Atara Biotherapeutics (ATRA) 10-K 27 Feb 20 2019 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2019	Feb. 18, 2020	Jun. 28, 2019
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	FY
Trading Symbol	ATRA
Entity Registrant Name	ATARA BIOTHERAPEUTICS, INC.
Entity Central Index Key	0001604464
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		58,571,550
Entity Public Float			$ 760,739,420
Entity Shell Company	false
Entity Small Business	false
Entity Emerging Growth Company	false
Entity File Number	001-36548
Entity Tax Identification Number	46-0920988
Entity Address, Address Line One	611 Gateway Blvd.
Entity Address, Address Line Two	Suite 900
Entity Address, City or Town	South San Francisco
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94080
City Area Code	650
Local Phone Number	278-8930
Entity Incorporation, State or Country Code	DE
Title of 12(b) Security	Common Stock, par value $0.0001 per share
Security Exchange Name	NASDAQ
Entity Interactive Data Current	Yes
Document Annual Report	true
Document Transition Report	false
Documents Incorporated by Reference	Portions of the Registrant’s definitive proxy statement relating to its 2020 Annual Meeting of Stockholders are incorporated by reference into Part III of this Report where indicated. Such proxy statement will be filed with the U.S. Securities and Exchange Commission within 120 days after the end of the fiscal year to which this report relates.

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 74,317	$ 60,698
Short-term investments	184,792	248,933
Restricted cash - short-term	194	194
Prepaid expenses and other current assets	13,689	11,664
Total current assets	272,992	321,489
Property and equipment, net	54,176	68,576
Operating lease assets	14,007	0
Restricted cash - long-term	1,200	1,200
Other assets	567	574
Total assets	342,942	391,839
Current liabilities:
Accounts payable	7,963	3,719
Accrued compensation	14,706	10,636
Accrued research and development expenses	8,341	19,210
Other current liabilities	5,733	6,414
Total current liabilities	36,743	39,979
Operating lease liabilities - long-term	14,136	0
Other long-term liabilities	1,282	13,003
Total liabilities	52,161	52,982
Commitments and contingencies (Note 8)
Stockholders’ equity:
Common stock—$0.0001 par value, 500,000 shares authorized as of December 31, 2019 and 2018, respectively; 56,806 and 45,951 shares issued and outstanding as of December 31, 2019 and 2018, respectively	6	5
Additional paid-in capital	1,108,516	866,541
Accumulated other comprehensive income (loss)	220	(340)
Accumulated deficit	(817,961)	(527,349)
Total stockholders’ equity	290,781	338,857
Total liabilities and stockholders’ equity	$ 342,942	$ 391,839

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2019	Dec. 31, 2018
Statement Of Financial Position [Abstract]
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	500,000,000	500,000,000
Common stock, shares issued	56,806,000	45,951,000
Common stock, shares outstanding	56,806,000	45,951,000

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating expenses:
Research and development	$ 216,097	$ 167,457	$ 81,206
General and administrative	79,584	69,654	40,326
Total operating expenses	295,681	237,111	121,532
Loss from operations	(295,681)	(237,111)	(121,532)
Interest and other income, net	4,717	6,368	2,027
Loss before provision for income taxes	(290,964)	(230,743)	(119,505)
Provision for (benefit from) income taxes	12	(44)	(14)
Net loss	(290,976)	(230,699)	(119,491)
Other comprehensive gain (loss):
Unrealized gain (loss) on available-for-sale securities	560	(189)	32
Comprehensive loss	$ (290,416)	$ (230,888)	$ (119,459)
Net loss per common share:
Basic and diluted net loss per common share	$ (5.67)	$ (5.27)	$ (4)
Weighted-average shares outstanding used to calculate basic and diluted net loss per common share	51,308	43,811	29,863

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Underwritten Public Offering	At The Market Offering	Common Stock	Common StockUnderwritten Public Offering	Common StockAt The Market Offering	Additional Paid-in Capital	Additional Paid-in CapitalUnderwritten Public Offering	Additional Paid-in CapitalAt The Market Offering	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Beginning balance at Dec. 31, 2016	$ 253,736			$ 3			$ 431,075			$ (183)	$ (177,159)
Beginning balance (in shares) at Dec. 31, 2016				28,933
Issuance of common stock, net of commissions and offering costs, value			$ 19,156						$ 19,156
Issuance of common stock, net of commissions and offering costs, shares						1,350
RSU settlements, net of shares withheld	(357)						(357)
RSU settlements, net of shares withheld, shares				305
Issuance of common stock pursuant to employee stock awards	1,688						1,688
Issuance of common stock pursuant to employee stock awards, shares				142
Stock-based compensation expense	23,100						23,100
Net loss	(119,491)										(119,491)
Unrealized gain (loss) on available-for-sale securities	32									32
Ending balance at Dec. 31, 2017	177,864			$ 3			474,662			(151)	(296,650)
Ending balance (in shares) at Dec. 31, 2017				30,730
Issuance of common stock, net Of offering costs, values		$ 293,290			$ 2			$ 293,288
Issuance of common stock, net Of offering costs, shares					12,604
Issuance of common stock, net of commissions and offering costs, value			47,586						47,586
Issuance of common stock, net of commissions and offering costs, shares						1,008
RSU settlements, net of shares withheld	(7,503)						(7,503)
RSU settlements, net of shares withheld, shares				449
Issuance of common stock pursuant to employee stock awards	24,691						24,691
Issuance of common stock pursuant to employee stock awards, shares				1,160
Stock-based compensation expense	33,817						33,817
Net loss	(230,699)										(230,699)
Unrealized gain (loss) on available-for-sale securities	(189)									(189)
Ending balance at Dec. 31, 2018	338,857			$ 5			866,541			(340)	(527,349)
Ending balance (in shares) at Dec. 31, 2018				45,951
Effect of the adoption of ASC topic 842 (Leases) at Jan. 1, 2019	364										364
Balance at Jan. 1, 2019	339,221			$ 5			866,541			(340)	(526,985)
Issuance of common stock and pre-funded warrants through underwritten offering, net of offering costs	140,716			$ 1			140,715
Issuance of common stock and pre-funded warrants, net of offering costs, shares				6,872
Issuance of common stock, net of commissions and offering costs, value			$ 48,909						$ 48,909
Issuance of common stock, net of commissions and offering costs, shares						3,135
RSU settlements, net of shares withheld	(6,695)						(6,695)
RSU settlements, net of shares withheld, shares				361
Issuance of common stock pursuant to employee stock awards	7,350						7,350
Issuance of common stock pursuant to employee stock awards, shares				487
Stock-based compensation expense	51,696						51,696
Net loss	(290,976)										(290,976)
Unrealized gain (loss) on available-for-sale securities	560									560
Ending balance at Dec. 31, 2019	$ 290,781			$ 6			$ 1,108,516			$ 220	$ (817,961)
Ending balance (in shares) at Dec. 31, 2019				56,806

Consolidated Statements of St_2

Consolidated Statements of Stockholders' Equity (Parenthetical) - Common Stock - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
At The Market Offering
Stock issuance, discounts, commissions and offering costs	$ 1,553	$ 1,310	$ 844
Underwritten Public Offering
Stock issuance, discounts, commissions and offering costs		$ 526

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating activities
Net loss	$ (290,976)	$ (230,699)	$ (119,491)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	51,696	33,817	23,100
Depreciation and amortization expense	7,070	3,732	956
(Accretion) amortization of investment (discounts) premiums	(1,330)	(1,885)	732
Loss on disposals of property and equipment	1,027	0	0
Non-cash operating lease expense	964	0	0
Non-cash interest expense	0	211	0
Asset retirement obligation accretion expense	71	49	0
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(998)	(5,764)	(784)
Operating lease assets	239	0	0
Other assets	322	(314)	2
Accounts payable	4,213	(1,958)	2,163
Accrued compensation	4,070	4,972	1,919
Accrued research and development expenses	(10,869)	15,204	1,598
Other current liabilities	(394)	2,491	1,896
Operating lease liabilities	(731)	0	0
Other long-term liabilities	0	372	407
Net cash used in operating activities	(235,626)	(179,772)	(87,502)
Investing activities
Purchases of short-term investments	(270,230)	(466,489)	(176,459)
Proceeds from maturities and sales of short-term investments	336,261	306,125	296,600
Purchases of property and equipment	(5,733)	(35,925)	(20,232)
Proceeds from sale of property and equipment	161	0	0
Net cash provided by (used in) investing activities	60,459	(196,289)	99,909
Financing activities
Proceeds from sale of common stock in underwritten offerings, net	140,888	293,290	0
Proceeds from issuance of common stock through ATM facilities, net	47,729	47,586	19,156
Proceeds from employee stock awards	7,350	24,691	1,249
Taxes paid related to net share settlement of restricted stock units	(6,695)	(7,503)	(357)
Principal payments on finance and capital lease obligations	(486)	(528)	0
Net cash provided by financing activities	188,786	357,536	20,048
Increase (decrease) in cash, cash equivalents and restricted cash	13,619	(18,525)	32,455
Cash, cash equivalents and restricted cash at beginning of period	62,092	80,617	48,162
Cash, cash equivalents and restricted cash at end of period	75,711	62,092	80,617
Non-cash investing and financing activities
Proceeds from issuance of common stock through ATM facilities received subsequent to December 31, 2019	1,185	0	0
Property and equipment purchases included in accounts payable and other accrued liabilities	276	1,579	10,122
Accrued costs related to underwritten public offering	172	0	160
Capitalized lease obligations	0	441	9,904
Property and equipment acquired under capital leases	0	191	1,076
Asset retirement costs	0	88	580
Interest capitalized during construction period for build-to-suit lease arrangement	0	77	264
Proceeds from options exercised received subsequent to December 31, 2017	0	0	439
Supplemental cash flow disclosure
Cash paid for interest	50	240	0
Cash paid for taxes	$ 0	$ 0	$ 0

Description of Business

Description of Business	12 Months Ended
	Dec. 31, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Description of Business	1. Description of Business Atara Biotherapeutics, Inc. (“Atara”, “we”, “our” or “the Company”) was incorporated in August 2012 in Delaware. Atara is a leading off-the-shelf, allogeneic T-cell immunotherapy company that is cancer, autoimmune and viral diseases. We have several T-cell immunotherapies in clinical development and are progressing a next-generation allogeneic chimeric antigen receptor T-cell (“CAR T”) program. We have licensed rights to T-cell product candidates from Memorial Sloan Kettering Cancer Center (“MSK”), rights related to our next-generation CAR T programs from MSK and from Moffitt Cancer Center, and rights to know-how and technology from the Council of the Queensland Institute of Medical Research

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation We prepare our consolidated financial statements in accordance with accounting principles generally accepted in the United States of America (“U.S. GAAP”) and follow the rules and regulations of the U.S. Securities and Exchange Commission (“SEC”). Principles of Consolidation The consolidated financial statements include the accounts of Atara and our wholly owned subsidiaries. All intercompany balances and transactions are eliminated in consolidation. Segment and Geographic Information We operate and manage our business as one operating and reportable segment, which is the business of developing and commercializing therapeutics. Our Chief Executive Officer, who is our chief operating decision maker, reviews financial information on an aggregate basis for purposes of allocating resources and evaluating financial performance. Substantially all of our assets are located in the U.S. Liquidity Risk We have incurred significant operating losses since inception and have relied on public and private equity financings to fund our operations. As of December 31, 2019, we had an accumulated deficit of $818.0 million. As we continue to incur losses, our transition to profitability will depend on the successful development, approval and commercialization of product candidates and on the achievement of sufficient revenues to support our cost structure. We may never achieve profitability, and unless and until we do, we will need to continue to raise additional capital. We expect that existing cash, cash equivalents and short-term investments as of December 31, 2019, together with net proceeds from the sale of common stock from our 2019 ATM Facility, as defined in Note 9, in January 2020, will be sufficient to fund our planned operations into the second quarter of 2021. Concentration of Credit Risk and Other Uncertainties We place cash and cash equivalents in the custody of financial institutions that management believes are of high credit quality, the amount of which at times, may be in excess of the amount insured by the Federal Deposit Insurance Corporation. We also make short-term investments in money market funds; U.S. Treasury, government agency and corporate debt obligations; commercial paper; certificates of deposit; and asset-backed securities, which can be subject to certain credit risk. However, we mitigate the risks by investing in high-grade instruments, limiting our exposure to any one issuer and monitoring the ongoing creditworthiness of the financial institutions and issuers. We are subject to certain risks and uncertainties and believe that changes in any of the following areas could have a material adverse effect on future financial position or results of operations: our ability to obtain future financing; regulatory approval and market acceptance of, and reimbursement for, our product candidates, if approved by applicable regulatory authorities; performance of third-party clinical research organizations and manufacturers upon which we rely; development of sales channels; protection of our intellectual property; litigation or claims against us based on intellectual property, patent, product, regulatory or other factors; and our ability to attract and retain employees necessary to support our growth. Use of Estimates The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates, assumptions and judgments that affect the amounts reported in the financial statements and accompanying notes. Significant estimates relied upon in preparing these financial statements include estimates related to clinical study and other accruals, lease assets and liabilities, stock-based compensation expense and income taxes. Actual results could differ materially from those estimates. Leases We determine if an arrangement is a lease at inception. Operating leases are included in operating lease assets, other current liabilities, and operating lease liabilities on our consolidated balance sheets. Leases with an initial term of 12 months or less are not recorded on the balance sheet; we recognize lease expense for these leases on a straight-line basis over the lease term. Finance leases are included in other assets, other current liabilities, and other long-term liabilities on our consolidated balance sheets. Lease assets and lease liabilities are recognized based on the present value of the future minimum lease payments over the lease term at commencement date. The lease term includes renewal options that we are reasonably certain of exercising as of the commencement date. None of the lease terms used to calculate the future minimum lease payments at commencement date include renewal options. As most of our leases do not provide an implicit rate, we use our incremental borrowing rate based on the information available at commencement date in determining the present value of future payments. The incremental borrowing rate for our leases is determined based on lease term and currency in which lease payments are made, adjusted for impacts of collateral. Lease assets also includes any lease payments made and excludes lease incentives and initial direct costs incurred. Operating lease expense for minimum lease payments is recognized on a straight-line basis over the lease term. Finance lease assets are amortized over the shorter of the lease term or the asset’s estimated useful life. Our facilities and equipment operating leases have lease and non-lease components and we have made a policy election to account for the lease and non-lease components as a single lease component. Through December 31, 2018, the leases were reviewed for classification as operating, capital or build-to-suit leases. For operating leases, rent was recognized on a straight-line basis over the lease period. For capital leases, we recorded the leased asset with a corresponding liability for principal and interest. Payments were recorded as reductions to these liabilities with interest being charged to interest expense in our consolidated statements of operations and comprehensive loss. We analyzed the nature of the renovations and our involvement during the construction period of our manufacturing facility and determined that we were the deemed “owner” of the construction project during the construction period. As a result, we were required to capitalize the fair value of the building as well as the construction costs incurred on our consolidated balance sheet along with a corresponding financing liability for landlord-paid construction costs (i.e. “build-to-suit” accounting). Once construction was complete, the Company considered the requirements for sale-leaseback accounting treatment, including evaluating whether all risks of ownership have been transferred back to the landlord, as evidenced by a lack of continuing involvement in the leased property. Since the arrangement did not qualify for sale-leaseback accounting treatment, the building asset remained on the Company’s consolidated balance sheets at its historical cost, and such asset was depreciated over its estimated useful life. The Company bifurcated its lease payments into a portion allocated to the building and a portion allocated to the parcel of land on which the building has been built. The portion of the lease payments allocated to the land was treated for accounting purposes as operating lease payments, and therefore was recorded as rent expense in the consolidated statements of operations and comprehensive loss. The portion of the lease payments allocated to the building was further bifurcated into a portion allocated to interest expense and a portion allocated to reduce the build-to-suit lease obligation. The initial recording of these assets and liabilities were classified as non-cash investing and financing items, respectively, for purposes of the consolidated statements of cash flows. The build-to-suit asset and corresponding lease obligation was derecognized upon adoption of the new lease standard as we did not control the building during the construction period. Asset Retirement Obligations (“ARO”) ARO are legal obligations associated with the retirement of long-lived assets pertaining to leasehold improvements. These liabilities are initially recorded at fair value and the related asset retirement costs are capitalized by increasing the carrying amount of the related assets by the same amount as the liability. Asset retirement costs are subsequently depreciated over the useful lives of the related assets. Subsequent to initial recognition, the Company records period-to-period changes in the ARO liability resulting from the passage of time and revisions to either the timing or the amount of the original estimate of undiscounted cash flows. The Company derecognizes ARO liabilities when the related obligations are settled. Foreign Currency Transactions and monetary assets and liabilities that are denominated in a foreign currency are translated into U.S. dollars at the current exchange rate on the transaction date and as of each balance sheet date, respectively, with gains or losses on foreign exchange changes recognized in interest and other income (expense), net in the consolidated statements of operations and comprehensive loss. Foreign currency-denominated monetary assets and liabilities as of December 31, 2019 were not material. Cash Equivalents and Short-Term Investments Cash equivalents are defined as short-term, highly liquid investments with original maturities of 90 days or less at the date of purchase, and generally consist of money market funds, U.S. Treasury, government agency and corporate debt obligations, and commercial paper. Investments with original maturities of greater than 90 days are classified as short-term investments on the balance sheet, and consist primarily of U.S. Treasury, government agency and corporate debt obligations, commercial paper and asset-backed securities. As our entire investment portfolio is considered available for use in current operations, we classify all investments as available-for-sale and as current assets, even though the stated maturity may be more than one year from the current balance sheet date. Available-for-sale securities are carried at fair value, with unrealized gains and losses reported in accumulated other comprehensive loss, which is a separate component of stockholders’ equity in the consolidated balance sheet. The amortized cost of securities is adjusted for amortization of premiums and accretion of discounts to maturity, which are both recorded to interest and other income (expense), net in the consolidated statements of operations and comprehensive loss. Changes in the fair value of available-for-sale securities impact the consolidated statements of operations and comprehensive loss only when such securities are sold or if an other-than-temporary impairment is recognized. Realized gains and losses on the sale of securities are determined by specific identification of each security’s cost basis. We regularly review our investment portfolio to determine if any security is other-than-temporarily impaired, which would require us to record an impairment charge in the period any such determination is made. In making this judgment, we evaluate, among other things, the duration and extent to which the fair value of a security is less than its cost, the financial condition of the issuer and any changes thereto, our intent to sell, or whether it is more likely than not that we will be required to sell the security before recovery of its amortized cost basis. Our assessment on whether a security is other-than-temporarily impaired could change in the future due to new developments or changes in assumptions related to any particular security. Realized gains and losses and declines in value judged to be other-than-temporary on available-for-sale securities, if any, are recorded to interest and other income (expense), net in the statements of operations and comprehensive loss. Fair Value Measurement The carrying amounts of certain of our financial instruments including cash equivalents, prepaid expenses and other current assets, accounts payable and accrued liabilities approximate fair value due to their short maturities. Short-term investments are comprised of available-for-sale securities, which are carried at fair value. Fair Value of Financial Instruments Our financial assets are measured at fair value on a recurring basis using the following hierarchy to prioritize valuation inputs, Level1: Quoted prices in active markets for identical assets or liabilities that we have the ability to access Level2: Observable market-based inputs or unobservable inputs that are corroborated by market data such as quoted prices, interest rates and yield curves Level 3: Inputs that are unobservable data points that are not corroborated by market data We review the fair value hierarchy classification on a quarterly basis. Changes in the ability to observe valuation inputs may result in a reclassification of levels of certain securities within the fair value hierarchy. We recognize transfers into and out of levels within the fair value hierarchy in the period in which the actual event or change in circumstances that caused the transfer occurs. There have been no transfers between Level 1, Level 2, and Level 3 in any periods presented. Financial assets and liabilities are considered Level 2 when their fair values are determined using inputs that are observable in the market or can be derived principally from or corroborated by observable market data such as pricing for similar securities, recently executed transactions, cash flow models with yield curves, and benchmark securities. In addition, Level 2 financial instruments are valued using comparisons to like-kind financial instruments and models that use readily observable market data as their basis. U.S. Treasury, government agency and corporate debt obligations, commercial paper and asset-backed securities are valued primarily using market prices of comparable securities, bid/ask quotes, interest rate yields and prepayment spreads and are included in Level 2. Financial assets and liabilities are considered Level 3 when their fair values are determined using pricing models, discounted cash flow methodologies, or similar techniques, and at least one significant model assumption or input is unobservable. We have no Level 3 financial assets or liabilities. Property and Equipment, net Property and equipment are stated at cost and depreciated using the straight-line method over the estimated useful lives of the assets, ranging from three to five years. Costs incurred to acquire, construct or install property and equipment during the construction stage of a capital project or costs incurred to purchase and develop internal use software during the application development stage are recorded as construction in progress. Leasehold improvements are amortized over the lesser of the life of the leasehold improvements or the lease term. Maintenance and repairs are charged to operations as incurred. Long-lived Assets We evaluate the carrying amount of our long-lived assets whenever events or changes in circumstances indicate that the assets may not be recoverable. An impairment loss would be recognized when estimated future cash flows expected to result from the use of the asset and its eventual disposition are less than the carrying amount of the asset. To date, there have been no such impairment losses. Stock-Based Compensation Expense We account for stock-based compensation expense, including the expense of restricted common stock awards (“RSAs”), grants of restricted stock units (“RSUs”), and stock options that may be settled in shares of our common stock, based on the fair values of the equity instruments issued. The fair value is determined on the measurement date, which is generally the date of grant. The fair value for our RSAs is their intrinsic value, which is the difference between the fair value of the underlying stock at the measurement date and the purchase price. The fair value of our RSUs is the fair value of the underlying stock at the measurement date. The fair value for our stock option awards is determined at the grant date using the Black-Scholes valuation model. For awards with performance-based vesting criteria, we assess the probability of the achievement of the performance conditions at the end of each reporting period and recognize the share-based compensation costs when it becomes probable that the performance conditions will be met. For awards that are subject to both service and performance conditions, no expense is recognized until it is probable that performance conditions will be met. Stock-based compensation expense for awards with time-based vesting criteria is recognized as expense on a straight-line basis over the requisite service period. Stock-based compensation expense for awards with performance and other vesting criteria is recognized as expense under an accelerated graded vesting model. Key assumptions used in the Black-Scholes valuation model used for employee stock awards include: Expected term – We derived the expected term using the “simplified” method (the expected term is determined as the average of the time-to-vesting and the contractual life of the options), as we have limited historical information to develop expectations about future exercise patterns and post vesting employment termination behavior. Expected volatility – Expected volatility is estimated using comparable public companies’ volatility for similar terms. Expected dividend – We have not historically declared or paid dividends to our stockholders and have no plans to pay dividends; therefore, we assumed an expected dividend yield of 0%. Risk-free interest rate – The risk-free interest rate is based on the yield on U.S. Treasury securities with the expected term of the associated award. The fair value of our common stock is based on observable market prices. We account for forfeitures of stock-based awards as they occur. Research and Development Expense Research and development expense consists of costs incurred in performing research and development activities, including compensation and benefits for research and development employees, including stock-based compensation; expenses incurred under agreements with contract research organizations and investigative sites that conduct clinical and preclinical studies; the costs of acquiring and manufacturing clinical study materials and other supplies; payments under licensing and research and development agreements; other outside services and consulting costs, and an allocation of facility, information technology and overhead expenses. Research and development costs are expensed as incurred. Clinical Study Accruals Costs for preclinical studies, clinical studies and manufacturing activities are recognized based on an evaluation of our vendors’ progress towards completion of specific tasks, using data such as patient enrollment, clinical site activations or information provided to us by our vendors regarding their actual costs incurred. Payments for these activities are based on the terms of individual contracts and payment timing may differ significantly from the period in which the services are performed. We determine accrual estimates through reports from and discussions with applicable personnel and outside service providers as to the progress or state of completion of studies, or the services completed. Our estimates of accrued expenses as of each balance sheet date are based on the facts and circumstances known at the time. Costs that are paid in advance of performance are deferred as a prepaid expense and amortized over the service period as the services are provided. Defined Contribution Plan We have one qualified 401(k) plan covering all eligible employees. Under the plan, employees may contribute up to the statutory allowable amount for any calendar year. Beginning in 2019, we make matching contributions, equal to 50% of each dollar contributed up to the first 6% of an individual’s eligible earnings, up to the annual IRS maximum. For the year ended December 31, 2019, we recorded matching contributions of approximately $1.6 million. Other Current Liabilities Other current liabilities consisted of the following as of each period end: December 31, December 31, 2019 2018 (in thousands) Accrued operating expenses $ 3,900 $ 5,627 Current portion of operating lease liabilities 1,312 — Current portion of finance and capital lease liabilities 269 587 Other accrued liabilities 252 200 Total other current liabilities $ 5,733 $ 6,414 Income Taxes We use the asset and liability method to account for income taxes. We record deferred tax assets and liabilities for the expected future tax consequences of temporary differences between the financial statement carrying amounts and the tax basis of assets and liabilities using enacted tax rates expected to be in effect when the differences are expected to reverse. Valuation allowances are provided when necessary to reduce net deferred tax assets to the amount that is more likely than not to be realized. Based on the available evidence, we are unable, at this time, to support the determination that it is more likely than not that our deferred tax assets will be utilized in the future. Accordingly, we recorded a full valuation allowance as of December 31, 2019 and 2018. We intend to maintain valuation allowances until sufficient evidence exists to support their reversal. Tax benefits related to uncertain tax positions are recognized when it is more likely than not that a tax position will be sustained during an audit. Interest and penalties related to unrecognized tax benefits are included within the provision for income tax. Comprehensive Income (Loss) Comprehensive income (loss) is defined as a change in equity of a business enterprise during a period resulting from transactions from non-owner sources. Our other comprehensive income (loss) is comprised solely of unrealized gains (losses) on available-for-sale securities and is presented net of taxes. We have not recorded any reclassifications from other comprehensive income (loss) to net loss during any period presented. Recent Accounting Pronouncements The Company considers the applicability and impact of any Accounting Standards Update (“ASU”) issued by the Financial Accounting Standards Board (“FASB”). ASUs not listed below were assessed and determined to be either not applicable or are expected to have minimal impact on our consolidated financial statements. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments - Credit Losses: Measurement of Credit Losses on Financial Instruments In August 2018, the FASB issued ASU No. 2018-15, Intangibles—Goodwill and Other—Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract In December 2019, the FASB issued ASU No. 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes Adoption of New Accounting Pronouncements We adopted ASU No. 2016-02, Leases (Topic 842), as of January 1, 2019, using the optional transition method, which allows for the initial application of the new accounting standard at the adoption date and the recognition of a cumulative-effect adjustment to the opening balance of retained earnings as of the beginning of the period of adoption. In addition, we elected the package of practical expedients permitted under the transition guidance within the new standard, which among other things, allowed us to carry forward the historical lease classification. In addition, we elected the hindsight practical expedient to determine the lease term for existing leases. Adoption of the new standard resulted in the recording of additional operating lease assets and operating lease liabilities of $14.3 million and $15.3 million, respectively, as of January 1, 2019. This was partially offset by de-recognition of the build-to-suit asset and corresponding lease obligation of $10.3 million for our Thousand Oaks manufacturing facility lease, as we did not control the building during the construction period (see Note 7). We evaluated the impact of the new standard for deferred tax accounting purposes and recorded corresponding deferred tax assets and liabilities upon adoption of ASC 842. The deferred tax assets are offset by the valuation allowance on deferred taxes. The cumulative effect adjustment to the opening balance of accumulated deficit was a decrease of $0.4 million. The standard did not have a significant impact on our consolidated statements of operations and comprehensive loss, changes in stockholders’ equity, and cash flow for the years ended December 31, 2019, 2018 and 2017. We adopted ASU No. 2018-02, Income Statement – Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income

Net Loss per Common Share

Net Loss per Common Share	12 Months Ended
	Dec. 31, 2019
Earnings Per Share [Abstract]
Net Loss per Common Share	3. Net Loss per Common Share Basic net loss per common share is calculated by dividing net loss by the weighted-average number of shares of common stock and pre-funded warrants outstanding during the period, without consideration of common share equivalents. Diluted net loss per common share is computed by dividing net loss by the weighted-average number of shares of common stock, pre-funded warrants and common share equivalents outstanding for the period. The pre-funded warrants are included in the computation of basic and diluted net loss per common share as the exercise price is negligible and the pre-funded warrants are fully vested and exercisable. Common share equivalents are only included in the calculation of diluted net loss per common share when their effect is dilutive. Potential dilutive securities, which include unvested restricted stock units (“RSUs”), vested and unvested options to purchase common stock and shares to be issued under our employee stock purchase plan (“ESPP”), have been excluded from the computation of diluted net loss per share as the effect is antidilutive. Therefore, the denominator used to calculate both basic and diluted net loss per common share is the same in all periods presented. The following table represents the potential common shares issuable pursuant to outstanding securities as of the related period end dates that were excluded from the computation of diluted net loss per common share as their inclusion would have an antidilutive effect: As of December 31, 2019 2018 2017 Unvested RSUs 1,910,764 1,405,460 1,685,000 Vested and unvested options 6,934,262 6,276,999 5,229,648 ESPP share purchase rights 20,438 7,974 14,905 Total 8,865,464 7,690,433 6,929,553

Financial Instruments

Financial Instruments	12 Months Ended
	Dec. 31, 2019
Financial Instruments Disclosure [Abstract]
Financial Instruments	4. Financial Instruments The following tables summarize the estimated fair value and related valuation input hierarchy of our available-for-sale securities as of each period end: Total Total Total Total Amortized Unrealized Unrealized Estimated As of December 31, 2019: Input Level Cost Gain Loss Fair Value (in thousands) Money market funds Level 1 $ 63,554 $ — $ — $ 63,554 U.S. Treasury obligations Level 2 52,805 46 (1 ) 52,850 Government agency obligations Level 2 6,151 1 (1 ) 6,151 Corporate debt obligations Level 2 100,512 180 (10 ) 100,682 Commercial paper Level 2 26,290 — — 26,290 Asset-backed securities Level 2 7,266 6 — 7,272 Certificate of deposit Level 2 500 — — 500 Total available-for-sale securities 257,078 233 (12 ) 257,299 Less amounts classified as cash equivalents (72,507 ) — — (72,507 ) Amounts classified as short-term investments $ 184,571 $ 233 $ (12 ) $ 184,792 Total Total Total Total Amortized Unrealized Unrealized Estimated As of December 31, 2018: Input Level Cost Gain Loss Fair Value (in thousands) Money market funds Level 1 $ 38,708 $ — $ — $ 38,708 U.S. Treasury obligations Level 2 111,164 4 (80 ) 111,088 Government agency obligations Level 2 15,206 1 (32 ) 15,175 Corporate debt obligations Level 2 121,017 15 (217 ) 120,815 Commercial paper Level 2 12,935 — — 12,935 Asset-backed securities Level 2 11,894 — (31 ) 11,863 Total available-for-sale securities 310,924 20 (360 ) 310,584 Less amounts classified as cash equivalents (61,651 ) — — (61,651 ) Amounts classified as short-term investments $ 249,273 $ 20 $ (360 ) $ 248,933 The amortized cost and fair value of our available-for-sale securities by contractual maturity were as follows: As of December 31, 2019 As of December 31, 2018 Amortized Estimated Amortized Estimated Cost Fair Value Cost Fair Value (in thousands) (in thousands) Maturing within one year $ 214,085 $ 214,199 $ 287,755 $ 287,469 Maturing in one to five years 42,993 43,100 23,169 23,115 Total available-for-sale securities $ 257,078 $ 257,299 $ 310,924 $ 310,584 As of December 31, 2019 , certain available-for-sale securities had been in a continuous unrealized loss position, each for less than twelve months. As of this date, no significant facts or circumstances were present to indicate a deterioration in the creditworthiness of the respective issuers . Because we do not intend to sell these securities and it is not more likely than not that we will be required to sell these securities before recovery of their amortized cost bases, which may be maturity, we do not consider these investments to be other-than-temporarily impaired at December 31, 2019 . During the years ended December 31, 2019 , 2018 and 2017 , we did not recognize any other-than-temporary impairment losses . In addition, restricted cash collateralized by money market funds is a financial asset measured at fair value and is a Level 1 financial instrument under the fair value hierarchy. As of December 31, 2019 and 2018, restricted cash totaled $1.4 million. The following table provides a reconciliation of cash, cash equivalents and restricted cash within the consolidated balance sheets that sum to the total of the same such amounts in the consolidated statement of cash flows: December 31, December 31, 2019 2018 (in thousands) Cash and cash equivalents $ 74,317 $ 60,698 Restricted cash - short-term 194 194 Restricted cash - long-term 1,200 1,200 Total cash, cash equivalents and restricted cash $ 75,711 $ 62,092

Property and Equipment

Property and Equipment	12 Months Ended
	Dec. 31, 2019
Property Plant And Equipment [Abstract]
Property and Equipment	5. Property and Equipment Property and equipment consisted of the following as of each period end: December 31, December 31, 2019 2018 (in thousands) Leasehold improvements $ 49,028 $ 47,609 Build-to-suit asset (see Note 7) — 10,686 Lab equipment 6,815 3,019 Machinery and equipment 3,832 2,980 Computer equipment and software 3,299 3,049 Furniture and fixtures 1,764 1,628 Construction in progress 1,116 4,682 Property and equipment, gross 65,854 73,653 Less accumulated depreciation and amortization (11,678 ) (5,077 ) Property and equipment, net $ 54,176 $ 68,576 Construction in progress represents capitalized costs for our manufacturing facility in Thousand Oaks, California and capitalizable costs incurred for equipment held at our facilities. Depreciation and amortization expense was $7.1 million, $3.7 million and $1.0 million for the years ended December 31, 2019 , 2018 and 2017

License, Collaboration and Manu

License, Collaboration and Manufacturing Agreements	12 Months Ended
	Dec. 31, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
License, Collaboration and Manufacturing Agreements	6. License, Collaboration and Manufacturing Agreements MSK Agreements – In June 2015, we entered into an exclusive license agreement with MSK for three clinical stage T-cell therapies. In connection with the execution of the agreement, the Company paid $4.5 million in cash to MSK. We are required to make additional payments of up to $33.0 million to MSK based on achievement of specified regulatory and sales-related milestones, mid-single-digit percentage tiered royalty payments based on In addition, under certain circumstances, we are required to make certain minimum annual royalty payments to MSK, which are creditable against earned royalties owed for the same annual period. We are also required to pay a low double-digit percentage of any consideration we receive for sublicensing the licensed rights. In May and December 2018, we licensed additional technology from MSK. In connection with the effectiveness of the December 2018 license agreement, we made upfront cash payments of $12.5 million in first quarter of 2019, which were recorded as research and development expense in our consolidated statement of operations and comprehensive loss in the fourth quarter of 2018. We are obligated to make additional milestone payments based on achievement of specified development, regulatory and sales-related milestones as well as mid-single-digit percentage tiered royalty payments based on future sales of products resulting from the development of the licensed product candidates, if any. QIMR Berghofer Agreements – In October 2015, we entered into an exclusive license agreement and a research and development collaboration agreement with QIMR Berghofer. In consideration for the exclusive license, the Company paid $3.0 million in cash to QIMR Berghofer. Under the terms of the license agreement, we possess an exclusive, worldwide license to develop and commercialize allogeneic T-cell therapy programs utilizing technology and know-how developed by QIMR Berghofer. In September 2016, the exclusive license agreement and research and development collaboration agreement were amended and restated. Under the amended and restated agreements, we obtained an exclusive, worldwide license to develop and commercialize additional T-cell programs as well as the option to license additional technology in exchange for $3.3 million in cash, which was recorded as research and development expense in our consolidated statement of operations and comprehensive loss in the third quarter of 2016. We exercised this option in June 2018. We further amended and restated our license agreement and research and development collaboration agreements with QIMR Berghofer in August 2019 to eliminate our license to certain rights related to cytomegalovirus. Our current license agreement also provides for various milestone and royalty payments to QIMR Berghofer based on future product sales, if any. Under the terms of our current research and development collaboration agreement, we are also required to reimburse the cost of agreed-upon development activities related to programs developed under the collaboration. These payments are expensed on a straight-line basis over the related development periods. The agreement also provides for various milestone payments to QIMR Berghofer based on achievement of certain developmental and regulatory milestones. From time to time, we have entered into other license and collaboration agreements with other parties. For example, we licensed additional rights related to our MSK-partnered next-generation CAR T programs from MSK in May 2018 and from the National Institutes of Health in December 2018 and we licensed rights related to our next-generation CAR T programs from Moffitt Cancer Center in August 2018, and we agreed to collaborate in connection with each of these licenses. Milestones and royalties under each of the above agreements are contingent upon future events and will be recorded as expense when it is probable that the milestones will be achieved or royalties are due. As of December 31, 2019 and 2018, there were no outstanding obligations for milestones and royalties under our license and collaboration agreements. Cognate Agreement In December 2019, Atara entered into a Commercial Manufacturing Services Agreement (the “Manufacturing Agreement”) with Cognate Bioservices, Inc. (“Cognate”) to supersede the Development and Manufacturing Agreement that was entered into with Cognate in August 2015 and amended in December 2017, May 2018, November 2018, June 2019 and November 2019. Pursuant to the Manufacturing Agreement, Cognate provides manufacturing services for certain Atara product candidates. The initial term of the Manufacturing Agreement is from January 1, 2020 until December 31, 2021 and is renewable with Cognate’s approval for an additional one-year period. We may terminate the Manufacturing Agreement for convenience on six months’ written notice to Cognate, or immediately if Cognate is unable to perform the services under the Manufacturing Agreement or fails to obtain or maintain certain necessary approvals. The Manufacturing Agreement includes standard mutual termination rights for uncured breach or insolvency, or a force majeure event preventing the performance of services for at least ninety days.

Leases

Leases	12 Months Ended
	Dec. 31, 2019
Leases [Abstract]
Leases	7. Leases We lease our corporate headquarters in South San Francisco, California under a non-cancellable lease agreement that expires in April 2021 February 2026 April 2024 In February 2017, we entered into a lease agreement for approximately 90,580 square feet of office, lab and cellular therapy manufacturing space in Thousand Oaks, California. The initial 15-year term of the lease commenced on February 15, 2018, upon the substantial completion of landlord’s work as defined under the agreement. The contractual obligations during the initial term are $16.4 million in aggregate. Based on the terms of the lease agreement and on our involvement in certain aspects of the construction, we were deemed the owner of the building during the construction period in accordance with U.S. GAAP in effect prior to January 1, 2019. Under this build-to-suit lease arrangement, we recognized construction in progress based on all construction costs incurred by both us and the landlord. We also recognized a financing obligation equal to all costs funded by the landlord. Due to completion of the construction by the landlord and not having met the criteria for sale-lease back accounting, we transferred the $10.3 million of landlord’s construction costs previously capitalized as construction in progress to a build-to-suit asset, and recognized a corresponding long-term financing obligation for the same amount in long-term liabilities in our consolidated balance sheets. In addition, we recorded $0.3 million of capitalized interest during the construction period through December 31, 2018. A portion of the monthly lease payment was allocated to land rent and recorded as an operating lease expense and the non-interest portion of the amortized lease payments to the landlord related to rent of the building was applied to the lease financing liability. Further, we recorded ground lease expense of $0.4 and $0.3 million for the years ended December 31, 2018 and 2017, respectively, in our consolidated statement of operations and comprehensive loss, representing the estimated cost of renting the land during the construction period. Due to the adoption of ASU No. 2016-02, Leases (Topic 842), no ground lease expense was recognized for the year ended December 31, 2019. The maturities of lease liabilities under our operating and finance leases as of December 31, 2019 were as follows: Operating Leases Finance Leases Years Ending December 31, (in thousands) 2020 $ 2,867 $ 308 2021 2,740 127 2022 2,610 116 2023 2,685 29 2024 2,591 — Thereafter 11,887 — Total lease payments $ 25,380 $ 580 Less: amount representing interest (9,932 ) (67 ) Present value of lease liabilities $ 15,448 $ 513 Balance as of December 31, 2019 Other current liabilities $ 1,312 $ 269 Operating lease liabilities 14,136 — Other long-term liabilities — 244 Total $ 15,448 $ 513 The components of lease cost were as follows: Year Ended December 31, 2019 (in thousands) Operating lease cost: Operating lease cost $ 2,578 Short-term lease cost 770 Total operating lease cost $ 3,348 Finance lease cost: Amortization expense $ 324 Interest on lease liabilities 56 Total finance lease cost $ 380 Future minimum payments under our operating, finance and capital leases as of December 31, 2018 were as follows: Operating Leases Finance Leases Capital Leases Years Ending December 31, (in thousands) 2019 $ 1,107 $ 934 $ 540 2020 1,666 962 234 2021 1,555 991 29 2022 1,337 1,020 — 2023 1,375 1,051 — Thereafter 3,122 11,458 — Total minimum payments $ 10,162 $ 16,416 $ 803 Less: amount representing interest (65 ) Present value of capital lease obligations 738 Less: current portion (490 ) Capital lease obligation, net of current portion $ 248 Rent expense under operating leases for the years ended December 31, 2018 and 2017 was $2.2 million and $1.4 million, respectively. Other information related to leases was as follows: Year Ended December 31, 2019 (in thousands, except lease term and discount rate) Supplemental Cash Flows Information Cash paid for amounts included in the measurement of lease liabilities: Operating cash flows for operating leases $ 2,346 Operating cash flows for finance leases 50 Financing cash flows for finance leases 486 Operating lease assets obtained in exchange for lease obligations: $ 838 Finance lease assets obtained in exchange for lease obligations: 323 Weighted Average Remaining Lease Term Operating leases 10.3 years Finance leases 2.5 years Weighted Average Discount Rate Operating leases 10.4 % Finance leases 10.0 % Asset Retirement Obligation The Company’s ARO consists of a contractual requirement to remove the tenant improvements at our manufacturing facility in Thousand Oaks, California and restore the facility to a condition specified in the lease agreement. The Company records an estimate of the fair value of its ARO in long-term liabilities in the period incurred. The fair value of the ARO is also capitalized in property and equipment, net and depreciated over the lease term. The fair value of our ARO was estimated by discounting projected cash flows over the estimated life of the related assets using our credit adjusted risk-free rate. The following table presents the activity for our ARO liabilities: ARO Liability (In thousands) Balance as of December 31, 2018 $ 717 Accretion expense 71 Balance as of December 31, 2019 $ 788

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2019
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	8 . Commitments and Contingencies License and Collaboration Agreements Potential payments related to our license and collaboration agreements, including milestone and royalty payments, are detailed in Note 6. Other Research and Development Agreements We may enter into contracts in the normal course of business with clinical research organizations for clinical trials, with contract manufacturing clinical supplies, and with other vendors for preclinical studies, supplies and other services for our operating purposes. These contracts generally provide for termination on Indemnification Agreements In the normal course of business, we enter into contracts and agreements that contain a variety of representations and warranties and provide for indemnification for certain liabilities. The exposure under these agreements is unknown because it involves claims that may be made against us in the future but have not yet been made. To date, we have not paid any claims or been required to defend any action related to our indemnification obligations. However, we may record charges in the future as a result of these indemnification obligations. We also have indemnification obligations to our directors and executive officers for specified events or occurrences, subject to some limits, while they are serving at our request in such capacities. There have been no claims to date and we believe the fair value of these indemnification agreements is minimal. Accordingly, we did not record liabilities for these agreements as of December 31, 2019 and 2018. Contingencies From time to time, we may be involved in legal proceedings, as well as demands, claims and threatened litigation, which arise in the normal course of our business or otherwise. The ultimate outcome of any litigation is uncertain and unfavorable outcomes could have a negative impact on our results of operations and financial condition. Regardless of outcome, litigation can have an adverse impact on us because of the defense costs, diversion of management resources and other factors. We are not currently involved in any material legal proceedings.

Stockholders' Equity

Stockholders' Equity	12 Months Ended
	Dec. 31, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stockholders' Equity	9 . Stockholders’ Equity Our authorized capital stock consists of 520,000,000 shares, all with a par value of $0.0001 per share, of which 500,000,000 shares are designated as common stock and 20,000,000 shares are designated as preferred stock. There were no shares of preferred stock outstanding as of . Equity Offerings In January 2018, we completed an underwritten public offering of 7,675,072 shares of common stock at an offering price of $18.25 per share and received net proceeds of $131.4 million, after deducting underwriting discounts and commissions and offering expenses payable by us. Further, in March 2018, we completed an underwritten public offering of 4,928,571 shares of common stock at an offering price of $35.00 per share and received net proceeds of $161.9 million, after deducting underwriting discounts and commissions and offering expenses payable by us. In July 2019, we issued and sold 6,871,727 shares of common stock at a public offering price of $15.28 per share and pre-funded warrants to purchase 2,945,026 shares of common stock at an offering price of $15.2799 per warrant in an underwritten public offering pursuant to a shelf registration on Form S-3. The gross proceeds from this public offering were $150.0 million, resulting in aggregate net proceeds of $140.7 million, after deducting underwriting discounts and commissions and offering expenses payable by us. Each pre-funded warrant entitles the holder to purchase one share of common stock at an exercise price of $0.0001 per share and expires seven years from the date of issuance. These warrants were recorded as a component of stockholders’ equity within additional paid-in capital. Per the terms of the warrant agreement, a holder of the outstanding warrants is not entitled to exercise any portion of any pre-funded warrant if, upon exercise of the warrant, the holder’s ownership (together with its affiliates) of our common stock or combined voting power of our securities beneficially owned by such holder (together with its affiliates) would exceed 9.99% after giving effect to the exercise (“Maximum Ownership Percentage”). Upon at least 61 days’ prior notice to us by the holder, any holder may increase or decrease the Maximum Ownership Percentage to any other percentage not to exceed 19.99%. As of December 31, 2019, none of the pre-funded warrants had been exercised. ATM Facilities In March 2017, we entered into a sales agreement (the “2017 ATM Facility”) with Cowen and Company, LLC (“Cowen”), which provided for the sale, in our sole discretion, of shares of our common stock, in the aggregate offering price of up to $75.0 million through Cowen, as our sales agent. We paid a commission of up to 3.0% of the gross sales proceeds of any common stock sold under the 2017 ATM Facility. In February 2019, we terminated the 2017 ATM Facility and entered into a new sales agreement (the “2019 ATM Facility”) with Cowen, which provides for the sale, in our sole discretion, of shares of our common stock having an aggregate offering price of up to $100.0 million through Cowen, as our sales agent. During the fiscal year ended December 31, 2019 December 31, 2018 The issuance and sale of these shares by us pursuant to the 2019 ATM Facility and 2017 ATM Facility are deemed “at the market” offerings as defined in Rule 415 under the Securities Act of 1933, as amended (the “Securities Act”), and are registered under the Securities Act. As of December 31, 2019 In February 2020, we entered into a new sales agreement (the “2020 ATM Facility”) with Cowen, which provides for the sale, in our sole discretion, of shares of our common stock having an aggregate offering price of up to $100.0 million through Cowen, as our sales agent. The 2020 ATM Facility is separate from and does not replace the 2019 ATM Facility in any way. The issuance and sale of these shares by us pursuant to the 2020 ATM Facility are deemed “at the market” offerings and are registered under the Securities Act of 1933, as amended. We will pay a commission of up to 3.0% of gross sales proceeds of any common stock sold under the 2020 ATM Facility. Equity Incentive Plan s In March 2014, we adopted the 2014 Equity Incentive Plan (“2014 EIP”), which was amended and restated on October 15, 2014 upon the pricing of our initial public offering, or IPO. The 2014 EIP provides for annual increases in the number of shares available for issuance thereunder on the first business day of each fiscal year, beginning with 2015 and ending in 2024, equal to five percent of the number of shares of the Company’s common stock outstanding as of such date or a lesser number of shares as determined by our board of directors. Under the terms of the 2014 EIP, we may grant stock options, RSAs and RSUs to employees, directors, consultants and other service providers. RSUs generally vest over four years. Stock options are granted at prices no less than 100% of the estimated fair value of the shares on the date of grant as determined by the board of directors, provided, however, that the exercise price of an option granted to a 10% shareholder cannot be less than 110% of the estimated fair value of the shares on the date of grant. Options granted generally vest over four years and expire in seven to ten years. As of December 31, 2019 In February 2018, we adopted the 2018 Inducement Plan (“Inducement Plan”), under which we may grant options, stock appreciation rights, RSAs and RSUs to new employees. As of December 31, 2019 Restricted Stock Units The fair value of RSUs is determined as the closing stock price on the date of grant. The weighted average grant date fair value of RSUs granted during the years ended December 31, 2019 December 31, 2019 December 31, 2019 December 31, 2019 The following is a summary of RSU activity under our 2014 EIP and Inducement Plan: RSUs Shares Weighted Average Grant Date Fair Value Unvested as of December 31, 2018 1,405,460 $ 26.94 Granted 1,591,888 $ 27.04 Forfeited (514,516 ) $ 30.37 Vested (572,068 ) $ 24.16 Outstanding as of December 31, 2019 1,910,764 $ 26.93 Under our RSU net settlement procedures, for most of our employees, we withhold shares at settlement to cover the minimum payroll withholding tax obligations. During 2019 , we settled shares underlying RSUs, of which shares underlying RSUs were net settled by withholding shares. The value of the shares underlying RSUs withheld was $ million, based on the closing price of our common stock on the settlement date. During 2018 , we settled shares underlying RSUs, of which shares underlying RSUs were net settled by withholding shares. The value of the shares underlying RSUs withheld was $ million, based on the closing price of our common stock on the settlement date. The value of RSUs withheld in each period was remitted to the appropriate taxing authorities and has been reflected as a financing activity in our consolidated statements of cash flows. Stock Options The following is a summary of stock option activity under our 2014 EIP and Inducement Plan. The table below also includes the activity relating to options for 275,000 shares of our common stock which were issued in 2017 outside of these plans: Shares Weighted Weighted Average Remaining Contractual Term (Years) Aggregate Intrinsic Value (in thousands) Outstanding as of December 31, 2018 6,276,999 $ 28.15 Granted 2,535,425 26.97 Exercised (347,716 ) 14.84 Forfeited or expired (1,530,446 ) 28.76 Outstanding as of December 31, 2019 6,934,262 $ 28.25 5.9 $ 4,103 Vested and expected to vest as of December 31, 2019 6,934,262 $ 28.25 5.9 $ 4,103 Exercisable as of December 31, 2019 2,932,449 $ 26.89 4.0 $ 1,689 Aggregate intrinsic value represents the difference between the closing stock price of our common stock on December 31, 2019 December 31, 2019 Options for 347,716, 1,051,180 and 60,125 shares of our common stock were exercised during the years ended December 31, 2019 The fair value of each option issued was estimated at the date of grant using the Black-Scholes valuation model. The following table summarizes the weighted-average assumptions used as inputs to the Black-Scholes model and resulting weighted-average grant date fair values of stock options granted to employees during the periods indicated: Year ended December 31, 2019 2018 2017 Assumptions: Expected term (years) 5.9 4.6 4.5 Expected volatility 76.1 % 73.5 % 71.3 % Risk-free interest rate 2.1 % 2.7 % 1.9 % Expected dividend yield 0.0 % 0.0 % 0.0 % Fair Value: Weighted-average estimated grant date fair value per share $ 18.06 $ 22.96 $ 9.01 Options granted 2,535,425 2,998,650 1,694,000 Total estimated grant date fair value $ 45,790,000 $ 68,849,000 $ 15,263,000 The estimated fair value of stock options that vested in the years ended December 31, 2019 Employee Stock Purchase Plan In May 2014, we adopted the 2014 Employee Stock Purchase Plan (“2014 ESPP”), which became effective on October 15, 2014 upon the pricing of our IPO. The 2014 ESPP permits eligible employees to purchase common stock at a discount through payroll deductions during defined offering periods. Eligible employees can purchase shares of the Company’s common stock at 85% of the lower of the fair market value of the common stock at (i) the beginning of a 12-month offering period, or (ii) at the end of one of the two related 6-month purchase periods. No participant in the 2014 ESPP may purchase shares of common stock valued at more than $25,000 per calendar year. The first offering under the 2014 ESPP commenced on June 1, 2016, and subsequent offerings commence on each anniversary of this date. The Company recorded $1.3 million, $1.2 million and $0.6 million of expense related to the 2014 ESPP in the years ended December 31, 2019, December 31, 2019 As of December 31, 2019, there was $0.5 million of unrecognized stock-based compensation expense related to the ESPP that is expected to be recognized by the end of second quarter of 2020. The 2014 ESPP provides for annual increases in the number of shares available for issuance thereunder on the first business day of each fiscal year, beginning with 2015 and ending in 2024, equal to the lower of (i) one percent of the number of shares of our common stock outstanding as of such date, (ii) 230,769 shares of our common stock, or (iii) a lesser number of shares as determined by our board of directors. As of December 31, 2019, there were 1,355,973 shares authorized under the 2014 ESPP. Reserved Shares The following shares of common stock were reserved for future issuance as of December 31, 2019 Total Shares Reserved 2014 Equity Incentive Plan 11,935,558 2018 Inducement Plan 1,213,760 2014 Employee Stock Purchase Plan 1,002,548 Options outside the equity plans 109,666 Total reserved shares of common stock 14,261,532 Stock-based Compensation Expense Total stock-based compensation expense related to all stock awards was as follows: Year Ended December 31, 2019 2018 2017 (in thousands) Research and development $ 26,773 $ 16,211 $ 8,778 General and administrative 24,923 17,606 14,322 Total stock-based compensation expense $ 51,696 $ 33,817 $ 23,100

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Income Taxes	10 . Income Taxes Losses before provision for income taxes were as follows in each period presented: Year Ended December 31, 2019 2018 2017 (in thousands) United States $ (291,049 ) $ (230,765 ) $ (12,894 ) Foreign 85 22 (106,611 ) Total loss before provision for income taxes $ (290,964 ) $ (230,743 ) $ (119,505 ) The Company liquidated its Cayman Islands entity in 2018 and elected to treat the entity as disregarded for the fiscal year 2018. As such, the applicable 2018 losses are treated as losses in the United States. The components of provision for (benefit from) income taxes were as follows in each period presented: Year Ended December 31, 2019 2018 2017 (in thousands) Current provision for (benefit from) income taxes: Federal $ — $ (31 ) $ (14 ) State — (15 ) — Foreign 12 2 — Total current provision for (benefit from) income taxes $ 12 $ (44 ) $ (14 ) A reconciliation of statutory tax rates to effective tax rates were as follows in each of the periods presented: Year Ended December 31, 2019 2018 2017 Federal income taxes at statutory rate 21.0 % 21.0 % 34.0 % Impact of stock compensation 0.1 % — (1.5 %) Foreign income tax at different rate — — (30.3 %) Impact of U.S. tax reform — — (11.3 %) Non-deductible executive compensation (0.7 %) (0.7 %) — Capitalized research — 7.8 % — Other (0.2 %) (0.6 %) (3.8 %) Change in valuation allowance (20.2 %) (27.5 %) 12.9 % Effective tax rate 0.0 % 0.0 % 0.0 % Deferred tax assets and liabilities reflect the net tax effect of temporary differences between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts used for income tax purposes. Significant components of our deferred tax assets and liabilities were as follows for each of the dates presented: As of December 31, 2019 2018 (in thousands) Deferred tax assets: Net operating losses $ 162,436 $ 91,994 Capitalized expenses 14,129 16,019 Stock-based compensation 17,573 8,578 License fees 6,870 7,380 Operating lease liabilities 4,325 — Legal fees 1,683 1,375 Other 3,329 1,807 Total deferred tax assets 210,345 127,153 Valuation allowance (205,249 ) (127,153 ) Total deferred tax assets 5,096 — Deferred tax liabilities: Operating lease assets (3,921 ) — Other (1,175 ) — Total deferred tax liabilities (5,096 ) — Net deferred tax assets (liabilities) $ — $ — We recognize deferred income taxes for temporary differences between the basis of assets and liabilities for financial statement and income tax purposes, as well as for tax attribute carryforwards. We regularly evaluate the positive and negative evidence in determining the realizability of our deferred tax assets. Based upon the weight of available evidence, which includes our historical operating performance and reported cumulative net losses since inception, we maintained a full valuation allowance on the net deferred tax assets as of December 31, 2019 December 31, 2019 As of December 31, 2019 Under Section 382 of the Internal Revenue Code of 1986, as amended, our ability to utilize net operating loss carryforwards or other tax attributes in any taxable year may be limited if we have experienced an “ownership change.” Generally, a Section 382 “ownership change” occurs if one or more stockholders or groups of stockholders who owns at least 5% of a corporation’s stock increases its ownership by more than 50 percentage points over its lowest ownership percentage within a specified testing period. Similar rules may apply under state tax laws. We have completed a Section 382 study of transactions in our stock through December 31, 2019. The study concluded that we have experienced ownership changes since inception and that our utilization of net operating loss carryforwards will be subject to annual limitations. However, it is not expected that the annual limitations will result in the expiration of tax attribute carryforwards prior to utilization. On December 22, 2017, the Tax Act was enacted into law. The Tax Act, among other things, contains significant changes to corporate taxation, including reduction of the corporate tax rate from a top marginal rate of 35% to a flat rate of ; limitation of the tax deduction for interest expense to 30% of adjusted earnings (except for certain small businesses); limitation of the deduction of net operating losses generated in tax years beginning after December 31, 2017 to 80% of taxable income, indefinite carryforward of net operating losses generated in tax years after 2018 and elimination of net operating loss carrybacks; changes in the treatment of offshore earnings regardless of whether they are repatriated; current inclusion in U.S. federal taxable income of certain earnings of controlled foreign corporations, mandatory capitalization of research and development expenses beginning in 2022; immediate deductions for certain new investments instead of deductions for depreciation expense over time; further deduction limits on executive compensation; and modifying, repealing and creating many other business deductions and credits, including the reduction in the orphan drug credit from 50% to 25% of qualifying expenditures. The changes in the balance of gross unrecognized tax benefits, which excludes interest and penalties, for the years ended December 31, 2017, 2018 and 2019 are as follows: (In thousands) Balance as of January 1, 2017 $ 9,285 Gross increases for tax positions related to current year 16,371 Gross increases for tax positions related to prior year 9,534 Gross decreases for tax positions related to prior year (4,643 ) Impact of change in tax rate (496 ) Balance as of December 31, 2017 30,051 Gross increases for tax positions related to current year 12,927 Gross increases for tax positions related to prior year 704 Gross decreases for tax positions related to prior year (2,608 ) Balance as of December 31, 2018 41,074 Gross increases for tax positions related to current year 22,800 Gross increases for tax positions related to prior year 22,126 Gross decreases for tax positions related to prior year — Balance as of December 31, 2019 $ 86,000 The Company currently has a full valuation allowance against its U.S. net deferred tax assets, which would impact the timing of the effective tax rate benefit should any uncertain tax position be favorably settled in the future. Of the $86.0 million total unrecognized tax benefits as of December 31, 2019 The Company’s policy is to account for interest and penalties related to uncertain tax positions as a component of the income tax provision. The Company has not accrued interest and penalties as of December 31, 2019 and 2018 due to available tax losses. Our significant jurisdictions are the U.S. federal jurisdiction and the California state jurisdiction. All of our tax years remain open to examination by the U.S. federal and California tax authorities.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation We prepare our consolidated financial statements in accordance with accounting principles generally accepted in the United States of America (“U.S. GAAP”) and follow the rules and regulations of the U.S. Securities and Exchange Commission (“SEC”).
Principles of Consolidation	Principles of Consolidation The consolidated financial statements include the accounts of Atara and our wholly owned subsidiaries. All intercompany balances and transactions are eliminated in consolidation.
Segment and Geographic Information	Segment and Geographic Information We operate and manage our business as one operating and reportable segment, which is the business of developing and commercializing therapeutics. Our Chief Executive Officer, who is our chief operating decision maker, reviews financial information on an aggregate basis for purposes of allocating resources and evaluating financial performance. Substantially all of our assets are located in the U.S.
Liquidity Risk	Liquidity Risk We have incurred significant operating losses since inception and have relied on public and private equity financings to fund our operations. As of December 31, 2019, we had an accumulated deficit of $818.0 million. As we continue to incur losses, our transition to profitability will depend on the successful development, approval and commercialization of product candidates and on the achievement of sufficient revenues to support our cost structure. We may never achieve profitability, and unless and until we do, we will need to continue to raise additional capital. We expect that existing cash, cash equivalents and short-term investments as of December 31, 2019, together with net proceeds from the sale of common stock from our 2019 ATM Facility, as defined in Note 9, in January 2020, will be sufficient to fund our planned operations into the second quarter of 2021.
Concentration of Credit Risk and Other Uncertainties	Concentration of Credit Risk and Other Uncertainties We place cash and cash equivalents in the custody of financial institutions that management believes are of high credit quality, the amount of which at times, may be in excess of the amount insured by the Federal Deposit Insurance Corporation. We also make short-term investments in money market funds; U.S. Treasury, government agency and corporate debt obligations; commercial paper; certificates of deposit; and asset-backed securities, which can be subject to certain credit risk. However, we mitigate the risks by investing in high-grade instruments, limiting our exposure to any one issuer and monitoring the ongoing creditworthiness of the financial institutions and issuers. We are subject to certain risks and uncertainties and believe that changes in any of the following areas could have a material adverse effect on future financial position or results of operations: our ability to obtain future financing; regulatory approval and market acceptance of, and reimbursement for, our product candidates, if approved by applicable regulatory authorities; performance of third-party clinical research organizations and manufacturers upon which we rely; development of sales channels; protection of our intellectual property; litigation or claims against us based on intellectual property, patent, product, regulatory or other factors; and our ability to attract and retain employees necessary to support our growth.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates, assumptions and judgments that affect the amounts reported in the financial statements and accompanying notes. Significant estimates relied upon in preparing these financial statements include estimates related to clinical study and other accruals, lease assets and liabilities, stock-based compensation expense and income taxes. Actual results could differ materially from those estimates.
Leases	Leases We determine if an arrangement is a lease at inception. Operating leases are included in operating lease assets, other current liabilities, and operating lease liabilities on our consolidated balance sheets. Leases with an initial term of 12 months or less are not recorded on the balance sheet; we recognize lease expense for these leases on a straight-line basis over the lease term. Finance leases are included in other assets, other current liabilities, and other long-term liabilities on our consolidated balance sheets. Lease assets and lease liabilities are recognized based on the present value of the future minimum lease payments over the lease term at commencement date. The lease term includes renewal options that we are reasonably certain of exercising as of the commencement date. None of the lease terms used to calculate the future minimum lease payments at commencement date include renewal options. As most of our leases do not provide an implicit rate, we use our incremental borrowing rate based on the information available at commencement date in determining the present value of future payments. The incremental borrowing rate for our leases is determined based on lease term and currency in which lease payments are made, adjusted for impacts of collateral. Lease assets also includes any lease payments made and excludes lease incentives and initial direct costs incurred. Operating lease expense for minimum lease payments is recognized on a straight-line basis over the lease term. Finance lease assets are amortized over the shorter of the lease term or the asset’s estimated useful life. Our facilities and equipment operating leases have lease and non-lease components and we have made a policy election to account for the lease and non-lease components as a single lease component. Through December 31, 2018, the leases were reviewed for classification as operating, capital or build-to-suit leases. For operating leases, rent was recognized on a straight-line basis over the lease period. For capital leases, we recorded the leased asset with a corresponding liability for principal and interest. Payments were recorded as reductions to these liabilities with interest being charged to interest expense in our consolidated statements of operations and comprehensive loss. We analyzed the nature of the renovations and our involvement during the construction period of our manufacturing facility and determined that we were the deemed “owner” of the construction project during the construction period. As a result, we were required to capitalize the fair value of the building as well as the construction costs incurred on our consolidated balance sheet along with a corresponding financing liability for landlord-paid construction costs (i.e. “build-to-suit” accounting). Once construction was complete, the Company considered the requirements for sale-leaseback accounting treatment, including evaluating whether all risks of ownership have been transferred back to the landlord, as evidenced by a lack of continuing involvement in the leased property. Since the arrangement did not qualify for sale-leaseback accounting treatment, the building asset remained on the Company’s consolidated balance sheets at its historical cost, and such asset was depreciated over its estimated useful life. The Company bifurcated its lease payments into a portion allocated to the building and a portion allocated to the parcel of land on which the building has been built. The portion of the lease payments allocated to the land was treated for accounting purposes as operating lease payments, and therefore was recorded as rent expense in the consolidated statements of operations and comprehensive loss. The portion of the lease payments allocated to the building was further bifurcated into a portion allocated to interest expense and a portion allocated to reduce the build-to-suit lease obligation. The initial recording of these assets and liabilities were classified as non-cash investing and financing items, respectively, for purposes of the consolidated statements of cash flows. The build-to-suit asset and corresponding lease obligation was derecognized upon adoption of the new lease standard as we did not control the building during the construction period.
Asset Retirement Obligations ("ARO")	Asset Retirement Obligations (“ARO”) ARO are legal obligations associated with the retirement of long-lived assets pertaining to leasehold improvements. These liabilities are initially recorded at fair value and the related asset retirement costs are capitalized by increasing the carrying amount of the related assets by the same amount as the liability. Asset retirement costs are subsequently depreciated over the useful lives of the related assets. Subsequent to initial recognition, the Company records period-to-period changes in the ARO liability resulting from the passage of time and revisions to either the timing or the amount of the original estimate of undiscounted cash flows. The Company derecognizes ARO liabilities when the related obligations are settled.
Foreign Currency	Foreign Currency Transactions and monetary assets and liabilities that are denominated in a foreign currency are translated into U.S. dollars at the current exchange rate on the transaction date and as of each balance sheet date, respectively, with gains or losses on foreign exchange changes recognized in interest and other income (expense), net in the consolidated statements of operations and comprehensive loss. Foreign currency-denominated monetary assets and liabilities as of December 31, 2019 were not material.
Cash Equivalents and Short-Term Investments	Cash Equivalents and Short-Term Investments Cash equivalents are defined as short-term, highly liquid investments with original maturities of 90 days or less at the date of purchase, and generally consist of money market funds, U.S. Treasury, government agency and corporate debt obligations, and commercial paper. Investments with original maturities of greater than 90 days are classified as short-term investments on the balance sheet, and consist primarily of U.S. Treasury, government agency and corporate debt obligations, commercial paper and asset-backed securities. As our entire investment portfolio is considered available for use in current operations, we classify all investments as available-for-sale and as current assets, even though the stated maturity may be more than one year from the current balance sheet date. Available-for-sale securities are carried at fair value, with unrealized gains and losses reported in accumulated other comprehensive loss, which is a separate component of stockholders’ equity in the consolidated balance sheet. The amortized cost of securities is adjusted for amortization of premiums and accretion of discounts to maturity, which are both recorded to interest and other income (expense), net in the consolidated statements of operations and comprehensive loss. Changes in the fair value of available-for-sale securities impact the consolidated statements of operations and comprehensive loss only when such securities are sold or if an other-than-temporary impairment is recognized. Realized gains and losses on the sale of securities are determined by specific identification of each security’s cost basis. We regularly review our investment portfolio to determine if any security is other-than-temporarily impaired, which would require us to record an impairment charge in the period any such determination is made. In making this judgment, we evaluate, among other things, the duration and extent to which the fair value of a security is less than its cost, the financial condition of the issuer and any changes thereto, our intent to sell, or whether it is more likely than not that we will be required to sell the security before recovery of its amortized cost basis. Our assessment on whether a security is other-than-temporarily impaired could change in the future due to new developments or changes in assumptions related to any particular security. Realized gains and losses and declines in value judged to be other-than-temporary on available-for-sale securities, if any, are recorded to interest and other income (expense), net in the statements of operations and comprehensive loss.
Fair Value Measurement	Fair Value Measurement The carrying amounts of certain of our financial instruments including cash equivalents, prepaid expenses and other current assets, accounts payable and accrued liabilities approximate fair value due to their short maturities. Short-term investments are comprised of available-for-sale securities, which are carried at fair value.
Fair Value of Financial Instruments	Fair Value of Financial Instruments Our financial assets are measured at fair value on a recurring basis using the following hierarchy to prioritize valuation inputs, Level1: Quoted prices in active markets for identical assets or liabilities that we have the ability to access Level2: Observable market-based inputs or unobservable inputs that are corroborated by market data such as quoted prices, interest rates and yield curves Level 3: Inputs that are unobservable data points that are not corroborated by market data We review the fair value hierarchy classification on a quarterly basis. Changes in the ability to observe valuation inputs may result in a reclassification of levels of certain securities within the fair value hierarchy. We recognize transfers into and out of levels within the fair value hierarchy in the period in which the actual event or change in circumstances that caused the transfer occurs. There have been no transfers between Level 1, Level 2, and Level 3 in any periods presented. Financial assets and liabilities are considered Level 2 when their fair values are determined using inputs that are observable in the market or can be derived principally from or corroborated by observable market data such as pricing for similar securities, recently executed transactions, cash flow models with yield curves, and benchmark securities. In addition, Level 2 financial instruments are valued using comparisons to like-kind financial instruments and models that use readily observable market data as their basis. U.S. Treasury, government agency and corporate debt obligations, commercial paper and asset-backed securities are valued primarily using market prices of comparable securities, bid/ask quotes, interest rate yields and prepayment spreads and are included in Level 2. Financial assets and liabilities are considered Level 3 when their fair values are determined using pricing models, discounted cash flow methodologies, or similar techniques, and at least one significant model assumption or input is unobservable. We have no Level 3 financial assets or liabilities.
Property and Equipment, Net	Property and Equipment, net Property and equipment are stated at cost and depreciated using the straight-line method over the estimated useful lives of the assets, ranging from three to five years. Costs incurred to acquire, construct or install property and equipment during the construction stage of a capital project or costs incurred to purchase and develop internal use software during the application development stage are recorded as construction in progress. Leasehold improvements are amortized over the lesser of the life of the leasehold improvements or the lease term. Maintenance and repairs are charged to operations as incurred.
Long-lived Assets	Long-lived Assets We evaluate the carrying amount of our long-lived assets whenever events or changes in circumstances indicate that the assets may not be recoverable. An impairment loss would be recognized when estimated future cash flows expected to result from the use of the asset and its eventual disposition are less than the carrying amount of the asset. To date, there have been no such impairment losses.
Stock-Based Compensation Expense	Stock-Based Compensation Expense We account for stock-based compensation expense, including the expense of restricted common stock awards (“RSAs”), grants of restricted stock units (“RSUs”), and stock options that may be settled in shares of our common stock, based on the fair values of the equity instruments issued. The fair value is determined on the measurement date, which is generally the date of grant. The fair value for our RSAs is their intrinsic value, which is the difference between the fair value of the underlying stock at the measurement date and the purchase price. The fair value of our RSUs is the fair value of the underlying stock at the measurement date. The fair value for our stock option awards is determined at the grant date using the Black-Scholes valuation model. For awards with performance-based vesting criteria, we assess the probability of the achievement of the performance conditions at the end of each reporting period and recognize the share-based compensation costs when it becomes probable that the performance conditions will be met. For awards that are subject to both service and performance conditions, no expense is recognized until it is probable that performance conditions will be met. Stock-based compensation expense for awards with time-based vesting criteria is recognized as expense on a straight-line basis over the requisite service period. Stock-based compensation expense for awards with performance and other vesting criteria is recognized as expense under an accelerated graded vesting model. Key assumptions used in the Black-Scholes valuation model used for employee stock awards include: Expected term – We derived the expected term using the “simplified” method (the expected term is determined as the average of the time-to-vesting and the contractual life of the options), as we have limited historical information to develop expectations about future exercise patterns and post vesting employment termination behavior. Expected volatility – Expected volatility is estimated using comparable public companies’ volatility for similar terms. Expected dividend – We have not historically declared or paid dividends to our stockholders and have no plans to pay dividends; therefore, we assumed an expected dividend yield of 0%. Risk-free interest rate – The risk-free interest rate is based on the yield on U.S. Treasury securities with the expected term of the associated award. The fair value of our common stock is based on observable market prices. We account for forfeitures of stock-based awards as they occur.
Research and Development Expense	Research and Development Expense Research and development expense consists of costs incurred in performing research and development activities, including compensation and benefits for research and development employees, including stock-based compensation; expenses incurred under agreements with contract research organizations and investigative sites that conduct clinical and preclinical studies; the costs of acquiring and manufacturing clinical study materials and other supplies; payments under licensing and research and development agreements; other outside services and consulting costs, and an allocation of facility, information technology and overhead expenses. Research and development costs are expensed as incurred.
Clinical Study Accruals	Clinical Study Accruals Costs for preclinical studies, clinical studies and manufacturing activities are recognized based on an evaluation of our vendors’ progress towards completion of specific tasks, using data such as patient enrollment, clinical site activations or information provided to us by our vendors regarding their actual costs incurred. Payments for these activities are based on the terms of individual contracts and payment timing may differ significantly from the period in which the services are performed. We determine accrual estimates through reports from and discussions with applicable personnel and outside service providers as to the progress or state of completion of studies, or the services completed. Our estimates of accrued expenses as of each balance sheet date are based on the facts and circumstances known at the time. Costs that are paid in advance of performance are deferred as a prepaid expense and amortized over the service period as the services are provided.
Defined Contribution Plan	Defined Contribution Plan We have one qualified 401(k) plan covering all eligible employees. Under the plan, employees may contribute up to the statutory allowable amount for any calendar year. Beginning in 2019, we make matching contributions, equal to 50% of each dollar contributed up to the first 6% of an individual’s eligible earnings, up to the annual IRS maximum. For the year ended December 31, 2019, we recorded matching contributions of approximately $1.6 million.
Other Current Liabilities	Other Current Liabilities Other current liabilities consisted of the following as of each period end: December 31, December 31, 2019 2018 (in thousands) Accrued operating expenses $ 3,900 $ 5,627 Current portion of operating lease liabilities 1,312 — Current portion of finance and capital lease liabilities 269 587 Other accrued liabilities 252 200 Total other current liabilities $ 5,733 $ 6,414
Income Taxes	Income Taxes We use the asset and liability method to account for income taxes. We record deferred tax assets and liabilities for the expected future tax consequences of temporary differences between the financial statement carrying amounts and the tax basis of assets and liabilities using enacted tax rates expected to be in effect when the differences are expected to reverse. Valuation allowances are provided when necessary to reduce net deferred tax assets to the amount that is more likely than not to be realized. Based on the available evidence, we are unable, at this time, to support the determination that it is more likely than not that our deferred tax assets will be utilized in the future. Accordingly, we recorded a full valuation allowance as of December 31, 2019 and 2018. We intend to maintain valuation allowances until sufficient evidence exists to support their reversal. Tax benefits related to uncertain tax positions are recognized when it is more likely than not that a tax position will be sustained during an audit. Interest and penalties related to unrecognized tax benefits are included within the provision for income tax.
Comprehensive Income (Loss)	Comprehensive Income (Loss) Comprehensive income (loss) is defined as a change in equity of a business enterprise during a period resulting from transactions from non-owner sources. Our other comprehensive income (loss) is comprised solely of unrealized gains (losses) on available-for-sale securities and is presented net of taxes. We have not recorded any reclassifications from other comprehensive income (loss) to net loss during any period presented.
Recent Accounting Pronouncements	Recent Accounting Pronouncements The Company considers the applicability and impact of any Accounting Standards Update (“ASU”) issued by the Financial Accounting Standards Board (“FASB”). ASUs not listed below were assessed and determined to be either not applicable or are expected to have minimal impact on our consolidated financial statements. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments - Credit Losses: Measurement of Credit Losses on Financial Instruments In August 2018, the FASB issued ASU No. 2018-15, Intangibles—Goodwill and Other—Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract In December 2019, the FASB issued ASU No. 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes Adoption of New Accounting Pronouncements We adopted ASU No. 2016-02, Leases (Topic 842), as of January 1, 2019, using the optional transition method, which allows for the initial application of the new accounting standard at the adoption date and the recognition of a cumulative-effect adjustment to the opening balance of retained earnings as of the beginning of the period of adoption. In addition, we elected the package of practical expedients permitted under the transition guidance within the new standard, which among other things, allowed us to carry forward the historical lease classification. In addition, we elected the hindsight practical expedient to determine the lease term for existing leases. Adoption of the new standard resulted in the recording of additional operating lease assets and operating lease liabilities of $14.3 million and $15.3 million, respectively, as of January 1, 2019. This was partially offset by de-recognition of the build-to-suit asset and corresponding lease obligation of $10.3 million for our Thousand Oaks manufacturing facility lease, as we did not control the building during the construction period (see Note 7). We evaluated the impact of the new standard for deferred tax accounting purposes and recorded corresponding deferred tax assets and liabilities upon adoption of ASC 842. The deferred tax assets are offset by the valuation allowance on deferred taxes. The cumulative effect adjustment to the opening balance of accumulated deficit was a decrease of $0.4 million. The standard did not have a significant impact on our consolidated statements of operations and comprehensive loss, changes in stockholders’ equity, and cash flow for the years ended December 31, 2019, 2018 and 2017. We adopted ASU No. 2018-02, Income Statement – Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Summary of Other Current Liabilities	Other current liabilities consisted of the following as of each period end: December 31, December 31, 2019 2018 (in thousands) Accrued operating expenses $ 3,900 $ 5,627 Current portion of operating lease liabilities 1,312 — Current portion of finance and capital lease liabilities 269 587 Other accrued liabilities 252 200 Total other current liabilities $ 5,733 $ 6,414

Net Loss per Common Share (Tabl

Net Loss per Common Share (Tables)	12 Months Ended
	Dec. 31, 2019
Earnings Per Share [Abstract]
Antidilutive Securities Excluded From Computation of Diluted Net Loss per Common Share	The following table represents the potential common shares issuable pursuant to outstanding securities as of the related period end dates that were excluded from the computation of diluted net loss per common share as their inclusion would have an antidilutive effect: As of December 31, 2019 2018 2017 Unvested RSUs 1,910,764 1,405,460 1,685,000 Vested and unvested options 6,934,262 6,276,999 5,229,648 ESPP share purchase rights 20,438 7,974 14,905 Total 8,865,464 7,690,433 6,929,553

Financial Instruments (Tables)

Financial Instruments (Tables)	12 Months Ended
	Dec. 31, 2019
Financial Instruments Disclosure [Abstract]
Summary of Estimated Fair Value and Related Valuation Input Hierarchy of Available-for-Sale Securities	The following tables summarize the estimated fair value and related valuation input hierarchy of our available-for-sale securities as of each period end: Total Total Total Total Amortized Unrealized Unrealized Estimated As of December 31, 2019: Input Level Cost Gain Loss Fair Value (in thousands) Money market funds Level 1 $ 63,554 $ — $ — $ 63,554 U.S. Treasury obligations Level 2 52,805 46 (1 ) 52,850 Government agency obligations Level 2 6,151 1 (1 ) 6,151 Corporate debt obligations Level 2 100,512 180 (10 ) 100,682 Commercial paper Level 2 26,290 — — 26,290 Asset-backed securities Level 2 7,266 6 — 7,272 Certificate of deposit Level 2 500 — — 500 Total available-for-sale securities 257,078 233 (12 ) 257,299 Less amounts classified as cash equivalents (72,507 ) — — (72,507 ) Amounts classified as short-term investments $ 184,571 $ 233 $ (12 ) $ 184,792 Total Total Total Total Amortized Unrealized Unrealized Estimated As of December 31, 2018: Input Level Cost Gain Loss Fair Value (in thousands) Money market funds Level 1 $ 38,708 $ — $ — $ 38,708 U.S. Treasury obligations Level 2 111,164 4 (80 ) 111,088 Government agency obligations Level 2 15,206 1 (32 ) 15,175 Corporate debt obligations Level 2 121,017 15 (217 ) 120,815 Commercial paper Level 2 12,935 — — 12,935 Asset-backed securities Level 2 11,894 — (31 ) 11,863 Total available-for-sale securities 310,924 20 (360 ) 310,584 Less amounts classified as cash equivalents (61,651 ) — — (61,651 ) Amounts classified as short-term investments $ 249,273 $ 20 $ (360 ) $ 248,933
Amortized Cost and Fair Value of Available for Sale Securities by Contractual Maturity	The amortized cost and fair value of our available-for-sale securities by contractual maturity were as follows: As of December 31, 2019 As of December 31, 2018 Amortized Estimated Amortized Estimated Cost Fair Value Cost Fair Value (in thousands) (in thousands) Maturing within one year $ 214,085 $ 214,199 $ 287,755 $ 287,469 Maturing in one to five years 42,993 43,100 23,169 23,115 Total available-for-sale securities $ 257,078 $ 257,299 $ 310,924 $ 310,584
Reconciliation of Cash, Cash Equivalents and Restricted Cash	The following table provides a reconciliation of cash, cash equivalents and restricted cash within the consolidated balance sheets that sum to the total of the same such amounts in the consolidated statement of cash flows: December 31, December 31, 2019 2018 (in thousands) Cash and cash equivalents $ 74,317 $ 60,698 Restricted cash - short-term 194 194 Restricted cash - long-term 1,200 1,200 Total cash, cash equivalents and restricted cash $ 75,711 $ 62,092

Property and Equipment (Tables)

Property and Equipment (Tables)	12 Months Ended
	Dec. 31, 2019
Property Plant And Equipment [Abstract]
Schedule of Property and Equipment	Property and equipment consisted of the following as of each period end: December 31, December 31, 2019 2018 (in thousands) Leasehold improvements $ 49,028 $ 47,609 Build-to-suit asset (see Note 7) — 10,686 Lab equipment 6,815 3,019 Machinery and equipment 3,832 2,980 Computer equipment and software 3,299 3,049 Furniture and fixtures 1,764 1,628 Construction in progress 1,116 4,682 Property and equipment, gross 65,854 73,653 Less accumulated depreciation and amortization (11,678 ) (5,077 ) Property and equipment, net $ 54,176 $ 68,576

Leases (Tables)

Leases (Tables)	12 Months Ended
	Dec. 31, 2019
Leases [Abstract]
Schedule of Maturities of Lease Liabilities Under Operating and Finance Leases	The maturities of lease liabilities under our operating and finance leases as of December 31, 2019 were as follows: Operating Leases Finance Leases Years Ending December 31, (in thousands) 2020 $ 2,867 $ 308 2021 2,740 127 2022 2,610 116 2023 2,685 29 2024 2,591 — Thereafter 11,887 — Total lease payments $ 25,380 $ 580 Less: amount representing interest (9,932 ) (67 ) Present value of lease liabilities $ 15,448 $ 513 Balance as of December 31, 2019 Other current liabilities $ 1,312 $ 269 Operating lease liabilities 14,136 — Other long-term liabilities — 244 Total $ 15,448 $ 513
Components of Lease Cost	The components of lease cost were as follows: Year Ended December 31, 2019 (in thousands) Operating lease cost: Operating lease cost $ 2,578 Short-term lease cost 770 Total operating lease cost $ 3,348 Finance lease cost: Amortization expense $ 324 Interest on lease liabilities 56 Total finance lease cost $ 380
Schedule of Future Minimum Payments Under Operating, Finance and Capital Leases	Future minimum payments under our operating, finance and capital leases as of December 31, 2018 were as follows: Operating Leases Finance Leases Capital Leases Years Ending December 31, (in thousands) 2019 $ 1,107 $ 934 $ 540 2020 1,666 962 234 2021 1,555 991 29 2022 1,337 1,020 — 2023 1,375 1,051 — Thereafter 3,122 11,458 — Total minimum payments $ 10,162 $ 16,416 $ 803 Less: amount representing interest (65 ) Present value of capital lease obligations 738 Less: current portion (490 ) Capital lease obligation, net of current portion $ 248
Summary of Other Information Related to Leases	Other information related to leases was as follows: Year Ended December 31, 2019 (in thousands, except lease term and discount rate) Supplemental Cash Flows Information Cash paid for amounts included in the measurement of lease liabilities: Operating cash flows for operating leases $ 2,346 Operating cash flows for finance leases 50 Financing cash flows for finance leases 486 Operating lease assets obtained in exchange for lease obligations: $ 838 Finance lease assets obtained in exchange for lease obligations: 323 Weighted Average Remaining Lease Term Operating leases 10.3 years Finance leases 2.5 years Weighted Average Discount Rate Operating leases 10.4 % Finance leases 10.0 %
Summary of Activity for ARO Liabilities	The following table presents the activity for our ARO liabilities: ARO Liability (In thousands) Balance as of December 31, 2018 $ 717 Accretion expense 71 Balance as of December 31, 2019 $ 788

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	12 Months Ended
	Dec. 31, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Summary of RSU Activity	The following is a summary of RSU activity under our 2014 EIP and Inducement Plan: RSUs Shares Weighted Average Grant Date Fair Value Unvested as of December 31, 2018 1,405,460 $ 26.94 Granted 1,591,888 $ 27.04 Forfeited (514,516 ) $ 30.37 Vested (572,068 ) $ 24.16 Outstanding as of December 31, 2019 1,910,764 $ 26.93
Summary of Stock Option Activity	The following is a summary of stock option activity under our 2014 EIP and Inducement Plan. The table below also includes the activity relating to options for 275,000 shares of our common stock which were issued in 2017 outside of these plans: Shares Weighted Weighted Average Remaining Contractual Term (Years) Aggregate Intrinsic Value (in thousands) Outstanding as of December 31, 2018 6,276,999 $ 28.15 Granted 2,535,425 26.97 Exercised (347,716 ) 14.84 Forfeited or expired (1,530,446 ) 28.76 Outstanding as of December 31, 2019 6,934,262 $ 28.25 5.9 $ 4,103 Vested and expected to vest as of December 31, 2019 6,934,262 $ 28.25 5.9 $ 4,103 Exercisable as of December 31, 2019 2,932,449 $ 26.89 4.0 $ 1,689
Summary of Options Estimated with Weighted Average	The fair value of each option issued was estimated at the date of grant using the Black-Scholes valuation model. The following table summarizes the weighted-average assumptions used as inputs to the Black-Scholes model and resulting weighted-average grant date fair values of stock options granted to employees during the periods indicated: Year ended December 31, 2019 2018 2017 Assumptions: Expected term (years) 5.9 4.6 4.5 Expected volatility 76.1 % 73.5 % 71.3 % Risk-free interest rate 2.1 % 2.7 % 1.9 % Expected dividend yield 0.0 % 0.0 % 0.0 % Fair Value: Weighted-average estimated grant date fair value per share $ 18.06 $ 22.96 $ 9.01 Options granted 2,535,425 2,998,650 1,694,000 Total estimated grant date fair value $ 45,790,000 $ 68,849,000 $ 15,263,000
Schedule of Common Stock Reserved for Future Issuance	The following shares of common stock were reserved for future issuance as of December 31, 2019 Total Shares Reserved 2014 Equity Incentive Plan 11,935,558 2018 Inducement Plan 1,213,760 2014 Employee Stock Purchase Plan 1,002,548 Options outside the equity plans 109,666 Total reserved shares of common stock 14,261,532
Schedule of Stock-based Compensation Related to Stock Awards	Total stock-based compensation expense related to all stock awards was as follows: Year Ended December 31, 2019 2018 2017 (in thousands) Research and development $ 26,773 $ 16,211 $ 8,778 General and administrative 24,923 17,606 14,322 Total stock-based compensation expense $ 51,696 $ 33,817 $ 23,100

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Losses Before Provision for Income Taxes	Losses before provision for income taxes were as follows in each period presented: Year Ended December 31, 2019 2018 2017 (in thousands) United States $ (291,049 ) $ (230,765 ) $ (12,894 ) Foreign 85 22 (106,611 ) Total loss before provision for income taxes $ (290,964 ) $ (230,743 ) $ (119,505 )
Components of Provision for (Benefit from) Income Taxes	The components of provision for (benefit from) income taxes were as follows in each period presented: Year Ended December 31, 2019 2018 2017 (in thousands) Current provision for (benefit from) income taxes: Federal $ — $ (31 ) $ (14 ) State — (15 ) — Foreign 12 2 — Total current provision for (benefit from) income taxes $ 12 $ (44 ) $ (14 )
Reconciliation of Statutory Tax Rates to Effective Tax Rates	A reconciliation of statutory tax rates to effective tax rates were as follows in each of the periods presented: Year Ended December 31, 2019 2018 2017 Federal income taxes at statutory rate 21.0 % 21.0 % 34.0 % Impact of stock compensation 0.1 % — (1.5 %) Foreign income tax at different rate — — (30.3 %) Impact of U.S. tax reform — — (11.3 %) Non-deductible executive compensation (0.7 %) (0.7 %) — Capitalized research — 7.8 % — Other (0.2 %) (0.6 %) (3.8 %) Change in valuation allowance (20.2 %) (27.5 %) 12.9 % Effective tax rate 0.0 % 0.0 % 0.0 %
Components of Deferred Tax Assets and Liabilities	Significant components of our deferred tax assets and liabilities were as follows for each of the dates presented: As of December 31, 2019 2018 (in thousands) Deferred tax assets: Net operating losses $ 162,436 $ 91,994 Capitalized expenses 14,129 16,019 Stock-based compensation 17,573 8,578 License fees 6,870 7,380 Operating lease liabilities 4,325 — Legal fees 1,683 1,375 Other 3,329 1,807 Total deferred tax assets 210,345 127,153 Valuation allowance (205,249 ) (127,153 ) Total deferred tax assets 5,096 — Deferred tax liabilities: Operating lease assets (3,921 ) — Other (1,175 ) — Total deferred tax liabilities (5,096 ) — Net deferred tax assets (liabilities) $ — $ —
Reconciliation of Beginning and Ending Amount of Gross Unrecognized Tax Benefits	The changes in the balance of gross unrecognized tax benefits, which excludes interest and penalties, for the years ended December 31, 2017, 2018 and 2019 are as follows: (In thousands) Balance as of January 1, 2017 $ 9,285 Gross increases for tax positions related to current year 16,371 Gross increases for tax positions related to prior year 9,534 Gross decreases for tax positions related to prior year (4,643 ) Impact of change in tax rate (496 ) Balance as of December 31, 2017 30,051 Gross increases for tax positions related to current year 12,927 Gross increases for tax positions related to prior year 704 Gross decreases for tax positions related to prior year (2,608 ) Balance as of December 31, 2018 41,074 Gross increases for tax positions related to current year 22,800 Gross increases for tax positions related to prior year 22,126 Gross decreases for tax positions related to prior year — Balance as of December 31, 2019 $ 86,000

Description of Business (Detail

Description of Business (Detail)	12 Months Ended
	Dec. 31, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Entity incorporation state	DE
Entity incorporation date	Aug. 1, 2012

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Additional Information (Detail)	Jan. 01, 2019USD ($)	Dec. 31, 2019USD ($)Segment	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)
Significant Accounting Policies [Line Items]
Number of reportable segments | Segment		1
Number of operating segments | Segment		1
Accumulated deficit		$ 817,961,000	$ 527,349,000
Cash and cash equivalents maturity period		90 days
Investment maturity period		90 days
Impairment of long-lived assets		$ 0
Expected dividend yield		0.00%
Defined contribution plan, plan name		401(k)
Defined contribution plan, matching contribution percentage		50.00%
Defined contribution plan, maximum percentage of an individual's eligible earnings		6.00%
Defined contribution plan contributions by employer		$ 1,600,000
Additional operating lease assets		(239,000)	0	$ 0
Additional operating lease liabilities		$ (731,000)	$ 0	$ 0
Accounting Standards Update 2016-02
Significant Accounting Policies [Line Items]
Additional operating lease assets	$ 14,300,000
Additional operating lease liabilities	15,300,000
De-recognition of build-to-suit asset and corresponding lease obligation	10,300,000
Cumulative effect adjustment to opening balance of accumulated deficit, expected decrease	$ 400,000
Minimum
Significant Accounting Policies [Line Items]
Property, plant and equipment, useful life		3 years
Maximum
Significant Accounting Policies [Line Items]
Property, plant and equipment, useful life		5 years

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Summary of Other Current Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Accrued operating expenses	$ 3,900	$ 5,627
Current portion of operating lease liabilities	1,312	0
Current portion of finance and capital lease liabilities	269	587
Other accrued liabilities	252	200
Total other current liabilities	$ 5,733	$ 6,414

Net Loss per Common Share - Ant

Net Loss per Common Share - Antidilutive Securities Excluded From Computation of Diluted Net Loss per Common Share (Detail) - shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	8,865,464	7,690,433	6,929,553
Unvested RSUs
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	1,910,764	1,405,460	1,685,000
Vested and Unvested Options
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	6,934,262	6,276,999	5,229,648
ESPP Share Purchase Rights
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	20,438	7,974	14,905

Financial Instruments - Summary

Financial Instruments - Summary of Estimated Fair Value and Related Valuation Input Hierarchy of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Total Fair Value	$ 257,299	$ 310,584
Fair Value, Measurements, Recurring
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	257,078	310,924
Total Unrealized Gain	233	20
Total Unrealized Loss	(12)	(360)
Total Fair Value	257,299	310,584
Fair Value, Measurements, Recurring | Money Market Funds | Level 1
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	63,554	38,708
Total Unrealized Gain	0	0
Total Unrealized Loss	0	0
Total Fair Value	63,554	38,708
Fair Value, Measurements, Recurring | U.S. Treasury Obligations | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	52,805	111,164
Total Unrealized Gain	46	4
Total Unrealized Loss	(1)	(80)
Total Fair Value	52,850	111,088
Fair Value, Measurements, Recurring | Government Agency Obligations | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	6,151	15,206
Total Unrealized Gain	1	1
Total Unrealized Loss	(1)	(32)
Total Fair Value	6,151	15,175
Fair Value, Measurements, Recurring | Corporate Debt Obligations | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	100,512	121,017
Total Unrealized Gain	180	15
Total Unrealized Loss	(10)	(217)
Total Fair Value	100,682	120,815
Fair Value, Measurements, Recurring | Commercial Paper | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	26,290	12,935
Total Unrealized Gain	0	0
Total Unrealized Loss	0	0
Total Fair Value	26,290	12,935
Fair Value, Measurements, Recurring | Asset-Backed Securities | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	7,266	11,894
Total Unrealized Gain	6	0
Total Unrealized Loss	0	(31)
Total Fair Value	7,272	11,863
Fair Value, Measurements, Recurring | Certificate of Deposit | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	500
Total Unrealized Gain	0
Total Unrealized Loss	0
Total Fair Value	500
Fair Value, Measurements, Recurring | Amounts Classified As Cash Equivalents
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	72,507	61,651
Total Unrealized Gain	0	0
Total Unrealized Loss	0	0
Total Fair Value	72,507	61,651
Fair Value, Measurements, Recurring | Amounts Classified As Short-Term Investments
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	184,571	249,273
Total Unrealized Gain	233	20
Total Unrealized Loss	(12)	(360)
Total Fair Value	$ 184,792	$ 248,933

Financial Instruments - Amortiz

Financial Instruments - Amortized Cost and Fair Value of Available-for-Sale Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Amortized cost
Maturing within one year, Amortized cost	$ 214,085	$ 287,755
Maturing in one to five years, Amortized cost	42,993	23,169
Total available-for-sale securities, Amortized cost	257,078	310,924
Estimated Fair value
Maturing within one year, Estimated fair value	214,199	287,469
Maturing in one to five years, Estimated fair value	43,100	23,115
Total available-for-sale securities, Estimated fair value	$ 257,299	$ 310,584

Financial Instruments - Additio

Financial Instruments - Additional Information (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Financial Instruments Disclosure [Abstract]
Restricted Cash	$ 1.4	$ 1.4

Financial Instruments - Reconci

Financial Instruments - Reconciliation of Cash, Cash Equivalents and Restricted Cash (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Financial Instruments Disclosure [Abstract]
Cash and cash equivalents	$ 74,317	$ 60,698
Restricted cash - short-term	194	194
Restricted cash - long-term	1,200	1,200
Total cash, cash equivalents and restricted cash	$ 75,711	$ 62,092	$ 80,617	$ 48,162

Property and Equipment - Schedu

Property and Equipment - Schedule of Property and Equipment (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 65,854	$ 73,653
Less accumulated depreciation and amortization	(11,678)	(5,077)
Property and equipment, net	54,176	68,576
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	49,028	47,609
Build-to-Suit Asset
Property Plant And Equipment [Line Items]
Property and equipment, gross	0	10,686
Lab Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	6,815	3,019
Machinery and Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	3,832	2,980
Computer Equipment and Software
Property Plant And Equipment [Line Items]
Property and equipment, gross	3,299	3,049
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and equipment, gross	1,764	1,628
Construction in Progress
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 1,116	$ 4,682

Property and Equipment - Additi

Property and Equipment - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Property Plant And Equipment Net By Type [Abstract]
Depreciation and amortization expense	$ 7,070	$ 3,732	$ 956

License, Collaboration and Ma_2

License, Collaboration and Manufacturing Agreements - Additional Information (Detail) - USD ($)	1 Months Ended		3 Months Ended
	Oct. 31, 2015	Jun. 30, 2015	Mar. 31, 2019	Sep. 30, 2016	Dec. 31, 2019	Dec. 31, 2018
License, Collaboration and Manufacturing Agreements [Line Items]
Contractual obligations due to MSK and QIMR					$ 0	$ 0
MSK Agreements
License, Collaboration and Manufacturing Agreements [Line Items]
Payments for research and development		$ 4,500,000	$ 12,500,000
Potential milestone payments payable					$ 33,000,000
License Agreement | QIMR Berghofer
License, Collaboration and Manufacturing Agreements [Line Items]
Payments for research and development	$ 3,000,000			$ 3,300,000

Leases - Additional Information

Leases - Additional Information (Detail)	Feb. 15, 2018USD ($)ft²	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)
Lessee Lease Description [Line Items]
Ground lease expense			$ 400,000	$ 300,000
Rent expenses under operating leases			2,200,000	$ 1,400,000
Accounting Standards Update 2016-02
Lessee Lease Description [Line Items]
Ground lease expense		$ 0
Building | Build To Suit Lease Arrangement
Lessee Lease Description [Line Items]
Build-to-suit asset			10,300,000
Capitalized interest costs during the construction period			$ 300,000
South San Francisco California
Lessee Lease Description [Line Items]
Lease expiration date		Apr. 30, 2021
Letter of credit renewal term		12 months
South San Francisco California | Letter of Credit
Lessee Lease Description [Line Items]
Letter of credit maintained		$ 200,000
Thousand Oaks California
Lessee Lease Description [Line Items]
Lease agreement area of office, lab and cellular therapy manufacturing space | ft²	90,580
Lease commencement date	Feb. 15, 2018
Lease initial term	15 years
Contractual obligations	$ 16,400,000
Lease extension term, option one	10 years
Lease extension term, option two	9 years
Thousand Oaks California | Letter of Credit
Lessee Lease Description [Line Items]
Letter of credit issued, classified as long-term restricted cash	$ 1,200,000
Thousand Oaks California | Additional Office Space
Lessee Lease Description [Line Items]
Lease expiration date		Feb. 28, 2026
Aurora Colorado
Lessee Lease Description [Line Items]
Lease expiration date		Apr. 30, 2024

Leases - Schedule of Maturities

Leases - Schedule of Maturities of Lease Liabilities Under Operating and Finance Leases (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Leases [Abstract]
Operating leases, 2020	$ 2,867
Operating leases, 2021	2,740
Operating leases, 2022	2,610
Operating leases, 2023	2,685
Operating leases, 2024	2,591
Operating leases, Thereafter	11,887
Operating leases, Total minimum payments	25,380
Less: amount representing interest	(9,932)
Present value of lease liabilities	15,448
Other current liabilities	$ 1,312	$ 0
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesCurrent
Operating lease liabilities	$ 14,136	0
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesNoncurrent
Total	$ 15,448
Finance leases, 2020	308	934
Finance leases, 2021	127	962
Finance leases, 2022	116	991
Finance leases, 2023	29	1,020
Finance leases, 2024	0	1,051
Finance leases, Thereafter	0	11,458
Finance leases, Total minimum payments	580	16,416
Less: amount representing interest	(67)
Present value of lease liabilities	513
Other current liabilities	$ 269	$ 587
Finance Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesCurrent
Other long-term liabilities	$ 244
Finance Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesNoncurrent
Total	$ 513

Leases - Components of Lease Co

Leases - Components of Lease Cost (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2019USD ($)
Operating lease cost:
Operating lease cost	$ 2,578
Short-term lease cost	770
Total operating lease cost	3,348
Finance lease cost:
Amortization expense	324
Interest on lease liabilities	56
Total finance lease cost	$ 380

Leases - Schedule of Future Min

Leases - Schedule of Future Minimum Payments Under Operating, Finance and Capital Leases (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Leases [Abstract]
Operating leases, 2019		$ 1,107
Operating leases 2020		1,666
Operating leases, 2021		1,555
Operating leases, 2022		1,337
Operating leases, 2023		1,375
Operating leases, Thereafter		3,122
Operating leases, Total minimum payments		10,162
Finance leases, 2019	$ 308	934
Finance leases, 2020	127	962
Finance leases, 2021	116	991
Finance leases, 2022	29	1,020
Finance leases, 2023	0	1,051
Finance leases, Thereafter	0	11,458
Finance leases, Total minimum payments	$ 580	16,416
Capital leases, 2019		540
Capital leases, 2020		234
Capital leases, 2021		29
Capital leases, 2022		0
Capital leases, 2023		0
Capital leases, Thereafter		0
Capital leases, Total minimum payments		803
Less: amount representing interest		(65)
Present value of capital lease obligations		738
Less: current portion		(490)
Capital lease obligation, net of current portion		$ 248

Leases - Summary of Other Infor

Leases - Summary of Other Information Related to Leases (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2019USD ($)
Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flows for operating leases	$ 2,346
Operating cash flows for finance leases	50
Financing cash flows for finance leases	486
Operating lease assets obtained in exchange for lease obligations:	838
Finance lease assets obtained in exchange for lease obligations:	$ 323
Weighted Average Remaining Lease Term
Operating leases	10 years 3 months 18 days
Finance leases	2 years 6 months
Weighted Average Discount Rate
Operating leases	10.40%
Finance leases	10.00%

Leases - Summary of Activity fo

Leases - Summary of Activity for ARO Liabilities (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2019USD ($)
Leases [Abstract]
Balance as of December 31, 2018	$ 717
Accretion expense	71
Balance as of December 31, 2019	$ 788

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
Accrued termination charges	$ 0	$ 0
Liabilities related to indemnification agreements	$ 0	$ 0

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Detail) - USD ($)	Feb. 27, 2020	Jan. 31, 2020	Jan. 03, 2020	Jul. 31, 2019	Jul. 18, 2019	Feb. 25, 2019	Mar. 31, 2018	Jan. 31, 2018	Mar. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Authorized capital stock										520,000,000
Common stock, par value										$ 0.0001	$ 0.0001
Common stock, shares authorized										500,000,000	500,000,000
Preferred stock, par value										$ 0.0001
Preferred stock, shares authorized										20,000,000
Preferred stock, shares outstanding										0	0
Stock option granted description terms										the exercise price of an option granted to a 10% shareholder cannot be less than 110% of the estimated fair value of the shares on the date of grant. Options granted generally vest over four years and expire in seven to ten years.
Ownership percent										10.00%
Shares of common stock, reserved for issuance										14,261,532
Common stock, shares exercised										347,716	1,051,180	60,125
Option intrinsic value, exercised										$ 3,800,000	$ 19,200,000	$ 200,000
Net tax benefits related to exercised options										0
Estimated fair value of stock option, vested										31,600,000	16,200,000	14,000,000
Stock-based compensation expense										$ 51,696,000	$ 33,817,000	$ 23,100,000
Restricted Stock Units (RSUs)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted average grant date fair value per share										$ 27.04	$ 36.83	$ 15.07
Estimated fair value, vested in period										$ 13,800,000	$ 10,800,000	$ 3,700,000
Unrecognized stock-based compensation expense										$ 38,000,000
Unrecognized stock-based compensation weighted average recognition period										2 years 1 month 6 days
Aggregate intrinsic value										$ 31,500,000
Restricted stock units, settled										574,168	638,518
RSU settlements, net of shares withheld, shares										488,964	440,503
Restricted stock units withheld for tax obligations										212,879	190,205
Restricted stock units withheld for tax obligations, value										$ 6,700,000	$ 7,500,000
Restricted Stock Units (RSUs) | From Date Of Grant
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share based compensation, vesting period										4 years
Employees And Non Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share based compensation, vesting period										4 years
Vested and Unvested Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock-based compensation weighted average recognition period										2 years 9 months 18 days
Stock options, issued												275,000
Unrecognized stock-based compensation										$ 69,700,000
2014 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Annual increase in EIP/ESPP, subject to other limitations										5.00%
Shares of common stock, reserved for issuance										11,935,558
Aggregate number of awards available for grant to be issued										3,948,605
Outstanding options and RSUs										7,986,953
Inducement Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares of common stock, reserved for issuance										1,213,760
Aggregate number of awards available for grant to be issued										427,436
Outstanding options and RSUs										786,324
2014 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of employees purchase price of common stock										85.00%
Shares of common stock, reserved for issuance										1,002,548
Unrecognized stock-based compensation expense										$ 500,000
Employee stock purchase plan effective date										Oct. 15, 2014
Employee stock purchase plan description										(i) the beginning of a 12-month offering period, or (ii) at the end of one of the two related 6-month purchase periods
Employee stock purchase plan offering commenced start date										Jun. 1, 2016
Stock-based compensation expense										$ 1,300,000	$ 1,200,000	$ 600,000
Shares purchased										139,466	109,193	81,922
Aggregate number of shares authorized										1,355,973
Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of employees purchase price of common stock										100.00%
Minimum | 10% Shareholder
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of employees purchase price of common stock										110.00%
Minimum | Employees And Non Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share based compensation award expiration period										7 years
Maximum | Employees And Non Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share based compensation award expiration period										10 years
Maximum | 2014 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Annual increase in EIP/ESPP, subject to other limitations										1.00%
Stock may be issued or transferred, value										$ 25,000
Maximum increase in number of shares available for issuance										230,769
Underwritten Public Offering
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, shares issued				6,871,727			4,928,571	7,675,072
Shares issued, price per share				$ 15.28			$ 35	$ 18.25
Net proceeds from sale of common stock				$ 140,700,000			$ 161,900,000	$ 131,400,000
Proceeds from sale of common stock, gross				150,000,000
Exercise of warrants										$ 0
Proceeds from sale of common stock, gross				$ 150,000,000
Underwritten Public Offering | Warrant
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of warrants issued				2,945,026
Warrant, price per share				$ 15.2799
Number of securities called by each warrant					1
Warrants, exercise price					$ 0.0001
Warrants, term					7 years
Maximum ownership Percentage					9.99%
Underwritten Public Offering | Warrant | Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Prior notice period					61 days
Underwritten Public Offering | Warrant | Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Maximum ownership Percentage					19.99%
At The Market Offering | Cowen
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, shares issued										3,135,347	1,007,806
Net proceeds from sale of common stock										$ 48,900,000	$ 47,600,000
Proceeds from sale of common stock, gross										$ 50,500,000	$ 48,900,000
Common stock average price										$ 16.09	$ 48.52
Proceeds from sale of common stock, gross										$ 50,500,000	$ 48,900,000
Common stock value available to be sold										$ 49,500,000
At The Market Offering | Cowen | Subsequent Event
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, shares issued		1,371,216
Net proceeds from sale of common stock		$ 21,100,000	$ 1,200,000
Proceeds from sale of common stock, gross		$ 21,600,000
Common stock average price		$ 15.77
Proceeds from sale of common stock, gross		$ 21,600,000
Common stock value available to be sold		$ 27,900,000
At The Market Offering | Maximum | Cowen
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock aggregate offering price						$ 100,000,000			$ 75,000,000
Percentage of commission to be paid on gross sales proceeds of common stock sold						3.00%			3.00%
At The Market Offering | Maximum | Cowen | Subsequent Event
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock aggregate offering price	$ 100,000,000
Percentage of commission to be paid on gross sales proceeds of common stock sold	3.00%

Stockholders' Equity - Summary

Stockholders' Equity - Summary of RSU Activity (Detail) - Unvested RSUs - $ / shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares, Unvested as of December 31, 2018	1,405,460
Shares, Granted	1,591,888
Shares, Forfeited	(514,516)
Shares, Vested	(572,068)
Shares, Outstanding as of December 31, 2019	1,910,764	1,405,460
Weighted Average Grant Date Fair Value, Unvested as of December 31, 2018	$ 26.94
Weighted Average Grant Date Fair Value, Granted	27.04	$ 36.83	$ 15.07
Weighted Average Grant Date Fair Value, Forfeited	30.37
Weighted Average Grant Date Fair Value, Vested	24.16
Weighted Average Grant Date Fair Value, Outstanding as of December 31, 2019	$ 26.93	$ 26.94

Stockholders' Equity - Summar_2

Stockholders' Equity - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Exercised, Shares	(347,716)	(1,051,180)	(60,125)
2014 EIP and Inducement Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Outstanding, Shares, beginning balance	6,276,999
Granted, Shares	2,535,425
Exercised, Shares	(347,716)
Forfeited or expired, Shares	(1,530,446)
Outstanding, Shares, ending balance	6,934,262	6,276,999
Vested and expected to vest, Shares	6,934,262
Exercisable, Shares	2,932,449
Outstanding, Weighted Average Exercise Price, beginning balance	$ 28.15
Granted, Weighted Average Exercise Price	26.97
Exercised, Weighted Average Exercise Price	14.84
Forfeited or expired, Weighted Average Exercise price	28.76
Outstanding, Weighted Average Exercise Price, ending balance	28.25	$ 28.15
Stock options vested and expected to vest, Weighted Average Exercise Price	28.25
Exercisable, Weighted Average Exercise Price	$ 26.89
Outstanding, Weighted Average Remaining Contractual Term	5 years 10 months 24 days
Stock options vested and expected to vest, Weighted Average Remaining Contractual Term	5 years 10 months 24 days
Exercisable, Weighted Average Remaining Contractual Term	4 years
Aggregate intrinsic value	$ 4,103
Vested and expected to vest, Aggregate Intrinsic Value	4,103
Exercisable, Aggregate Intrinsic Value	$ 1,689

Stockholders' Equity - Summar_3

Stockholders' Equity - Summary of Estimated Weighted-Average Assumptions (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected dividend yield	0.00%
Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (years)	5 years 10 months 24 days	4 years 7 months 6 days	4 years 6 months
Expected volatility	76.10%	73.50%	71.30%
Risk-free interest rate	2.10%	2.70%	1.90%
Expected dividend yield	0.00%	0.00%	0.00%
Weighted-average estimated grant date fair value per share	$ 18.06	$ 22.96	$ 9.01
Options granted	2,535,425	2,998,650	1,694,000
Total estimated grant date fair value	$ 45,790,000	$ 68,849,000	$ 15,263,000

Stockholders' Equity - Schedule

Stockholders' Equity - Schedule of Common Stock Reserved for Future Issuance (Detail)	Dec. 31, 2019shares
Class Of Stock [Line Items]
Total reserved shares of common stock	14,261,532
2014 Equity Incentive Plan
Class Of Stock [Line Items]
Total reserved shares of common stock	11,935,558
2018 Inducement Plan
Class Of Stock [Line Items]
Total reserved shares of common stock	1,213,760
2014 Employee Stock Purchase Plan
Class Of Stock [Line Items]
Total reserved shares of common stock	1,002,548
Options Outside the Equity Plans
Class Of Stock [Line Items]
Total reserved shares of common stock	109,666

Stockholders' Equity - Schedu_2

Stockholders' Equity - Schedule of Stock-based Compensation Related to Stock Awards (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 51,696	$ 33,817	$ 23,100
Research and development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	26,773	16,211	8,778
General and administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 24,923	$ 17,606	$ 14,322

Income Taxes - Losses Before Pr

Income Taxes - Losses Before Provision for Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
United States	$ (291,049)	$ (230,765)	$ (12,894)
Foreign	85	22	(106,611)
Total loss before provision for income taxes	$ (290,964)	$ (230,743)	$ (119,505)

Income Taxes - Components of Pr

Income Taxes - Components of Provision for (Benefit from) Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Current provision for (benefit from) income taxes:
Federal	$ 0	$ (31)	$ (14)
State	0	(15)	0
Foreign	12	2	0
Total current provision for (benefit from) income taxes	$ 12	$ (44)	$ (14)

Income Taxes - Reconciliation o

Income Taxes - Reconciliation of Statutory Tax Rates to Effective Tax Rates (Detail)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Effective Income Tax Rate Continuing Operations Tax Rate Reconciliation [Abstract]
Federal income taxes at statutory rate	21.00%	21.00%	34.00%
Impact of stock compensation	0.10%	0.00%	(1.50%)
Foreign income tax at different rate	0.00%	0.00%	(30.30%)
Impact of U.S. tax reform	0.00%	0.00%	(11.30%)
Non-deductible executive compensation	(0.70%)	(0.70%)	0.00%
Capitalized research	0.00%	7.80%	0.00%
Other	(0.20%)	(0.60%)	(3.80%)
Change in valuation allowance	(20.20%)	(27.50%)	12.90%
Effective tax rate	0.00%	0.00%	0.00%

Income Taxes - Components of De

Income Taxes - Components of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Net operating losses	$ 162,436	$ 91,994
Capitalized expenses	14,129	16,019
Stock-based compensation	17,573	8,578
License fees	6,870	7,380
Operating lease liabilities	4,325	0
Legal fees	1,683	1,375
Other	3,329	1,807
Total deferred tax assets	210,345	127,153
Valuation allowance	(205,249)	(127,153)
Total deferred tax assets	5,096	0
Deferred tax liabilities:
Operating lease assets	(3,921)	0
Other	(1,175)	0
Total deferred tax liabilities	(5,096)	0
Net deferred tax assets (liabilities)	$ 0	$ 0

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Jan. 01, 2019	Dec. 31, 2017	Dec. 31, 2016
Income Taxes [Line Items]
Increase in valuation allowance tax	$ 78,100,000	$ 79,900,000
Net operating loss carryforwards, federal	547,700,000
Net operating loss carryforwards, state	$ 695,400,000
Net tax operating losses, expiration	begin to expire in 2032 in various amounts if not utilized
Unrecognized tax benefits	$ 86,000,000	$ 41,074,000		$ 30,051,000	$ 9,285,000
Unrecognized tax benefits if recognized would impact effective tax rate	$ 0
Domestic
Income Taxes [Line Items]
Net operating loss carryforwards, federal			$ 482,200

Income Taxes - Reconciliation_2

Income Taxes - Reconciliation of Beginning and Ending Amount of Gross Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Beginning Balance	$ 41,074	$ 30,051	$ 9,285
Gross increases for tax positions related to current year	22,800	12,927	16,371
Gross increases for tax positions related to prior year	22,126	704	9,534
Gross decreases for tax positions related to prior year	0	(2,608)	(4,643)
Impact of change in tax rate			(496)
Ending Balance	$ 86,000	$ 41,074	$ 30,051