelled (112,324 ) 76.78 Unvested — December 31, 2017 821,560 $ 70.71 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 fair value of the 156,021 shares vested during 2017 was approximately $11.2 million with an aggregate intrinsic value of the shares of $9.2 million. Performance Stock Units In December 2017, the Company granted 508,850 performance stock units (PSUs) to certain employees at a grant date fair value of $48.03 per share of the Company’s common stock, of which no shares vested during 2017. These 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. 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. In December 2017, the Company began recognizing stock compensation expense on one of the criteria as the outcome was deemed probable. Employee Stock Purchase Plan In January 2014, the Company adopted the 2014 Employee Stock Purchase Plan (ESPP) and had 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, 2017, the Company issued 62,412 shares of common stock under the ESPP. Stock-Based Compensation Expense Total stock-based compensation recognized was as follows (in thousands): Year Ended December 31, 2017 2016 2015 Research and development $ 38,212 $ 29,412 $ 17,100 Selling, general and administrative 29,802 18,897 7,784 Total stock-based compensation expense $ 68,014 $ 48,309 $ 24,884 As of December 31, 2017, the total unrecognized compensation expense related to unvested equity awards, net of estimated forfeitures, was $165.3 million, which the Company expects to recognize over an estimated weighted-average period of 2.52 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 —As the Company does not have sufficient historical stock price information from the Company to meet the expected life of the stock-based awards, our approach to estimating expected volatility is to phase in our own common stock trading history and supplement the remaining historical information with a blended volatility from the trading history from the common stock of the set of comparable publicly traded biopharmaceutical companies. When selecting comparable publicly traded biopharmaceutical companies on which to base the expected stock price volatility, we selected companies with comparable characteristics to it, including enterprise value, risk profiles, position within the industry, and with historical share price information sufficient to meet the expected life of the stock-based awards. The historical volatility data was computed using the daily closing prices for the selected companies’ shares during the equivalent period of the calculated expected term of the stock-based awards. The average volatility for comparable publicly traded biopharmaceutical companies over a period equal to the expected term of the stock option grants is used to supplement the Company’s historical volatility. We will continue to apply this process until a sufficient amount of historical information regarding the volatility of our own stock price becomes available. 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, 2017 2016 2015 Expected term (years) 6.23 6.23 6.23 Expected volatility 65% 65% 65% Risk-free interest rate 2.1% 1.5% 1.8% Expected dividend rate 0.0% 0.0% 0.0%

Defined Contribution Plan

Defined Contribution Plan	12 Months Ended
	Dec. 31, 2017
Defined Contribution Pension And Other Postretirement Plans Disclosure [Abstract]
Defined Contribution Plan	10. Defined Contribution Plan Since 2013, the Company has sponsored a 401(k) retirement plan, in which substantially all of its full-time employees are eligible to participate. Eligible participants may contribute a percentage of their annual compensation to this plan, subject to statutory limitations. Prior to 2015, the Company had not provided any contributions to the plan. The Company recorded $2.1 million, $1.5 million, and $0.6 million as contribution expenses for the years ended December 31, 2017, 2016, and 2015, respectively.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income Taxes	11. Income Taxes The components of the Company’s loss before income taxes were as follows (in thousands): Year Ended December 31, 2017 2016 2015 Domestic $ 250,917 $ 192,287 $ 145,618 Foreign 67,421 53,552 — Total loss before income taxes $ 318,338 $ 245,839 $ 145,618 The components of the Company’s income tax provision were as follows (in thousands): Year Ended December 31, 2017 2016 2015 Current provision for income taxes: Federal $ — $ — $ — State 5 — — International 42 35 — Total current tax provision 47 35 — Deferred tax benefit: Federal (16,243 ) — — State (3 ) — — International — — — Total deferred tax benefit (16,246 ) — — Total (benefit from) provision for income taxes $ (16,199 ) $ 35 $ — 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, 2017 2016 2015 Federal statutory income tax rate 34.0 % 34.0 % 34.0 % State income taxes, net of federal benefit — 1.3 7.5 Federal tax credits 9.0 13.7 5.9 Other (0.9 ) (0.3 ) 0.8 Nondeductible permanent items — — — Stock-based compensation (0.6 ) (1.4 ) (1.3 ) Uncertain tax positions (1.8 ) 2.0 (7.7 ) Change in valuation allowance (32.5 ) (41.9 ) (39.2 ) Foreign rate differential (7.2 ) (7.4 ) — Change in federal tax rate 5.1 — — Provision for income taxes 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, 2017 2016 Deferred tax assets: Loss carryforwards $ 154,949 $ 102,960 Tax credits 119,542 78,324 Stock options 21,336 16,084 Accruals and reserves 5,939 10,039 Fixed assets and intangibles 1,194 3,014 Other 1,286 1,950 Gross deferred tax assets 304,246 212,371 Valuation allowance (304,246 ) (212,371 ) Total deferred tax assets — — Deferred tax liabilities: In-process research and development (31,166 ) — Net deferred tax assets (liabilities) $ (31,166 ) $ — As of December 31, 2017 and 2016, the Company had $422.3 million and $315.5 million of federal net operating loss carryforwards available to reduce future taxable income that will begin to expire in 2030. As of December 31, 2017 and 2016, the Company had $457.7 million and $259.8 million of state net operating loss carryforwards available to reduce future taxable income that will begin to expire in 2030. As of December 31, 2017 and 2016, the Company had federal research tax credit carryforwards of $4.5 million and $2.5 million available to reduce future tax liabilities that will begin to expire in 2030. As of December 31, 2017 and 2016, the Company had state research credit carryforwards of $12.4 million and $6.7 million available to reduce future tax liabilities that will be carried forward indefinitely. As of December 31, 2017 and 2016, the Company had federal Orphan Drug Credits of $124.6 million and $84.3 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 million of federal net operating loss carryforwards, $3.6 million of state net operating loss carryforwards, and $0.2 million of federal tax credits are permanently limited. Deferred tax assets for net operating losses and tax credits have been reduced and a corresponding adjustment to the valuation allowance has been recorded. On November 7, 2017, the Company acquired Dimension (see Note 3 “Acquisition”). The Company recorded a $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. The valuation allowance increased by $91.9 million and $102.9 million during the year ended December 31, 2017 and 2016, respectively. On December 22, 2017, the U.S. government enacted comprehensive tax legislation commonly referred to as the Tax Cuts and Jobs Act (Tax Act). The Company has calculated its best estimate of the impact of the Tax Act in its year end income tax provision in accordance with its understanding of the Tax Act and guidance available as of the date of this filing. The provisional amount related to the remeasurement of certain deferred tax assets and liabilities based on the rates at which they are expected to reverse in the future was a net decrease related to deferred tax assets and deferred tax liabilities of $70.5 million, with a corresponding net adjustment to benefit from income taxes of $16.1 million and offsetting change in valuation allowance of $86.6 million for the year ended December 31, 2017. The Company does not expect a material impact related to the one-time transition tax on the mandatory deemed repatriation of foreign earnings. On December 22, 2017, Staff Accounting Bulletin No. 118 (SAB 118) was issued to address the application of GAAP in situations when a registrant does not have the necessary information available, prepared, or analyzed (including computations) in reasonable detail to complete the accounting for certain income tax effects of the Act. In accordance with SAB 118, the Company has determined that the adjustment to deferred taxes was a provisional amount and a reasonable estimate at December 31, 2017. The determination of the benefit from (provision for) income taxes requires complex estimations, significant judgments and significant knowledge and experience concerning the applicable tax laws. Given that the Company is still in the transition period for the accounting for income tax effects of the Tax Act, the current assessment on deferred tax assets (liabilities) is based on the currently available information and guidance. If in the future any element of the tax reform changes the related accounting guidance for income tax, it could affect the Company’s income tax position and the Company may need to adjust the benefit from (provision for) income taxes accordingly. The Company recorded unrecognized tax benefits for uncertainties in income taxes. A reconciliation of the Company’s unrecognized tax benefits for the years ended December 31, 2017, 2016, and 2015 is as follows (in thousands): December 31, 2017 2016 2015 Balance at beginning of year $ 13,505 $ 24,010 $ 7,275 Additions based on tax positions related to current year 9,338 6,777 15,628 Additions for tax positions of prior years 5,534 877 5,505 Reductions for tax positions of prior years — (18,159 ) (4,398 ) Balance at end of year $ 28,377 $ 13,505 $ 24,010 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, 2017 and 2016, 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, 2017, the Company had not made a provision for U.S. income taxes or foreign withholding taxes on a nominal amount 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. Generally, such amounts become subject to U.S. taxation upon repatriation as dividends and under certain other circumstances. If these earnings were repatriated to the U.S., the deferred tax liability associated with these temporary differences would result in a nominal amount. 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, 2017 remain subject to examination.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2017
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	12. Commitments and Contingencies Facilities The Company leases office space and research, testing and manufacturing laboratory space in various facilities in Novato and Brisbane, California and in Cambridge and Woburn, Massachusetts, under operating agreements expiring at various dates through 2026. Certain of the leases provide for options by the Company to extend the lease for multiple five-year renewal periods and also provide for annual minimum increases in rent, usually based on a consumer price index or annual minimum increases. The Company recognizes rent expense on a straight-line basis over the noncancelable term of its operating lease. Rent expense was $4.5 million, $3.3 million, and $1.4 million during the years ended December 31, 2017, 2016 and 2015, respectively. Other Commitments 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, 2017, the aggregate future minimum lease payments under the noncancelable operating lease arrangements and future payments under contractually binding manufacturing and service agreements are as follows (in thousands): Year Ending December 31, Leases Manufacturing and Services 2018 $ 6,210 $ 7,542 2019 5,293 1,224 2020 3,727 62 2021 2,477 — 2022 2,413 — Thereafter 9,033 — $ 29,153 $ 8,828 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, 2017
Earnings Per Share [Abstract]
Net Loss per Share	13. 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, 2017, 2016 and 2015 (in thousands, except share and per share data): Year Ended December 31, 2017 2016 2015 Numerator: Net loss $ (302,139 ) $ (245,874 ) $ (145,618 ) Denominator: Weighted-average shares used to compute net loss per share, basic and diluted 42,453,135 39,586,908 36,782,603 Net loss per share, basic and diluted $ (7.12 ) $ (6.21 ) $ (3.96 ) 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, 2017 2016 2015 Options to purchase common stock, RSUs, and PSUs 5,862,784 4,699,111 3,368,507 Employee stock purchase plan 2,728 7,933 — Common stock warrants 149,700 149,700 195,762 6,015,212 4,856,744 3,564,269

Accumulated Other Comprehensive

Accumulated Other Comprehensive Income (Loss)	12 Months Ended
	Dec. 31, 2017
Other Comprehensive Income Loss Net Of Tax Period Increase Decrease [Abstract]
Accumulated Other Comprehensive Income (Loss)	14. Accumulated Other Comprehensive Income (Loss) Total accumulated other comprehensive income (loss) consisted of the following (in thousands) : Unrealized Gain (Loss) on Securities Available-for-Sale Foreign Currency Translation Adjustments Accumulated Other Comprehensive Income (Loss) December 31, 2014 $ (174 ) $ — $ (174 ) Current period other comprehensive loss (694 ) — (694 ) December 31, 2015 (868 ) — (868 ) Current period other comprehensive income 451 1,322 1,773 December 31, 2016 (417 ) 1,322 905 Current period other comprehensive income (loss) 35 (10,110 ) (10,075 ) Reclassification from accumulated other comprehensive income — 3,490 3,490 December 31, 2017 $ (382 ) $ (5,298 ) $ (5,680 )

Quarterly Financial Data (unaud

Quarterly Financial Data (unaudited)	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Data [Abstract]
Quarterly Financial Data (unaudited)	15. 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 share and per share data): 2017 March 31, June 30, September 30, December 31, Revenue $ — $ — $ 198 $ 2,414 Operating expenses 69,954 78,441 83,911 99,248 Net loss (68,290 ) (72,891 ) (79,227 ) (81,731 ) Net loss per share, basic and diluted $ (1.63 ) $ (1.72 ) $ (1.87 ) $ (1.89 ) 2016 March 31, June 30, September 30, December Revenue $ — $ 17 $ 111 $ 5 Operating expenses 53,622 58,070 65,894 70,554 Net loss (52,757 ) (56,923 ) (64,907 ) (71,287 ) Net loss per share, basic and diluted $ (1.35 ) $ (1.46 ) $ (1.64 ) $ (1.75 )

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2017
Subsequent Events [Abstract]
Subsequent Events	ULTRAGENYX PHARMACEUTICAL INC. Notes to Consolidated Financial Statements 16. Subsequent Events ATM Offering During the period of January 1, 2018 through February 21, 2018, the Company sold an additional 240,417 shares of common stock, resulting in net proceeds of approximately $11.8 million, after commissions and other offering costs. Priority Review Voucher sale On January 10, 2018, the Company completed the sale of a PRV to Novartis Pharma AG for $130.0 million. The Company had received the PRV from the FDA in connection with the approval of Mepsevii 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 included 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.

Summary of Significant Accoun24

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2017
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 and Cash Equivalents	Cash and Cash Equivalents 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.
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.
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, 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 bond issuers and other financial instruments to the extent recorded in the balance sheets . Concentrations on credit risk with respect to accounts receivable are limited due to the Company’s limited number of customers. 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. Inventories consist of currently approved product. 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.
Restricted Cash	Restricted Cash Restricted cash primarily consists of money market accounts as collateral for its obligations under its facility leases of the Company’s corporate headquarters in Novato, California and for its facilities in Brisbane, California and Woburn and Cambridge, Massachusetts.
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 the 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. The Company also reviews the useful lives of its assets periodically to determine whether events and circumstances warrant a revision to the remaining useful life. Changes in the useful life are adjusted prospectively by revising the remaining period over which the asset is amortized. The Company has not recorded any impairments nor made any adjustments to the useful lives of its assets since inception.
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. The Company has not recorded any impairments since inception.
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 since inception.
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 In May 2014, the Financial Revenue from Contracts with Customers (ASC 606) Product sales The Company sells Mepsevii The Company also recognizes revenue from sales of Mepsevii on a “named patient” basis, which are allowed in certain countries prior to the commercial approval of the product in the territory. Collaboration and license revenue The Company has certain license and collaboration agreements that are within the scope of ASC 808, Collaborative Agreements The Company 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 under these license or collaborations agreements, royalty revenue is recorded as the underlying sales occur. The Company also recognizes collaboration and license revenue under certain of the Company’s license or collaboration agreements that are within the scope of ASC 606. The terms of these agreements may contain multiple performance obligations, which may include licenses and research and development activities. The Company evaluates these agreements under ASC 606 to determine the distinct performance obligations. 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. Variable 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. Revenue is recognized by measuring the progress toward complete satisfaction of the performance obligations using an input measure.
Leases	Leases The Company enters into lease agreements for its office and laboratory facilities. These leases are classified as operating leases. Rent expense is recognized on a straight-line basis over the term of the lease and, accordingly, the Company records the difference between cash rent payments and the recognition of rent expense as a deferred rent liability. Incentives granted under the Company’s facilities leases, including allowances to fund leasehold improvements, are deferred and are recognized as adjustments to rental expense on a straight-line basis over the term of the lease.
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 and capitalized. The capitalized 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. Noncash stock compensation expense on awards expected to vest are recognized net of estimated forfeitures. In March 2016, the FASB issued ASU 2016-09, Compensation — Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting
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 Topic 805, “Business Combinations” (Topic 805) allocates
Recent Accounting Pronouncements	Recent Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) In October 2016, the FASB issued ASU 2016-16, Income Taxes - Intra-Entity Transfers of Assets Other Than Inventory In November 2016, the FASB issued ASU 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash In January 2017, the FASB issued ASU 2017-04, Intangibles – Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment

Summary of Significant Accoun25

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Summary of Useful Lives of Property and Equipment	The useful lives of the 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

Acquisition (Tables)

Acquisition (Tables)	12 Months Ended
	Dec. 31, 2017
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
Summary of Supplemental Unaudited Pro Forma	The following supplemental unaudited pro forma information presents the financial results as if the acquisition had occurred on January 1, 2016 (in thousands): For the year ended December 31, 2017 2016 Total revenues $ 18,528 $ 12,684 Net loss 341,737 306,596

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017 Level 1 Level 2 Level 3 Total Financial Assets: Money market funds $ 79,670 $ — $ — $ 79,670 Corporate bonds — 39,240 — 39,240 U.S. Government Treasury and agency securities — 104,739 — 104,739 Total financial assets $ 79,670 $ 143,979 $ — $ 223,649 December 31, 2016 Level 1 Level 2 Level 3 Total Financial Assets: Money market funds $ 123,536 $ — $ — $ 123,536 Corporate bonds — 207,726 — 207,726 Commercial paper — 11,970 — 11,970 Asset-backed securities — 27,713 — 27,713 U.S. Government Treasury and agency securities — 111,931 — 111,931 Total financial assets $ 123,536 $ 359,340 $ — $ 482,876

Balance Sheet Components (Table

Balance Sheet Components (Tables)	12 Months Ended
	Dec. 31, 2017
Investments Debt And Equity Securities [Abstract]
Summary of Fair Values of Cash Equivalents, Short-Term Investments and Long-Term Investments Classified as Available-for-Sale Securities	The fair values of cash equivalents, short-term investments, and long-term investments classified as available-for-sale securities, consisted of the following (in thousands): December 31, 2017 Gross Unrealized Amortized Cost Gains Losses Estimated Fair Value Money market funds $ 79,670 $ — $ — $ 79,670 Corporate bonds 39,330 — (90 ) 39,240 U.S. Government Treasury and agency securities 105,029 — (290 ) 104,739 Total $ 224,029 $ — $ (380 ) $ 223,649 December 31, 2016 Gross Unrealized Amortized Cost Gains Losses Estimated Fair Value Money market funds $ 123,536 $ — $ — $ 123,536 Corporate bonds 207,909 14 (197 ) 207,726 Commercial paper 11,970 — — 11,970 Asset-backed securities 27,712 3 (2 ) 27,713 U.S. Government Treasury and agency securities 112,166 10 (245 ) 111,931 Total $ 483,293 $ 27 $ (444 ) $ 482,876
Summary of Inventory	Inventory consists of the following (in thousands): December 31, 2017 2016 Work-in-process $ 737 $ — Finished goods 20 — Total inventory $ 757 $ —
Summary of Property and Equipment	Property and equipment, net consists of the following (in thousands): December 31, 2017 2016 Research and development equipment $ 7,696 $ 2,154 Furniture and office equipment 2,873 2,595 Computer equipment 2,563 2,013 Software 4,182 3,931 Leasehold improvements 15,085 12,486 Construction-in-progress 429 37 Property and equipment, gross 32,828 23,216 Less accumulated depreciation (10,991 ) (6,161 ) Property and equipment, net $ 21,837 $ 17,055
Accrued Liabilities	Accrued liabilities consists of the following (in thousands): December 31, 2017 2016 Research and clinical study expenses $ 17,141 $ 18,593 Payroll and related expenses 26,527 17,226 Repayment liability under collaboration agreement 3,681 13,650 Contract liability 5,986 — Other 8,092 5,085 Total accrued liabilities $ 61,427 $ 54,554

Stock-Based Awards (Tables)

Stock-Based Awards (Tables)	12 Months Ended
	Dec. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
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, 2014 2,739,475 $ 16.15 8.43 $ 79,840 Options granted 2,013,350 90.60 Options exercised (795,825 ) 8.65 Options cancelled (130,037 ) 31.49 Outstanding — December 31, 2015 3,826,963 $ 56.36 8.58 $ 217,386 Options granted 1,435,995 67.00 Options exercised (425,922 ) 21.21 Options cancelled (407,564 ) 70.87 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 Vested and exercisable — December 31, 2017 2,709,510 $ 53.53 6.22 $ 34,558 Vested and expected to vest — December 31, 2017 5,216,110 $ 62.50 7.35 $ 37,528
Summary of Activity Under Restricted Stock Units (RSUs) 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, 2014 31,000 $ 53.23 RSUs granted 187,260 89.67 RSUs released (18,209 ) 54.72 RSUs cancelled (2,900 ) 84.89 Unvested — December 31, 2015 197,151 $ 87.24 RSUs granted 477,816 66.83 RSUs released (52,273 ) 82.59 RSUs cancelled (48,950 ) 78.04 Unvested — December 31, 2016 573,744 $ 71.45 RSUs granted 516,161 71.58 RSUs released (156,021 ) 71.93 RSUs cancelled (112,324 ) 76.78 Unvested — December 31, 2017 821,560 $ 70.71
Summary of Stock-Based Compensation Recognized	Total stock-based compensation recognized was as follows (in thousands): Year Ended December 31, 2017 2016 2015 Research and development $ 38,212 $ 29,412 $ 17,100 Selling, general and administrative 29,802 18,897 7,784 Total stock-based compensation expense $ 68,014 $ 48,309 $ 24,884
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, 2017 2016 2015 Expected term (years) 6.23 6.23 6.23 Expected volatility 65% 65% 65% Risk-free interest rate 2.1% 1.5% 1.8% Expected dividend rate 0.0% 0.0% 0.0%

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017 2016 2015 Domestic $ 250,917 $ 192,287 $ 145,618 Foreign 67,421 53,552 — Total loss before income taxes $ 318,338 $ 245,839 $ 145,618
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, 2017 2016 2015 Current provision for income taxes: Federal $ — $ — $ — State 5 — — International 42 35 — Total current tax provision 47 35 — Deferred tax benefit: Federal (16,243 ) — — State (3 ) — — International — — — Total deferred tax benefit (16,246 ) — — Total (benefit from) provision for income taxes $ (16,199 ) $ 35 $ —
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, 2017 2016 2015 Federal statutory income tax rate 34.0 % 34.0 % 34.0 % State income taxes, net of federal benefit — 1.3 7.5 Federal tax credits 9.0 13.7 5.9 Other (0.9 ) (0.3 ) 0.8 Nondeductible permanent items — — — Stock-based compensation (0.6 ) (1.4 ) (1.3 ) Uncertain tax positions (1.8 ) 2.0 (7.7 ) Change in valuation allowance (32.5 ) (41.9 ) (39.2 ) Foreign rate differential (7.2 ) (7.4 ) — Change in federal tax rate 5.1 — — Provision for income taxes 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, 2017 2016 Deferred tax assets: Loss carryforwards $ 154,949 $ 102,960 Tax credits 119,542 78,324 Stock options 21,336 16,084 Accruals and reserves 5,939 10,039 Fixed assets and intangibles 1,194 3,014 Other 1,286 1,950 Gross deferred tax assets 304,246 212,371 Valuation allowance (304,246 ) (212,371 ) Total deferred tax assets — — Deferred tax liabilities: In-process research and development (31,166 ) — Net deferred tax assets (liabilities) $ (31,166 ) $ —
Unrecognized Tax Benefits	A reconciliation of the Company’s unrecognized tax benefits for the years ended December 31, 2017, 2016, and 2015 is as follows (in thousands): December 31, 2017 2016 2015 Balance at beginning of year $ 13,505 $ 24,010 $ 7,275 Additions based on tax positions related to current year 9,338 6,777 15,628 Additions for tax positions of prior years 5,534 877 5,505 Reductions for tax positions of prior years — (18,159 ) (4,398 ) Balance at end of year $ 28,377 $ 13,505 $ 24,010

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2017
Commitments And Contingencies Disclosure [Abstract]
Summary of Future Minimum Lease Payments under Non-Cancellable Operating Leases Arrangements and Future Payments under Contractually Binding Manufacturing and Service Agreements	As of December 31, 2017, the aggregate future minimum lease payments under the noncancelable operating lease arrangements and future payments under contractually binding manufacturing and service agreements are as follows (in thousands): Year Ending December 31, Leases Manufacturing and Services 2018 $ 6,210 $ 7,542 2019 5,293 1,224 2020 3,727 62 2021 2,477 — 2022 2,413 — Thereafter 9,033 — $ 29,153 $ 8,828

Net Loss per Share (Tables)

Net Loss per Share (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017, 2016 and 2015 (in thousands, except share and per share data): Year Ended December 31, 2017 2016 2015 Numerator: Net loss $ (302,139 ) $ (245,874 ) $ (145,618 ) Denominator: Weighted-average shares used to compute net loss per share, basic and diluted 42,453,135 39,586,908 36,782,603 Net loss per share, basic and diluted $ (7.12 ) $ (6.21 ) $ (3.96 )
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, 2017 2016 2015 Options to purchase common stock, RSUs, and PSUs 5,862,784 4,699,111 3,368,507 Employee stock purchase plan 2,728 7,933 — Common stock warrants 149,700 149,700 195,762 6,015,212 4,856,744 3,564,269

Accumulated Other Comprehensi33

Accumulated Other Comprehensive Income (Loss) (Tables)	12 Months Ended
	Dec. 31, 2017
Other Comprehensive Income Loss Net Of Tax Period Increase Decrease [Abstract]
Schedule of Total Accumulated Other Comprehensive Income (Loss)	Total accumulated other comprehensive income (loss) consisted of the following (in thousands) : Unrealized Gain (Loss) on Securities Available-for-Sale Foreign Currency Translation Adjustments Accumulated Other Comprehensive Income (Loss) December 31, 2014 $ (174 ) $ — $ (174 ) Current period other comprehensive loss (694 ) — (694 ) December 31, 2015 (868 ) — (868 ) Current period other comprehensive income 451 1,322 1,773 December 31, 2016 (417 ) 1,322 905 Current period other comprehensive income (loss) 35 (10,110 ) (10,075 ) Reclassification from accumulated other comprehensive income — 3,490 3,490 December 31, 2017 $ (382 ) $ (5,298 ) $ (5,680 )

Quarterly Financial Data (una34

Quarterly Financial Data (unaudited) (Tables)	12 Months Ended
	Dec. 31, 2017
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 share and per share data): 2017 March 31, June 30, September 30, December 31, Revenue $ — $ — $ 198 $ 2,414 Operating expenses 69,954 78,441 83,911 99,248 Net loss (68,290 ) (72,891 ) (79,227 ) (81,731 ) Net loss per share, basic and diluted $ (1.63 ) $ (1.72 ) $ (1.87 ) $ (1.89 ) 2016 March 31, June 30, September 30, December Revenue $ — $ 17 $ 111 $ 5 Operating expenses 53,622 58,070 65,894 70,554 Net loss (52,757 ) (56,923 ) (64,907 ) (71,287 ) Net loss per share, basic and diluted $ (1.35 ) $ (1.46 ) $ (1.64 ) $ (1.75 )

Organization and Basis of Pre35

Organization and Basis of Presentation - Additional Information (Details)	12 Months Ended
	Dec. 31, 2017Segment
United States of America
Organization And Nature Of Business [Line Items]
Number of reportable segments	1

Summary of Significant Accoun36

Summary of Significant Accounting Policies - Summary of Useful Lives of Property and Equipment (Details)	12 Months Ended
	Dec. 31, 2017
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

Summary of Significant Accoun37

Summary of Significant Accounting Policies - Additional Information (Details) - USD ($)	3 Months Ended					12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2017	Dec. 31, 2015	Dec. 31, 2014
Summary Of Significant Accounting Policies [Line Items]
Revenue recognized	$ 2,414,000	$ 198,000	$ 5,000	$ 111,000	$ 17,000	$ 476,000	$ 133,000
Unrecognized excess tax benefits which was fully offset by increase in valuation allowance	$ 28,377,000		$ 13,505,000			28,377,000	$ 13,505,000		$ 24,010,000	$ 7,275,000
Unrecognized tax benefits related to interest or penalties						0
Accounting Standards Update 2014-09 | New Accounting Pronouncement, Early Adoption, Effect
Summary Of Significant Accounting Policies [Line Items]
Revenue recognized						$ 100,000
Accounting Standards Update 2016-09
Summary Of Significant Accounting Policies [Line Items]
Unrecognized excess tax benefits which was fully offset by increase in valuation allowance								$ 19,700,000

Acquisition - Additional Inform

Acquisition - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	Nov. 07, 2017	Dec. 31, 2017	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Acquisition [Line Items]
Total revenues											$ 2,612	$ 133
Net loss			$ (81,731)	$ (79,227)	$ (72,891)	$ (68,290)	$ (71,287)	$ (64,907)	$ (56,923)	$ (52,757)	(302,139)	(245,874)	$ (145,618)
Dimension
Business Acquisition [Line Items]
Business acquisition date	Nov. 7, 2017
Business acquisition, purchase price per share	$ 6
Business acquisition, cash	$ 152,292
REGENX termination fee paid	2,850
Business acquisition, assumed equity awards value	15,400
Business acquisition, acceleration of certain awards recognized as expense in post-combination financial statements	2,200
Business acquisition, expected to be recognized as expense over employee's remaining service period	4,200
Business acquisition, consideration transferred	164,121
Non-current deferred tax liability	47,412
Notes payable	4,944
Transaction costs	6,000
Total revenues		$ 2,100
Net loss		$ 7,500
Transaction costs related to acquisition											9,600
Amortization of definite-lived intangible assets											$ 1,800	$ 11,100
Stock Options | Dimension
Business Acquisition [Line Items]
Business acquisition, assumed equity awards value	$ 8,979

Acquisition - Summary of Fair V

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

Acquisition - Summary of Fair40

Acquisition - Summary of Fair Values of Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Nov. 07, 2017
Business Acquisition [Line Items]
Goodwill	$ 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

Acquisition - Summary of Supple

Acquisition - Summary of Supplemental Unaudited Pro Forma (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Business Combinations [Abstract]
Total revenues	$ 18,528	$ 12,684
Net loss	$ 341,737	$ 306,596

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Financial Assets and Liabilities Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financial Assets:
Fair value	$ 223,649	$ 482,876
Money Market Funds
Financial Assets:
Fair value	79,670	123,536
Asset-backed Securities
Financial Assets:
Fair value		27,713
Corporate Bonds
Financial Assets:
Fair value	39,240	207,726
Commercial Paper
Financial Assets:
Fair value		11,970
U.S. Government Treasury and Agency Securities
Financial Assets:
Fair value	104,739	111,931
Level 1
Financial Assets:
Fair value	79,670	123,536
Level 1 | Money Market Funds
Financial Assets:
Fair value	79,670	123,536
Level 2
Financial Assets:
Fair value	143,979	359,340
Level 2 | Asset-backed Securities
Financial Assets:
Fair value		27,713
Level 2 | Corporate Bonds
Financial Assets:
Fair value	39,240	207,726
Level 2 | Commercial Paper
Financial Assets:
Fair value		11,970
Level 2 | U.S. Government Treasury and Agency Securities
Financial Assets:
Fair value	$ 104,739	$ 111,931

Balance Sheet Components - Summ

Balance Sheet Components - Summary of Fair Values of Cash Equivalents, Short-Term Investments and Long-Term Investments Classified as Available-for-Sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 224,029	$ 483,293
Gross Unrealized Gains		27
Gross Unrealized Losses	(380)	(444)
Estimated Fair Value	223,649	482,876
Money Market Funds
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	79,670	123,536
Estimated Fair Value	79,670	123,536
Commercial Paper
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost		11,970
Estimated Fair Value		11,970
Corporate Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	39,330	207,909
Gross Unrealized Gains		14
Gross Unrealized Losses	(90)	(197)
Estimated Fair Value	39,240	207,726
U.S. Government Treasury and Agency Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	105,029	112,166
Gross Unrealized Gains		10
Gross Unrealized Losses	(290)	(245)
Estimated Fair Value	$ 104,739	111,931
Asset-backed Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost		27,712
Gross Unrealized Gains		3
Gross Unrealized Losses		(2)
Estimated Fair Value		$ 27,713

Balance Sheet Components - Addi

Balance Sheet Components - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Available For Sale Securities [Line Items]
Significant realized gains or losses on available-for-sale securities	$ 0
Depreciation expense	$ 4,800,000	$ 3,400,000	$ 1,400,000
Maximum
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale securities remaining contractual maturities	2 years

Balance Sheet Components - Su45

Balance Sheet Components - Summary of Inventory (Details) $ in Thousands	Dec. 31, 2017USD ($)
Investments Debt And Equity Securities [Abstract]
Work-in-process	$ 737
Finished goods	20
Total inventory	$ 757

Balance Sheet Components - Su46

Balance Sheet Components - Summary of Property and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 32,828	$ 23,216
Less accumulated depreciation	(10,991)	(6,161)
Property and equipment, net	21,837	17,055
Research and Development Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	7,696	2,154
Furniture and Office Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	2,873	2,595
Computer Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	2,563	2,013
Software
Property Plant And Equipment [Line Items]
Property and equipment, gross	4,182	3,931
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	15,085	12,486
Construction-in-progress
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 429	$ 37

Balance Sheet Components - Accr

Balance Sheet Components - Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Payables And Accruals [Abstract]
Research and clinical study expenses	$ 17,141	$ 18,593
Payroll and related expenses	26,527	17,226
Repayment liability under collaboration agreement	3,681	13,650
Contract liability	5,986
Other	8,092	5,085
Total accrued liabilities	$ 61,427	$ 54,554

Intangible Assets, Net - Additi

Intangible Assets, Net - Additional Information (Details) - USD ($) $ in Thousands	2 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Nov. 07, 2017
Finite Lived Intangible Assets [Line Items]
Research and development expense related to amortization of assets		$ 231,644	$ 183,204	$ 114,737
Dimension
Finite Lived Intangible Assets [Line Items]
Business combination recognized intangible assets					$ 129,000
Research and development expense related to amortization of assets	$ 1,000
Estimated future amortization expense in 2018	10,500	10,500
Estimated future amortization expense in 2019	$ 2,000	$ 2,000
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

License and Research Agreemen49

License and Research Agreements - Additional Information (Details)	Nov. 07, 2017USD ($)	Oct. 31, 2016USD ($)shares	Jul. 31, 2016USD ($)$ / sharesshares	Jun. 30, 2016USD ($)	Oct. 31, 2015USD ($)	Dec. 31, 2017USD ($)shares	Dec. 31, 2017USD ($)DiseaseIndicationshares	Dec. 31, 2016USD ($)shares	Dec. 31, 2015USD ($)shares
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Research and development							$ 231,644,000	$ 183,204,000	$ 114,737,000
Common stock purchased | shares						44,167,071	44,167,071	41,240,230
Common stock issued upon option exercised in period, shares | shares							478,470	425,922	795,825
Contract liability						$ 5,986,000	$ 5,986,000
Dimension
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Research and development						1,000,000
Business acquisition date	Nov. 7, 2017
Business combination recognized intangible assets	$ 129,000,000
Amortization completion year							2,019
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]
Contract liability	$ 2,500,000					5,400,000	$ 5,400,000
Collaboration and license revenue						2,100,000
Saint Louis University License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payments paid							$ 100,000
St. Jude Children's Research Hospital License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Payments for royalty on net sales							1.00%
Baylor Research Institute License Agreement | Development Milestones
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Future contingent milestone payments						10,500,000	$ 10,500,000
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
Kyowa Hakko Kirin Collaboration | License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Reduction in research and development cost							31,200,000	$ 25,300,000
Reduction in selling, general and administrative expenses							4,500,000	1,500,000
License agreement receivable						10,300,000	$ 10,300,000	8,600,000
Kyowa Hakko Kirin Collaboration | License Agreement | Profit Share Territory
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 Hakko Kirin Collaboration | License Agreement | Maximum | European Territory
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Percentage of royalty on net sales entitled to receive							10.00%
Arcturus Research Collaboration and License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Research and development					$ 10,000,000
Takeda License and Collaboration and Purchase Agreements
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Estimated fair value of license recorded as research and development expense				$ 700,000
Research collaboration agreement, period				5 years
Common stock purchased | shares			374,590
Common stock value			$ 40,000,000
Common closing stock price per share | $ / shares			$ 72.95
Common stock shares issued premium amount			$ 12,700,000
Option to purchase additional common stock, value			25,000,000
Common stock purchase agreement, terms							Takeda is subject to a five-year standstill (subject to customary exceptions or release).
Total arrangement consideration received			14,300,000
Estimated transaction price			3,100,000
Reduction in research and development expenses			2,500,000
Repayment liability						3,700,000	$ 3,700,000	14,300,000
Contract liability						600,000	600,000	$ 0
Takeda License and Collaboration and Purchase Agreements | Put Option
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Estimated fair value of future options			900,000
Common stock issued upon option exercised in period, shares | shares		352,530
Common stock issued upon option exercised in period, value		$ 25,000,000
Takeda License and Collaboration and Purchase Agreements | Common Stock
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Fair market value of shares issued			27,300,000
Takeda License and Collaboration and Purchase Agreements | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Estimated approximate cost for expected work performed			11,200,000
Takeda License and Collaboration and Purchase Agreements | Minimum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Estimated approximate cost for expected work performed			$ 10,000,000
Research, Collaboration, and License Agreement | Maximum | University of Pennsylvania School of Medicine (Penn)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Future contingent milestone payments						5,000,000	$ 5,000,000
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. The exercise of the remaining option will require payment of a $1.0 million option fee, is subject to availability and if not exercised, will automatically terminate in 2019 (which may be extended for additional time for a fee).
Number of disease indications for exclusive license | DiseaseIndication							3
Payment of licensed option exercise fee required, subject to availability						1,000,000	$ 1,000,000
Option and license agreement, option termination year							2,019
Annual maintenance fees for each option exercised							$ 100,000
Option and License Agreement | Maximum | REGENXBIO, Inc.
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 invoiced						5,000,000	5,000,000
Research, Develop and Commercialize AAV Gene Therapy Products | Maximum | Bayer HealthCare LLC
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Future contingent milestone payments						$ 232,000,000	$ 232,000,000

Equity - Additional Information

Equity - Additional Information (Details) - USD ($)	1 Months Ended		12 Months Ended
	Jul. 31, 2017	Jul. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stockholders Equity [Line Items]
Net proceeds from sale of common stock					$ 461,136,000
June 2010 Common Stock Warrant
Stockholders Equity [Line Items]
Common stock warrants outstanding			149,700	149,700
Common stock warrants, issued date			2010-06	2010-06
Common stock warrants, contractual term			10 years	10 years
Common stock warrants, exercise price			$ 3.01	$ 3.01
June 2011 Common Stock Warrant
Stockholders Equity [Line Items]
Common stock warrants outstanding			149,700	149,700
Common stock warrants, issued date			2011-06	2011-06
Common stock warrants, contractual term			10 years	10 years
Common stock warrants, exercise price			$ 3.01	$ 3.01
ATM Sales Agreement
Stockholders Equity [Line Items]
Option to sell common stock for cash		$ 150,000,000
Common stock shares sold			912,351	1,159,415
Net proceeds from sale of common stock			$ 67,600,000	$ 79,500,000
Additional ATM Sales Agreement
Stockholders Equity [Line Items]
Common stock shares sold			1,338,866
Net proceeds from sale of common stock			$ 64,300,000
Additional ATM Sales Agreement | Maximum
Stockholders Equity [Line Items]
Option to sell common stock for cash	$ 150,000,000

Stock-Based Awards - Additional

Stock-Based Awards - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions	Nov. 07, 2017	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Feb. 05, 2014	Jan. 31, 2014
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total intrinsic value of options exercised			$ 20.4	$ 22.2	$ 59
Cash received from exercise of options			$ 7.8	$ 9	$ 6.9
Weighted-average estimated fair value of stock option			$ 44.20	$ 40.49	$ 54.90
Total estimated fair value of options vested			$ 49.1	$ 43.8	$ 10.7
Total unrecognized compensation cost		$ 165.3	$ 165.3
Weighted-average period to recognize cost			2 years 6 months 7 days
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			156,021	52,273	18,209
Total fair value of shares vested			$ 11.2
Aggregate intrinsic value			$ 9.2
Performance stock units granted			516,161	477,816	187,260
Performance stock units grant date fair value per share			$ 71.58	$ 66.83	$ 89.67
Performance stock units vested			156,021	52,273	18,209
Performance Stock Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares vested during the period			0
Performance stock units granted		508,850
Performance stock units grant date fair value per share		$ 48.03
Performance stock units vested			0
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		9,732,069	9,732,069
2014 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
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			62,412

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, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Options, Outstanding, Beginning Balance	4,429,472	3,826,963	2,739,475
Number of Options, Options granted	1,328,860	1,435,995	2,013,350
Number of Options, Options assumed	639,897
Number of Options, Options exercised	(478,470)	(425,922)	(795,825)
Number of Options, Options cancelled	(520,349)	(407,564)	(130,037)
Number of Options, Outstanding, Ending Balance	5,399,410	4,429,472	3,826,963	2,739,475
Number of Options, Vested and exercisable — December 31, 2017	2,709,510
Number of Options, Vested and expected to vest — December 31, 2017	5,216,110
Weighted Average Exercise Price, Outstanding, Beginning Balance	$ 61.85	$ 56.36	$ 16.15
Weighted Average Exercise Price, Options granted	71.99	67	90.60
Weighted Average Exercise Price, Options assumed	27.97
Weighted Average Exercise Price, Options exercised	16.25	21.21	8.65
Weighted Average Exercise Price, Options cancelled	78.70	70.87	31.49
Weighted Average Exercise Price, Outstanding, Ending Balance	62.75	$ 61.85	$ 56.36	$ 16.15
Weighted Average Exercise Price, Vested and exercisable — December 31, 2017	53.53
Weighted Average Exercise Price, Vested and expected to vest — December 31, 2017	$ 62.50
Weighted Average Remaining Contractual Term (Years), Outstanding	7 years 4 months 24 days	8 years 2 months 19 days	8 years 6 months 29 days	8 years 5 months 4 days
Weighted Average Remaining Contractual Term (Years), Vested and exercisable — December 31, 2017	6 years 2 months 19 days
Weighted Average Remaining Contractual Term (Years), Vested and expected to vest — December 31, 2017	7 years 4 months 6 days
Aggregate Intrinsic Value, Outstanding	$ 37,687	$ 79,135	$ 217,386	$ 79,840
Aggregate Intrinsic Value, Vested and exercisable — December 31, 2017	34,558
Aggregate Intrinsic Value, Vested and expected to vest — December 31, 2017	$ 37,528

Stock-Based Awards - Summary 53

Stock-Based Awards - Summary of Activity Under Restricted Stock Units (RSUs) from 2014 Plan (Details) - Restricted Stock Units - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
RSUs Outstanding, Number of Shares, Unvested, Beginning Balance	573,744	197,151	31,000
Number of Shares, RSUs granted	516,161	477,816	187,260
Number of Shares, RSUs released	(156,021)	(52,273)	(18,209)
Number of Shares, RSUs cancelled	(112,324)	(48,950)	(2,900)
RSUs Outstanding, Number of Shares, Unvested, Ending Balance	821,560	573,744	197,151
RSUs Outstanding, Weighted-Average Grant Date Fair Value, Unvested, Beginning Balance	$ 71.45	$ 87.24	$ 53.23
Weighted-Average Grant Date Fair Value, RSUs granted	71.58	66.83	89.67
Weighted-Average Grant Date Fair Value, RSUs released	71.93	82.59	54.72
Weighted-Average Grant Date Fair Value, RSUs cancelled	76.78	78.04	84.89
RSUs Outstanding, Weighted-Average Grant Date Fair Value, Unvested, Ending Balance	$ 70.71	$ 71.45	$ 87.24

Stock-Based Awards - Summary 54

Stock-Based Awards - Summary of Stock-Based Compensation Recognized (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 68,014	$ 48,309	$ 24,884
Research and development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	38,212	29,412	17,100
Selling, general and administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 29,802	$ 18,897	$ 7,784

Stock-Based Awards - Fair Value

Stock-Based Awards - Fair Value of Stock Option Awards Granted Estimated Using Black-Scholes Option-Pricing Model (Details) - Employee Stock Option	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (years)	6 years 2 months 23 days	6 years 2 months 23 days	6 years 2 months 23 days
Expected volatility	65.00%	65.00%	65.00%
Risk-free interest rate	2.10%	1.50%	1.80%
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, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Contribution Pension And Other Postretirement Plans Disclosure [Abstract]
Contribution expenses	$ 2.1	$ 1.5	$ 0.6

Income Taxes - Components of Co

Income Taxes - Components of Company's Loss Before Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Domestic	$ 250,917	$ 192,287	$ 145,618
Foreign	67,421	53,552
Total loss before income taxes	$ 318,338	$ 245,839	$ 145,618

Income Taxes - Components of 58

Income Taxes - Components of Company's Income Tax Provision (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Current provision for income taxes:
State	$ 5
International	42	$ 35
Total current tax provision	47	35
Deferred tax benefit:
Federal	(16,243)
State	(3)
Total deferred tax benefit	(16,246)
Total (benefit from) provision for income taxes	$ (16,199)	$ 35

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, 2017	Dec. 31, 2016	Dec. 31, 2015
Effective Income Tax Rate Continuing Operations Tax Rate Reconciliation [Abstract]
Federal statutory income tax rate	34.00%	34.00%	34.00%
State income taxes, net of federal benefit		1.30%	7.50%
Federal tax credits	9.00%	13.70%	5.90%
Other	(0.90%)	(0.30%)	0.80%
Stock-based compensation	(0.60%)	(1.40%)	(1.30%)
Uncertain tax positions	(1.80%)	2.00%	(7.70%)
Change in valuation allowance	(32.50%)	(41.90%)	(39.20%)
Foreign rate differential	(7.20%)	(7.40%)
Change in federal tax rate	5.10%
Provision for income taxes	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, 2017	Dec. 31, 2016	Dec. 31, 2012	Dec. 31, 2011
Deferred tax assets:
Loss carryforwards	$ 154,949	$ 102,960	$ 200	$ 200
Tax credits	119,542	78,324
Stock options	21,336	16,084
Accruals and reserves	5,939	10,039
Fixed assets and intangibles	1,194	3,014
Other	1,286	1,950
Gross deferred tax assets	304,246	212,371
Valuation allowance	(304,246)	$ (212,371)
Deferred tax liabilities:
Net deferred tax assets (liabilities)	(31,166)
In-process Research and Development
Deferred tax liabilities:
In-process research and development	$ (31,166)

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Nov. 07, 2017	Dec. 31, 2012	Dec. 31, 2011
Tax Credit Carryforward [Line Items]
Deferred tax assets net operating loss carryforwards	$ 154,949,000	$ 102,960,000		$ 200,000	$ 200,000
Deferred tax liability decrease, net	16,200,000
Benefit from (provision for) income taxes	16,199,000	(35,000)
Valuation allowance increased	91,900,000	102,900,000
Net decrease to deferred tax assets and deferred tax liabilities	70,500,000
Net adjustment to benefit from income taxes	16,100,000
Change in valuation allowance	86,600,000
Unrecognized tax benefits, income tax penalties and interest accrued	0	0
Dimension
Tax Credit Carryforward [Line Items]
Deferred tax liability			$ 47,412,000
Federal
Tax Credit Carryforward [Line Items]
Deferred tax assets operating loss carryforwards subject to expiration	$ 422,300,000	315,500,000
Operating loss carryforwards expiration year	2,030
Tax credit carryforwards	$ 4,500,000	2,500,000
Tax credit carryforwards expiration year	2,030
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	$ 124,600,000	84,300,000
Operating loss carryforwards expiration year	2,031
State
Tax Credit Carryforward [Line Items]
Deferred tax assets operating loss carryforwards subject to expiration	$ 457,700,000	259,800,000
Operating loss carryforwards expiration year	2,030
Tax credit carryforwards	$ 12,400,000	$ 6,700,000
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, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Balance at beginning of year	$ 13,505	$ 24,010	$ 7,275
Additions based on tax positions related to current year	9,338	6,777	15,628
Additions for tax positions of prior years	5,534	877	5,505
Reductions for tax positions of prior years		(18,159)	(4,398)
Balance at end of year	$ 28,377	$ 13,505	$ 24,010

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure Commitments And Contingencies Additional Information Details [Line Items]
Rent expense	$ 4.5	$ 3.3	$ 1.4
Novato Office Facility
Disclosure Commitments And Contingencies Additional Information Details [Line Items]
Operating lease expiration year	2,026
Lease renewal term	5 years

Commitments and Contingencies64

Commitments and Contingencies - Summary of Future Minimum Lease Payments under Non-Cancellable Operating Leases Arrangements and Future Payments under Contractually Binding Manufacturing and Service Agreements (Details) $ in Thousands	Dec. 31, 2017USD ($)
Commitments And Contingencies Disclosure [Abstract]
Leases, 2018	$ 6,210
Leases, 2019	5,293
Leases, 2020	3,727
Leases, 2021	2,477
Leases, 2022	2,413
Leases, Thereafter	9,033
Leases, Total	29,153
Manufacturing and Services, 2018	7,542
Manufacturing and Services, 2019	1,224
Manufacturing and Services, 2020	62
Manufacturing and Services, Total	$ 8,828

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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Net loss	$ (81,731)	$ (79,227)	$ (72,891)	$ (68,290)	$ (71,287)	$ (64,907)	$ (56,923)	$ (52,757)	$ (302,139)	$ (245,874)	$ (145,618)
Denominator:
Weighted-average shares used to compute net loss per share, basic and diluted									42,453,135	39,586,908	36,782,603
Net loss per share, basic and diluted	$ (1.89)	$ (1.87)	$ (1.72)	$ (1.63)	$ (1.75)	$ (1.64)	$ (1.46)	$ (1.35)	$ (7.12)	$ (6.21)	$ (3.96)

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, 2017	Dec. 31, 2016	Dec. 31, 2015
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from computation of diluted net loss per share	6,015,212	4,856,744	3,564,269
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	5,862,784	4,699,111	3,368,507
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	195,762
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	2,728	7,933

Accumulated Other Comprehensi67

Accumulated Other Comprehensive Income (Loss) - Schedule of Total Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	$ 473,974	$ 531,090	$ 184,945
Ending balance	383,454	473,974	531,090
Unrealized Gain (Loss) on Securities Available-for-Sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(417)	(868)	(174)
Current period other comprehensive income (loss)	35	451	(694)
Ending balance	(382)	(417)	(868)
Foreign Currency Translation Adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	1,322
Current period other comprehensive income (loss)	(10,110)	1,322
Reclassification from accumulated othercomprehensive income	3,490
Ending balance	(5,298)	1,322
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	905	(868)	(174)
Current period other comprehensive income (loss)	(10,075)	1,773	(694)
Reclassification from accumulated othercomprehensive income	3,490
Ending balance	$ (5,680)	$ 905	$ (868)

Quarterly Financial Data (una68

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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Selected Quarterly Financial Information [Abstract]
Revenue	$ 2,414	$ 198			$ 5	$ 111	$ 17		$ 476	$ 133
Operating expenses	99,248	83,911	$ 78,441	$ 69,954	70,554	65,894	58,070	$ 53,622	331,554	248,140	$ 147,738
Net loss	$ (81,731)	$ (79,227)	$ (72,891)	$ (68,290)	$ (71,287)	$ (64,907)	$ (56,923)	$ (52,757)	$ (302,139)	$ (245,874)	$ (145,618)
Net loss per share, basic and diluted	$ (1.89)	$ (1.87)	$ (1.72)	$ (1.63)	$ (1.75)	$ (1.64)	$ (1.46)	$ (1.35)	$ (7.12)	$ (6.21)	$ (3.96)

Subsequent Events - Additional

Subsequent Events - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	Jan. 10, 2018	Jan. 31, 2018	Feb. 21, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Subsequent Event [Line Items]
Net proceeds from sale of common stock						$ 461,136
ATM Sales Agreement
Subsequent Event [Line Items]
Common stock shares sold				912,351	1,159,415
Net proceeds from sale of common stock				$ 67,600	$ 79,500
Common stock shares sold				912,351	1,159,415
Subsequent Event
Subsequent Event [Line Items]
Sale of PRV	$ 130,000
Subsequent Event | ATM Sales Agreement
Subsequent Event [Line Items]
Common stock shares sold			240,417
Net proceeds from sale of common stock			$ 11,800
Common stock shares sold			240,417
Subsequent Event | Underwritten Public Offering
Subsequent Event [Line Items]
Common stock shares sold		5,043,860
Net proceeds from sale of common stock		$ 271,000
Common stock shares sold		5,043,860
Shares purchased by underwriters		657,895
Public offering price		$ 57