Atara Biotherapeutics (ATRA) 10-K 1 Mar 21 2020 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2020	Feb. 18, 2021	Jun. 30, 2020
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2020
Document Fiscal Year Focus	2020
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		83,614,853
Entity Public Float			$ 906,895,284
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
ICFR Auditor Attestation Flag	true
Documents Incorporated by Reference	Portions of the Registrant’s definitive proxy statement relating to its 2021 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, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 200,404	$ 74,317
Short-term investments	300,255	184,792
Restricted cash - short-term	194	194
Accounts receivable	1,250	0
Prepaid expenses and other current assets	21,170	13,689
Total current assets	523,273	272,992
Property and equipment, net	50,517	54,176
Operating lease assets	12,303	14,007
Restricted cash - long-term	1,200	1,200
Other assets	827	567
Total assets	588,120	342,942
Current liabilities:
Accounts payable	7,118	7,963
Accrued compensation	20,458	14,706
Accrued research and development expenses	15,813	8,341
Deferred revenue	33,455	0
Other current liabilities	6,057	5,733
Total current liabilities	82,901	36,743
Deferred revenue - long-term	27,795	0
Operating lease liabilities - long-term	13,041	14,136
Other long-term liabilities	2,044	1,282
Total liabilities	125,781	52,161
Commitments and contingencies (Note 9)
Stockholders’ equity:
Common stock—$0.0001 par value, 500,000 shares authorized as of December 31, 2020 and 2019, respectively; 83,372 and 56,806 shares issued and outstanding as of December 31, 2020 and 2019, respectively	8	6
Additional paid-in capital	1,586,616	1,108,516
Accumulated other comprehensive income	296	220
Accumulated deficit	(1,124,581)	(817,961)
Total stockholders’ equity	462,339	290,781
Total liabilities and stockholders’ equity	$ 588,120	$ 342,942

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2020	Dec. 31, 2019
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	83,372,000	56,806,000
Common stock, shares outstanding	83,372,000	56,806,000

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating expenses:
Research and development	$ 244,650	$ 216,097	$ 167,457
General and administrative	64,402	79,584	69,654
Total operating expenses	309,052	295,681	237,111
Loss from operations	(309,052)	(295,681)	(237,111)
Interest and other income, net	2,447	4,717	6,368
Loss before provision for income taxes	(306,605)	(290,964)	(230,743)
Provision for (benefit from) income taxes	15	12	(44)
Net loss	(306,620)	(290,976)	(230,699)
Other comprehensive gain (loss):
Unrealized gain (loss) on available-for-sale securities	76	560	(189)
Comprehensive loss	$ (306,544)	$ (290,416)	$ (230,888)
Net loss per common share:
Basic and diluted net loss per common share	$ (4.15)	$ (5.67)	$ (5.27)
Weighted-average shares outstanding used to calculate basic and diluted net loss per common share	73,973	51,308	43,811

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Cumulative Effect, Period of Adoption, Adjustment	Cumulative Effect, Period of Adoption, Adjusted Balance	At The Market Offering	Underwritten Public Offering	Common Stock	Common StockCumulative Effect, Period of Adoption, Adjusted Balance	Common StockAt The Market Offering	Common StockUnderwritten Public Offering	Additional Paid-in Capital	Additional Paid-in CapitalCumulative Effect, Period of Adoption, Adjusted Balance	Additional Paid-in CapitalAt The Market Offering	Additional Paid-in CapitalUnderwritten Public Offering	Accumulated Other Comprehensive Income (Loss)	Accumulated Other Comprehensive Income (Loss)Cumulative Effect, Period of Adoption, Adjusted Balance	Accumulated Deficit	Accumulated DeficitCumulative Effect, Period of Adoption, Adjustment	Accumulated DeficitCumulative Effect, Period of Adoption, Adjusted Balance
Beginning balance at Dec. 31, 2017	$ 177,864					$ 3				$ 474,662				$ (151)		$ (296,650)
Beginning 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	$ 364	$ 339,221			$ 5	$ 5			866,541	$ 866,541			(340)	$ (340)	(527,349)	$ 364	$ (526,985)
Ending balance (in shares) at Dec. 31, 2018						45,951	45,951
Accounting Standards Update Extensible List	us-gaap:AccountingStandardsUpdate201602Member
Issuance of common stock and pre-funded warrants, 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
Issuance of common stock and pre-funded warrants, net of offering costs					$ 353,588				$ 2				$ 353,586
Issuance of common stock and pre-funded warrants, net of offering costs, shares									20,060
Issuance of common stock, net of commissions and offering costs, value				$ 68,004								$ 68,004
Issuance of common stock, net of commissions and offering costs, shares								4,786
Exercise of pre-funded warrants, shares						57
RSU settlements, net of shares withheld	(1,521)									(1,521)
RSU settlements, net of shares withheld, shares						1,112
Issuance of common stock pursuant to employee stock awards	6,680									6,680
Issuance of common stock pursuant to employee stock awards, shares						551
Stock-based compensation expense	51,351									51,351
Net loss	(306,620)															(306,620)
Unrealized gain (loss) on available-for-sale securities	76													76
Ending balance at Dec. 31, 2020	$ 462,339					$ 8				$ 1,586,616				$ 296		$ (1,124,581)
Ending balance (in shares) at Dec. 31, 2020						83,372

Consolidated Statements of St_2

Consolidated Statements of Stockholders' Equity (Parenthetical) - Common Stock - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Underwritten Public Offering
Stock issuance, discounts, commissions and offering costs	$ 583	$ 284	$ 526
At The Market Offering
Stock issuance, discounts, commissions and offering costs	$ 1,887	$ 1,553	$ 1,310

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating activities
Net loss	$ (306,620)	$ (290,976)	$ (230,699)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	51,351	51,696	33,817
Depreciation and amortization expense	8,332	7,070	3,732
Non-cash operating lease expense	1,457	964	0
Amortization (accretion) of investment premiums (discounts)	828	(1,330)	(1,885)
Loss on disposals of property and equipment	130	1,027	0
Asset retirement obligation accretion expense	78	71	49
Non-cash interest expense	0	0	211
Changes in operating assets and liabilities:
Accounts receivable	(1,250)	0	0
Prepaid expenses and other current assets	(8,666)	(998)	(5,764)
Operating lease assets	886	239	0
Other assets	(219)	322	(314)
Accounts payable	(815)	4,213	(1,958)
Accrued compensation	5,752	4,070	4,972
Accrued research and development expenses	7,472	(10,869)	15,204
Other current liabilities	(187)	(394)	2,491
Deferred revenue	61,250	0	0
Operating lease liabilities	(1,316)	(731)	0
Other long-term liabilities	778	0	372
Net cash used in operating activities	(180,759)	(235,626)	(179,772)
Investing activities
Purchases of short-term investments	(425,868)	(270,230)	(466,489)
Proceeds from maturities and sales of short-term investments	309,653	336,261	306,125
Purchases of property and equipment	(4,513)	(5,733)	(35,925)
Proceeds from sale of property and equipment	0	161	0
Net cash (used in) provided by investing activities	(120,728)	60,459	(196,289)
Financing activities
Proceeds from sale of common stock and pre-funded warrants in underwritten offerings, net	353,780	140,888	293,290
Proceeds from issuance of common stock through ATM facilities, net	69,189	47,729	47,586
Proceeds from employee stock awards	6,680	7,350	24,691
Taxes paid related to net share settlement of restricted stock units	(1,521)	(6,695)	(7,503)
Principal payments on finance and capital lease obligations	(389)	(486)	(528)
Other financing activities, net	(165)	0	0
Net cash provided by financing activities	427,574	188,786	357,536
Increase (decrease) in cash, cash equivalents and restricted cash	126,087	13,619	(18,525)
Cash, cash equivalents and restricted cash at beginning of period	75,711	62,092	80,617
Cash, cash equivalents and restricted cash at end of period	201,798	75,711	62,092
Non-cash investing and financing activities
Property and equipment purchases included in accounts payable and other accrued liabilities	326	276	1,579
Accrued costs related to underwritten public offering	192	172	0
Proceeds from issuance of common stock through ATM facilities not yet received	0	1,185	0
Capitalized lease obligations	0	0	441
Property and equipment acquired under capital leases	0	0	191
Asset retirement costs	0	0	88
Interest capitalized during construction period for build-to-suit lease arrangement	0	0	77
Supplemental cash flow disclosure
Cash paid for interest	62	50	240
Cash paid for income taxes	$ 10	$ 0	$ 0

Description of Business

Description of Business	12 Months Ended
	Dec. 31, 2020
Description Of Business [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 pioneer in T-cell immunotherapy, leveraging its novel allogeneic EBV T-cell platform to develop transformative therapies for patients with serious diseases, including solid tumors, hematologic cancers and autoimmune disease. We have several T-cell immunotherapies in clinical development and are progressing multiple next-generation allogeneic chimeric antigen receptor T-cell (“CAR T”) programs. We have entered into a research, development and license agreement (“Bayer License Agreement”) with Bayer AG (“Bayer”) pursuant to which we granted to Bayer an exclusive, field-limited license under the applicable patents and know-how owned or controlled by us and our affiliates covering or related to ATA2271 and ATA3271. See Note 7 for further information. 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, 2020
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 primarily on public and private equity financings to fund our operations. 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, 2020 will be sufficient to fund our operations for at least the next twelve months from the date of issuance of these financial statements. 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. O ne research, development and license agreement entered into in 2020 accounted for all of the deferred revenue as of December 31, 2020 . For the year ended December 31, 2020 , we did not recognize any revenue from this agreement. Our accounts receivable balance consists of amounts due pursuant to this agreement. We believe the receivable balance is fully collectible. 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 revenue recognition, clinical study and other accruals, stock-based compensation expense and income taxes. Actual results could differ materially from those estimates. Leases On January 1, 2019, we adopted Accounting Standards Update (“ASU”) No. 2016-02, Leases (Topic 842) 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 short-term 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 in property and equipment, net, with the related amortization of the capital lease asset recorded in depreciation expense, and we recorded 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. See Note 8 for further information. 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, 2020 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, if an allowance for credit losses is recognized or if an 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 impaired, which would require us to record an allowance for credit losses or 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, 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, the financial condition of the issuer and any changes thereto, and, as necessary, the portion of a decline in fair value that is credit-related. This assessment could change in the future due to new developments or changes in assumptions related to any particular security. Realized gains and losses, allowances for credit losses and impairments on available-for-sale securities, if any, are recorded to interest and other income, net in the statements of operations and comprehensive loss. Fair Value Measurement The carrying amounts of certain of our financial instruments including cash equivalents, accounts receivable, 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, Level 1: Quoted prices in active markets for identical assets or liabilities that we have the ability to access Level 2: 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 begin to 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. Revenue Recognition At inception, we determine whether contracts are within the scope of Accounting Standards Codification Topic 606 (ASU No. 2014-09), Revenue from Contracts with Customers Performance obligations promised in a contract are identified based on the goods and services that will be transferred to the customer that are both capable of being distinct and are distinct in the context of the contract. To the extent a contract includes multiple promised goods and services, we apply judgment to determine whether promised goods and services are both capable of being distinct and distinct in the context of the contract. If these criteria are not met, the promised goods and services are accounted for as a combined performance obligation. The transaction price is determined based on the consideration to which we will be entitled in exchange for transferring goods and services to the customer. To the extent the transaction price includes variable consideration, we estimate the amount of variable consideration that should be included in the transaction price utilizing either the expected value method or the most likely amount method, depending on the nature of the variable consideration. Variable consideration is included in the transaction price if, in our judgment, it is probable that a significant future reversal of cumulative revenue under the contract will not occur. Any estimates, including the effect of the constraint on variable consideration, are evaluated at each reporting period for any changes. Determining the transaction price requires significant judgment, which is discussed in further detail for our research and license agreement in Note 7. Our research and license agreement did not contain a significant financing component. If the contract contains a single performance obligation, the entire transaction price is allocated to the single performance obligation. Contracts that contain multiple performance obligations require an allocation of the transaction price to each performance obligation on a relative standalone selling price basis unless the transaction price is variable and meets the criteria to be allocated entirely to a performance obligation or to a distinct service that forms part of a single performance obligation. The consideration to be received is allocated among the separate performance obligations based on relative standalone selling prices. We typically determine standalone selling prices using an adjusted market assessment approach model. We satisfy performance obligations either over time or at a point in time. Revenue is recognized over time if either (i) the customer simultaneously receives and consumes the benefits provided by our performance, (ii) our performance creates or enhances an asset that the customer controls as the asset is created or enhanced or (iii) our performance does not create an asset with an alternative use to the entity and we have an enforceable right to payment for performance completed to date. If we do not satisfy a performance obligation over time, the related performance obligation is satisfied at a point in time by transferring control of a promised good or service to a customer. We evaluate the measure of progress each reporting period and, if necessary, adjust the measure of performance and related revenue recognition. As of December 31, 2020 , our deferred revenue is related to the Bayer License Agreement, which is within the scope of ASC 606. A s discussed in further detail in Note 7 , the terms of this arrangement include potential payments to us for the following: nonrefundable, upfront fees; development, regulatory, and commercial milestone payments; research and development funding payments; and royalties on the net sales of licensed products. T hese payments relate to promised goods or services for which revenue will be recognized upon our satisfaction of the underlying performance obligations . Licenses of intellectual property Milestone payments Royalties We receive payments from our customer based on billing schedules established in each contract. Our contract liabilities consist of deferred revenue. Upfront payments and fees are recorded as deferred revenue upon receipt or when due and may require deferral of revenue recognition to a future period until the we have satisfied our obligations under these arrangements. Certain judgments affect the application of our revenue recognition policy. For example, we record short-term and long-term deferred revenue based on our best estimate of when such revenue will be recognized. Short-term deferred revenue consists of amounts that are expected to be recognized as revenue in the next 12 months, and long-term deferred revenue consists of amounts that we do not expect will be recognized in the next 12 months. This estimate is based on our current operating plan and, if our operating plan should change in the future, we may recognize a different amount of deferred revenue over the next 12-month period. Contract Balances Customer payments are recorded as deferred revenue upon receipt or when invoiced and may require deferral of revenue recognition to a future period until we satisfy its performance obligations under these arrangements. Amounts payable to us are recorded as accounts receivable when our right to consideration is unconditional. 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 facilities, 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 years ended December 31, 2020 and 2019, we recorded matching contributions of approximately $2.1 million and $1.6 million, respectively. Other Current Liabilities Other current liabilities consisted of the following as of each period end: December 31, December 31, 2020 2019 (in thousands) Accrued operating expenses $ 3,016 $ 3,900 Current portion of operating lease liabilities 1,730 1,312 Current portion of finance lease liabilities 255 269 Other accrued liabilities 1,056 252 Total other current liabilities $ 6,057 $ 5,733 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, 2020 and 2019. 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 ASUs issued by the Financial Accounting Standards Board (“FASB”). Other than the ASUs we adopted effective January 1, 2020 and listed below, all other ASUs were assessed and determined to be either not applicable or are expected to have minimal impact on our consolidated financial statements. Adoption of New Accounting Pronouncements We adopted ASU No. 2016-13 (as amended by ASUs 2018-19, 2019-04, 2019-05 and 2019-11), Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments We adopted 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 We adopted ASU No. 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes

Net Loss per Common Share

Net Loss per Common Share	12 Months Ended
	Dec. 31, 2020
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”), unvested performance-based RSUs for which established performance criteria have been achieved as of the end of the respective periods, 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, 2020 2019 2018 Unvested RSUs 2,868,407 1,910,764 1,405,460 Vested and unvested options 7,832,386 6,934,262 6,276,999 ESPP share purchase rights 26,349 20,438 7,974 Total 10,727,142 8,865,464 7,690,433

Financial Instruments

Financial Instruments	12 Months Ended
	Dec. 31, 2020
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, 2020: Input Level Cost Gain Loss Fair Value (in thousands) Money market funds Level 1 $ 168,343 $ — $ — $ 168,343 U.S. Treasury obligations Level 2 230,239 113 (6 ) 230,346 Government agency obligations Level 2 22,537 22 (3 ) 22,556 Corporate debt obligations Level 2 50,080 166 (1 ) 50,245 Commercial paper Level 2 17,990 — — 17,990 Asset-backed securities Level 2 9,860 10 (5 ) 9,865 Total available-for-sale securities 499,049 311 (15 ) 499,345 Less: amounts classified as cash equivalents (199,090 ) — — (199,090 ) Amounts classified as short-term investments $ 299,959 $ 311 $ (15 ) $ 300,255 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 The amortized cost and fair value of our available-for-sale securities by contractual maturity were as follows: As of December 31, 2020 As of December 31, 2019 Amortized Estimated Amortized Estimated Cost Fair Value Cost Fair Value (in thousands) (in thousands) Maturing within one year $ 434,828 $ 435,023 $ 214,085 $ 214,199 Maturing in one to five years 64,221 64,322 42,993 43,100 Total available-for-sale securities $ 499,049 $ 499,345 $ 257,078 $ 257,299 As of December 31, 2020, no significant facts or circumstances were present to indicate a deterioration in the creditworthiness of the issuers of the available-for-sale securities we hold, and the Company has no requirement or intention to sell these securities before maturity or recovery of their amortized cost basis. We considered the current and expected future economic and market conditions surrounding the COVID-19 pandemic and determined that our investments were not significantly impacted. For all securities with a fair value less than its amortized cost basis, we determined the decline in fair value below amortized cost basis to be immaterial and non-credit related, and therefore no allowance for losses has been recorded. During the years ended , 2019 and 2018, we did not recognize any impairment losses on our investments. We have elected the practical expedient to exclude the applicable accrued interest from both the fair value and the amortized cost basis of our available-for-sale securities for purposes of identifying and measuring an impairment. We present accrued interest receivable related to our available-for-sale securities in prepaid expenses and other current assets, separate from short-term investments on our consolidated balance sheet. As of December 31, 2020 and 2019, accrued interest receivable was $0.7 million and $0.9 million, respectively. Our accounting policy is to not measure an allowance for credit losses for accrued interest receivables and to write-off any uncollectible accrued interest receivable as a reversal of interest income in a timely manner, which we consider to be in the period in which we determine the accrued interest will not be collected by us. We have not written off any accrued interest receivables for the years ended December 31, 2020, 2019 and 2018. 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, 2020 and 2019, 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, 2020 2019 (in thousands) Cash and cash equivalents $ 200,404 $ 74,317 Restricted cash - short-term 194 194 Restricted cash - long-term 1,200 1,200 Total cash, cash equivalents and restricted cash $ 201,798 $ 75,711

Property and Equipment

Property and Equipment	12 Months Ended
	Dec. 31, 2020
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, 2020 2019 (in thousands) Leasehold improvements $ 50,132 $ 49,028 Lab equipment 8,033 6,815 Machinery and equipment 5,023 3,832 Computer equipment and software 4,060 3,299 Furniture and fixtures 2,066 1,764 Construction in progress 879 1,116 Property and equipment, gross 70,193 65,854 Less: accumulated depreciation and amortization (19,676 ) (11,678 ) Property and equipment, net $ 50,517 $ 54,176 Depreciation and amortization expense was $8.3 million, $7.1 million and $3.7 million for the years ended December 31, 2020 , 2019 and 2018

License, Collaboration and Manu

License, Collaboration and Manufacturing Agreements	12 Months Ended
	Dec. 31, 2020
License Collaboration And Manufacturing Agreements [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. We are required to make payments of up to $33.0 million to MSK based on achievement of specified 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. 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. The license agreement expires on a product-by-product and country-by-country basis on the latest of: (i) expiration of the last licensed patent rights related to each licensed product, (ii) expiration of any market exclusivity period granted by law with respect to each licensed product, and (iii) a specified number of years after the first commercial sale of the licensed product in each country. Upon expiration of the license agreement, Atara will retain non-exclusive rights to the licensed products. 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. Under the terms of the license agreement, we obtained 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 that we exercised in June 2018. We further amended and restated our license agreement and research and development collaboration agreements with QIMR Berghofer in August 2019 to terminate our license to certain rights related to cytomegalovirus and again in August 2020 to terminate our license to certain rights related to BK polyomavirus and JC polyomavirus. 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 we licensed rights related to our next-generation CAR T programs from Moffitt Cancer Center in August 2018, and we agreed to collaborate through sponsored research in connection with each of these licenses. We also licensed rights related to our MSK-partnered next-generation CAR T programs from the National Institutes of Health in December 2018. 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, 2020 and 2019, there were no outstanding obligations for milestones and royalties under our license and collaboration agreements. Cognate Agreement In December 2019, we entered into a Commercial Manufacturing Services Agreement (the “Manufacturing Agreement”) with Cognate Bioservices, Inc. (“Cognate”). Pursuant to the Manufacturing Agreement, Cognate provides manufacturing services for certain of our 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.

Research, Development and Licen

Research, Development and License Agreement	12 Months Ended
	Dec. 31, 2020
Research And Development [Abstract]
Research, Development and License Agreement	7. Research, Development and License Agreement In December 2020, we entered into the Bayer License Agreement to develop mesothelin-directed CAR T-cell therapies for the treatment of solid tumors, pursuant to which we granted to Bayer an exclusive, field-limited license under the applicable patents and know-how owned or controlled by us and our affiliates covering or related to ATA2271 and ATA3271 (the “Licensed Products”). Under the terms of the Bayer License Agreement, we will be responsible at our cost for all mutually agreed preclinical and clinical activities for ATA2271 through the first in human Phase 1 clinical study in collaboration with MSK, following which Bayer will be responsible for the further development of ATA2271 at its cost. Bayer will be responsible for the development of ATA3271, except for certain mutually agreed preclinical, translational, manufacturing and supply chain activities to be performed by us relating to ATA3271, in each case at Bayer’s cost. Bayer will also be solely responsible for commercializing the Licensed Products at its cost. In December 2020, we received an upfront cash payment of $45.0 million from Bayer for the exclusive license grant, net of applicable withholding taxes, which we believe are recoverable, and an additional $15.0 million upfront reimbursement payment for certain research and process development activities to be performed by us. We are also entitled to receive (i) up to an additional $5.0 million for additional, specified translational activities under the Bayer License Agreement, of which we have invoiced $1.3 million, and (ii) an aggregate of up to $610.0 million in milestone payments upon achieving certain development, regulatory and commercial milestones relating to the Licensed Products. In addition, we are eligible to receive from Bayer tiered royalties at percentages up to low double digits on worldwide net product sales of the Licensed Products on a country-by-country and product-by-product basis until the later of 12 years after the first commercial sale in such country or the expiration of specified patent rights in such country, subject to certain reductions and aggregate minimum floors. We will negotiate a separate manufacturing and supply agreement with Bayer for us to manufacture allogeneic mesothelin-directed CAR T-cell therapies for Bayer to use in clinical trials at a price based on our costs plus a margin, which is consistent with our standalone selling price. Bayer and we have formed a joint steering committee (“JSC”) that will provide oversight, decision making and implementation guidance regarding the collaboration activities covered under the agreement. We assessed this arrangement in accordance with ASC 606 and concluded that the promises in the Bayer License Agreement represent transactions with a customer. We concluded that the Bayer License Agreement contains the following promises: (i) a development and commercialization license; (ii) performance of early-stage research and development (“R&D”) services, including technology transfer services; (iii) JSC participation; and (iv) chemistry, manufacturing and control (“CMC”) services. In accordance with ASC 606, we determined that the license, early-stage R&D and CMC services were not distinct from each other, as the license, early-stage R&D and CMC services are highly interdependent upon one another. Participation on the JSC to oversee the research and development activities are combined into the single performance obligation as these activities are highly interdependent with the other R&D and CMC services. Accordingly, we determined that these promises should be combined into a single performance obligation. Under the Bayer License Agreement, in order to evaluate the appropriate transaction price, we determined that the $45.0 million upfront payment for the license, $15.0 million for certain research and process development activities and the $5.0 million for additional specified translational activities, to be billed based on certain criteria being met, constituted the entire consideration to be included in the transaction price at the outset of the arrangement, and this amount was allocated to the single performance obligation. The potential development and commercial milestone payments that we are eligible to receive were excluded from the transaction price, as all milestone amounts were fully constrained based on the probability of achievement. None of the future royalty and sales-based milestone payments were included in the transaction price, as the potential payments represent sales-based consideration. We will reevaluate the transaction price at the end of each reporting period and as uncertain events are resolved or other changes in circumstances occur, and, if necessary, adjust our estimate of the transaction price. We will utilize a cost-based input method to measure its progress toward completion of its performance obligation and to calculate the corresponding amount of revenue to recognize each period, as we determined the cost-based input method to be the best measure of progress, as other measures do not reflect how we transfer our performance obligation to Bayer. In applying the cost-based input method of revenue recognition, revenue will be recognized based on the amount of actual costs incurred relative to the total budgeted costs expected to be incurred for the combined performance obligation. A cost-based input method of revenue recognition requires us to make estimates of costs to complete our performance obligation. In making such estimates, significant judgment is required to evaluate assumptions related to cost estimates. The cumulative effect of revisions to estimated costs to complete our performance obligation will be recorded in the period in which changes are identified and amounts can be reasonably estimated. Revenue associated with the combined performance obligation is being recognized over the initial estimated contract term of three years. The transfer of control occurs over this time period and, in our judgment, is the best measure of progress towards satisfying the performance obligation. A significant change in these assumptions and estimates could have a material impact on the timing and amount of revenue recognized in future periods. During the year ended December 31, 2020, we did not recognize any revenue under Bayer License Agreement. Deferred revenue related to the agreement with Bayer amounted to $61.3 million as of December 31, 2020, of which $33.5 million is included in current liabilities and $27.8 million is included in long-term liabilities, is expected to be recognized over the next three years. No development or sales-based milestone payments were received during the year ended December 31, 2020.

Leases

Leases	12 Months Ended
	Dec. 31, 2020
Leases [Abstract]
Leases	8 . Leases We lease our corporate headquarters in South San Francisco, California under a non-cancellable lease agreement that expires in May 2022 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 million for the year ended December 31, 2018 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 years ended December 31, 2020 and 2019. The maturities of lease liabilities under our operating and finance leases as of December 31, 2020 were as follows: Operating Leases Finance Leases Years Ending December 31, (in thousands) 2021 $ 3,177 $ 282 2022 2,835 130 2023 2,696 29 2024 2,593 — 2025 2,587 — Thereafter 9,300 — Total lease payments $ 23,188 $ 441 Less: amount representing interest (8,417 ) (37 ) Present value of lease liabilities $ 14,771 $ 404 Balance as of December 31, 2020 Other current liabilities $ 1,730 $ 255 Operating lease liabilities - long-term 13,041 — Other long-term liabilities — 149 Total $ 14,771 $ 404 The components of lease cost were as follows: Year Ended Year Ended December 31, 2020 December 31, 2019 (in thousands) Operating lease cost: Operating lease cost $ 3,020 $ 2,578 Short-term lease cost 987 770 Total operating lease cost $ 4,007 $ 3,348 Finance lease cost: Amortization expense $ 389 $ 324 Interest on lease liabilities 60 56 Total finance lease cost $ 449 $ 380 Rent expense under operating leases for the year ended December 31, 2018 was $2.2 million. Other information related to leases was as follows: Year Ended Year Ended December 31, 2020 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 $ 2,878 $ 2,346 Operating cash flows for finance leases 62 50 Financing cash flows for finance leases 389 486 Operating lease assets obtained in exchange for lease obligations: $ — $ 838 Finance lease assets obtained in exchange for lease obligations: 281 323 Non-cash increase to operating lease assets due to remeasurement of lease liabilities: 639 — Weighted Average Remaining Lease Term Operating leases 9.4 years 10.3 years Finance leases 1.7 years 2.5 years Weighted Average Discount Rate Operating leases 10.3 % 10.4 % Finance leases 9.7 % 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, 2019 $ 788 Accretion expense 78 Balance as of December 31, 2020 $ 866

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2020
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	9 . 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 consider the fair value of these indemnification agreements to be minimal. Accordingly, we did not record liabilities for these agreements as of December 31, 2020 and 2019. 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, 2020
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stockholders' Equity	10 . 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, 2020, 2,888,526 of the pre-funded warrants from the July 2019 underwritten public offering were outstanding. In the second quarter of 2020, we issued and sold 12,633,039 shares of common stock at a public offering price of $11.32 per share and pre-funded warrants to purchase 2,866,961 shares of common stock at a public offering price of $11.3199 per warrant in an underwritten public offering pursuant to a shelf registration on Form S-3. We granted the underwriters an option to purchase up to 2,325,000 additional shares of our common stock at a public offering price of $11.32, less underwriting discounts and commissions. The full option was exercised by the underwriters in June 2020. The gross proceeds from this public offering were $201.8 million, resulting in net proceeds of $189.3 million, after deducting underwriting discounts and commissions and offering expenses payable by us. The terms of the pre-funded warrants issued and sold as part of this public offering were similar to those above. In December 2020, we issued and sold 5,102,041 shares of common stock at a public offering price of $24.50 per share and pre-funded warrants to purchase 2,040,816 shares of common stock at a public offering price of $24.4999 per warrant in an underwritten public offering pursuant to a shelf registration on Form S-3. The gross proceeds from this public offering were $175.0 million, resulting in net proceeds of $164.3 million, after deducting underwriting discounts and commissions and estimated offering expenses payable by us. As of December 31, 2020, all of the pre-funded warrants issued and sold as part of the 2020 underwritten public offerings were outstanding. 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. In February 2019, we terminated the 2017 ATM Facility and entered into a new sales agreement (the “2019 ATM Facility”) with Cowen, which provided 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. 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. We pay a commission of up to 3.0% of gross sales proceeds of any common stock sold under the 2020 ATM Facility. During the fiscal year ended December 31, 2020 December 31, 2019 The issuance and sale of these shares by us pursuant to the ATM facilities 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, 2020 fully utilized the 2019 ATM Facility, and remained available to be sold under the 2020 ATM Facility, subject to certain conditions as specified in the agreement Equity Incentive Plans 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 (“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. In 2020, we granted performance-based awards to certain of our employees that provide for the issuance of common stock if specified Company performance criteria related to our clinical programs are achieved. The number of performance-based awards that ultimately vests depends upon if, when and which performance criteria are achieved, as well as the employee’s continuous service, as defined in the 2014 EIP, through the date of vesting. The fair value of RSUs, including those with performance conditions, is determined as the closing stock price on the date of grant. 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, 2020 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. In September 2020, we amended the Inducement Plan to reserve an additional 1,500,000 shares of the Company’s common stock for issuance under the Inducement Plan, as amended. As of December 31, 2020 Restricted Stock Units The weighted average grant date fair value of RSUs granted during the years ended December 31, 2020 December 31, 2020 December 31, 2020 December 31, 2020 The following is a summary of RSU activity under our 2014 EIP and Inducement Plan: RSUs Shares Weighted Average Grant Date Fair Value Balance as of December 31, 2019 1,910,764 $ 26.93 Granted 3,973,991 $ 12.19 Forfeited (836,190 ) $ 18.39 Vested (1,218,945 ) $ 19.34 Balance as of December 31, 2020 3,829,620 $ 15.91 Under our RSU settlement procedures, for some of the RSUs granted to our employees, we withhold shares at settlement to cover the estimated payroll withholding tax obligations. During 2020, we settled 1,218,945 shares underlying RSUs, of which 276,822 shares underlying RSUs were net settled by withholding 106,459 shares. The value of the shares underlying RSUs withheld was $1.5 million, based on the closing price of our common stock on the settlement date. During 2019, we settled 574,168 shares underlying RSUs, of which 488,964 shares underlying RSUs were net settled by withholding 212,879 shares. The value of the shares underlying RSUs withheld was $6.7 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) Balance as of December 31, 2019 6,934,262 $ 28.25 5.9 $ 4,103 Granted 2,641,125 12.01 Exercised (268,938 ) 14.75 Forfeited or expired (1,454,563 ) 30.17 Balance as of December 31, 2020 7,851,886 $ 22.89 6.4 $ 26,834 Vested and expected to vest as of December 31, 2020 7,851,886 $ 22.89 6.4 $ 26,834 Exercisable as of December 31, 2020 3,747,718 $ 26.74 4.5 $ 6,214 Aggregate intrinsic value represents the difference between the closing stock price of our common stock on December 31, 2020 December 31, 2020 Options for 268,938, 347,716 and 1,051,180 shares of our common stock were exercised during the years ended December 31, 2020 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 during the periods indicated: Year ended December 31, 2020 2019 2018 Assumptions: Expected term (years) 6.0 5.9 4.6 Expected volatility 76.8 % 76.1 % 73.5 % Risk-free interest rate 0.8 % 2.1 % 2.7 % Expected dividend yield 0.0 % 0.0 % 0.0 % Fair Value: Weighted-average estimated grant date fair value per share $ 7.96 $ 18.06 $ 22.96 Options granted 2,641,125 2,535,425 2,998,650 Total estimated grant date fair value $ 21,023,000 $ 45,790,000 $ 68,849,000 The estimated fair value of stock options that vested in the years ended December 31, 2020 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 $1.8 million, $1.3 million and $1.2 million of expense related to the 2014 ESPP in the years ended December 31, 2020, December 31, 2020 As of December 31, 2020, there was $0.6 million of unrecognized stock-based compensation expense related to the ESPP that is expected to be recognized by the end of second quarter of 2021. 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, 2020, there were 1,586,742 shares authorized under the 2014 ESPP. Reserved Shares The following shares of common stock were reserved for future issuance under our equity incentive plans as of December 31, 2020 Total Shares Reserved 2014 Equity Incentive Plan 13,545,106 2018 Inducement Plan 2,634,836 2014 Employee Stock Purchase Plan 950,803 Total reserved shares of common stock 17,130,745 Stock-based Compensation Expense Total stock-based compensation expense related to all stock awards was as follows: Year Ended December 31, 2020 2019 2018 (in thousands) Research and development $ 31,527 $ 26,773 $ 16,211 General and administrative 19,824 24,923 17,606 Total stock-based compensation expense $ 51,351 $ 51,696 $ 33,817

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Income Taxes	1 1 . Income Taxes Losses before provision for income taxes were as follows in each period presented: Year Ended December 31, 2020 2019 2018 (in thousands) United States $ (306,758 ) $ (291,049 ) $ (230,765 ) Foreign 153 85 22 Total loss before provision for income taxes $ (306,605 ) $ (290,964 ) $ (230,743 ) 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, 2020 2019 2018 (in thousands) Current provision for (benefit from) income taxes: Federal $ — $ — $ (31 ) State 2 — (15 ) Foreign 13 12 2 Total current provision for (benefit from) income taxes $ 15 $ 12 $ (44 ) A reconciliation of statutory tax rates to effective tax rates were as follows in each of the periods presented: Year Ended December 31, 2020 2019 2018 Federal income taxes at statutory rate 21.0 % 21.0 % 21.0 % Impact of stock compensation (2.4 %) 0.1 % — Non-deductible executive compensation (0.1 %) (0.7 %) (0.7 %) Capitalized research — — 7.8 % Other 0.3 % (0.2 %) (0.6 %) Change in valuation allowance (18.8 %) (20.2 %) (27.5 %) 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, 2020 2019 (in thousands) Deferred tax assets: Net operating losses $ 237,010 $ 162,436 Stock-based compensation 20,672 17,573 Capitalized expenses 12,590 14,129 License fees 8,159 6,870 Operating lease liabilities 4,251 4,325 Legal fees 2,136 1,683 Tax credits 1,580 — Other 5,360 3,329 Total deferred tax assets 291,758 210,345 Valuation allowance (287,349 ) (205,249 ) Total deferred tax assets 4,409 5,096 Deferred tax liabilities: Operating lease assets (3,541 ) (3,921 ) Other (868 ) (1,175 ) Total deferred tax liabilities (4,409 ) (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, 2020 December 31, 2020 In response to the COVID-19 pandemic, the Coronavirus Aid, Relief, and Economic Security Act (the “CARES Act”) was signed into law on March 27, 2020, providing companies with various tax relief provisions and other stimulus measures. Such measures include, but are not limited to, temporary changes regarding the prior and future utilization of net operating losses, technical corrections to prior tax legislation for tax depreciation of certain qualified improvement property, acceleration of AMT credit refunds, and changes to business interest limitations. The Consolidated Appropriations Act was also signed into law on December 27, 2020 to provide further relief measures and renew various expiring tax provisions. Additionally, the IRS issued final regulations and proposed regulations on calculating the limitation on business interest expense, the allowance for the first-year depreciation deduction under IRC Section 168(k), as amended by the Tax Cuts and Jobs Act (the “Tax Act”), for qualified property acquired and placed in service after September 27, 2017, and meals and entertainment deductions. Based on our evaluation, these regulations did not have a material impact on the income tax provision for the years ended December 31, 2020, 2019 and 2018. Under the Tax Act, as modified by the CARES Act, federal net operating losses generated in tax years beginning on or after January 1, 2018 and in future years may be carried forward indefinitely, but the utilization of such federal net operating losses arising in taxable years beginning on or after January 1, 2021 is limited to 80% of a tax year’s taxable income. The CARES Act temporarily suspends this 80% taxable income limitation, allowing a net operating loss carryforward to fully offset taxable income in tax years beginning before January 1, 2021. Not all states conform to the Tax Act or CARES Act and some states have varying conformity to the Tax Act or CARES Act. As of December 31, 2020 As of December 31, 2020, we had federal research & development and orphan drug tax credits of $12.9 million and $75.1 million, respectively, to offset future taxes payable. These federal credits begin to expire in 2032 and 2035, respectively, if not utilized. As of December 31, 2020, we had California research & development tax credits of $21.6 million, which do not expire, to offset future taxes payable. 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, 2020. 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 . The changes in the balance of gross unrecognized tax benefits, which excludes interest and penalties, for the years ended December 31, 2018, 2019 and 2020 are as follows: (In thousands) Balance as of January 1, 2018 $ 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 Gross increases for tax positions related to current year 24,648 Gross increases for tax positions related to prior year — Gross decreases for tax positions related to prior year (47 ) Balance as of December 31, 2020 $ 110,601 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 $110.6 million total unrecognized tax benefits as of December 31, 2020 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, 2020 and 2019 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. We also file in other state, local and foreign jurisdictions in which we operate, and such tax years remain open to examination. As of December 31, 2020, we are permanently reinvested with respect to our foreign earnings, which are not material as our foreign operations are not significant.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2020
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 primarily on public and private equity financings to fund our operations. 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, 2020 will be sufficient to fund our operations for at least the next twelve months from the date of issuance of these financial statements.
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. O ne research, development and license agreement entered into in 2020 accounted for all of the deferred revenue as of December 31, 2020 . For the year ended December 31, 2020 , we did not recognize any revenue from this agreement. Our accounts receivable balance consists of amounts due pursuant to this agreement. We believe the receivable balance is fully collectible. 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 revenue recognition, clinical study and other accruals, stock-based compensation expense and income taxes. Actual results could differ materially from those estimates.
Leases	Leases On January 1, 2019, we adopted Accounting Standards Update (“ASU”) No. 2016-02, Leases (Topic 842) 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 short-term 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 in property and equipment, net, with the related amortization of the capital lease asset recorded in depreciation expense, and we recorded 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. See Note 8 for further information.
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, 2020 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, if an allowance for credit losses is recognized or if an 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 impaired, which would require us to record an allowance for credit losses or 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, 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, the financial condition of the issuer and any changes thereto, and, as necessary, the portion of a decline in fair value that is credit-related. This assessment could change in the future due to new developments or changes in assumptions related to any particular security. Realized gains and losses, allowances for credit losses and impairments on available-for-sale securities, if any, are recorded to interest and other income, 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, accounts receivable, 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, Level 1: Quoted prices in active markets for identical assets or liabilities that we have the ability to access Level 2: 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 begin to 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.
Revenue Recognition	Revenue Recognition At inception, we determine whether contracts are within the scope of Accounting Standards Codification Topic 606 (ASU No. 2014-09), Revenue from Contracts with Customers Performance obligations promised in a contract are identified based on the goods and services that will be transferred to the customer that are both capable of being distinct and are distinct in the context of the contract. To the extent a contract includes multiple promised goods and services, we apply judgment to determine whether promised goods and services are both capable of being distinct and distinct in the context of the contract. If these criteria are not met, the promised goods and services are accounted for as a combined performance obligation. The transaction price is determined based on the consideration to which we will be entitled in exchange for transferring goods and services to the customer. To the extent the transaction price includes variable consideration, we estimate the amount of variable consideration that should be included in the transaction price utilizing either the expected value method or the most likely amount method, depending on the nature of the variable consideration. Variable consideration is included in the transaction price if, in our judgment, it is probable that a significant future reversal of cumulative revenue under the contract will not occur. Any estimates, including the effect of the constraint on variable consideration, are evaluated at each reporting period for any changes. Determining the transaction price requires significant judgment, which is discussed in further detail for our research and license agreement in Note 7. Our research and license agreement did not contain a significant financing component. If the contract contains a single performance obligation, the entire transaction price is allocated to the single performance obligation. Contracts that contain multiple performance obligations require an allocation of the transaction price to each performance obligation on a relative standalone selling price basis unless the transaction price is variable and meets the criteria to be allocated entirely to a performance obligation or to a distinct service that forms part of a single performance obligation. The consideration to be received is allocated among the separate performance obligations based on relative standalone selling prices. We typically determine standalone selling prices using an adjusted market assessment approach model. We satisfy performance obligations either over time or at a point in time. Revenue is recognized over time if either (i) the customer simultaneously receives and consumes the benefits provided by our performance, (ii) our performance creates or enhances an asset that the customer controls as the asset is created or enhanced or (iii) our performance does not create an asset with an alternative use to the entity and we have an enforceable right to payment for performance completed to date. If we do not satisfy a performance obligation over time, the related performance obligation is satisfied at a point in time by transferring control of a promised good or service to a customer. We evaluate the measure of progress each reporting period and, if necessary, adjust the measure of performance and related revenue recognition. As of December 31, 2020 , our deferred revenue is related to the Bayer License Agreement, which is within the scope of ASC 606. A s discussed in further detail in Note 7 , the terms of this arrangement include potential payments to us for the following: nonrefundable, upfront fees; development, regulatory, and commercial milestone payments; research and development funding payments; and royalties on the net sales of licensed products. T hese payments relate to promised goods or services for which revenue will be recognized upon our satisfaction of the underlying performance obligations . Licenses of intellectual property Milestone payments Royalties We receive payments from our customer based on billing schedules established in each contract. Our contract liabilities consist of deferred revenue. Upfront payments and fees are recorded as deferred revenue upon receipt or when due and may require deferral of revenue recognition to a future period until the we have satisfied our obligations under these arrangements. Certain judgments affect the application of our revenue recognition policy. For example, we record short-term and long-term deferred revenue based on our best estimate of when such revenue will be recognized. Short-term deferred revenue consists of amounts that are expected to be recognized as revenue in the next 12 months, and long-term deferred revenue consists of amounts that we do not expect will be recognized in the next 12 months. This estimate is based on our current operating plan and, if our operating plan should change in the future, we may recognize a different amount of deferred revenue over the next 12-month period.
Contract Balances	Contract Balances Customer payments are recorded as deferred revenue upon receipt or when invoiced and may require deferral of revenue recognition to a future period until we satisfy its performance obligations under these arrangements. Amounts payable to us are recorded as accounts receivable when our right to consideration is unconditional.
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 facilities, 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 years ended December 31, 2020 and 2019, we recorded matching contributions of approximately $2.1 million and $1.6 million, respectively.
Other Current Liabilities	Other Current Liabilities Other current liabilities consisted of the following as of each period end: December 31, December 31, 2020 2019 (in thousands) Accrued operating expenses $ 3,016 $ 3,900 Current portion of operating lease liabilities 1,730 1,312 Current portion of finance lease liabilities 255 269 Other accrued liabilities 1,056 252 Total other current liabilities $ 6,057 $ 5,733
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, 2020 and 2019. 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 ASUs issued by the Financial Accounting Standards Board (“FASB”). Other than the ASUs we adopted effective January 1, 2020 and listed below, all other ASUs were assessed and determined to be either not applicable or are expected to have minimal impact on our consolidated financial statements. Adoption of New Accounting Pronouncements We adopted ASU No. 2016-13 (as amended by ASUs 2018-19, 2019-04, 2019-05 and 2019-11), Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments We adopted 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 We adopted ASU No. 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Summary of Other Current Liabilities	Other current liabilities consisted of the following as of each period end: December 31, December 31, 2020 2019 (in thousands) Accrued operating expenses $ 3,016 $ 3,900 Current portion of operating lease liabilities 1,730 1,312 Current portion of finance lease liabilities 255 269 Other accrued liabilities 1,056 252 Total other current liabilities $ 6,057 $ 5,733

Net Loss per Common Share (Tabl

Net Loss per Common Share (Tables)	12 Months Ended
	Dec. 31, 2020
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, 2020 2019 2018 Unvested RSUs 2,868,407 1,910,764 1,405,460 Vested and unvested options 7,832,386 6,934,262 6,276,999 ESPP share purchase rights 26,349 20,438 7,974 Total 10,727,142 8,865,464 7,690,433

Financial Instruments (Tables)

Financial Instruments (Tables)	12 Months Ended
	Dec. 31, 2020
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, 2020: Input Level Cost Gain Loss Fair Value (in thousands) Money market funds Level 1 $ 168,343 $ — $ — $ 168,343 U.S. Treasury obligations Level 2 230,239 113 (6 ) 230,346 Government agency obligations Level 2 22,537 22 (3 ) 22,556 Corporate debt obligations Level 2 50,080 166 (1 ) 50,245 Commercial paper Level 2 17,990 — — 17,990 Asset-backed securities Level 2 9,860 10 (5 ) 9,865 Total available-for-sale securities 499,049 311 (15 ) 499,345 Less: amounts classified as cash equivalents (199,090 ) — — (199,090 ) Amounts classified as short-term investments $ 299,959 $ 311 $ (15 ) $ 300,255 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
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, 2020 As of December 31, 2019 Amortized Estimated Amortized Estimated Cost Fair Value Cost Fair Value (in thousands) (in thousands) Maturing within one year $ 434,828 $ 435,023 $ 214,085 $ 214,199 Maturing in one to five years 64,221 64,322 42,993 43,100 Total available-for-sale securities $ 499,049 $ 499,345 $ 257,078 $ 257,299
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, 2020 2019 (in thousands) Cash and cash equivalents $ 200,404 $ 74,317 Restricted cash - short-term 194 194 Restricted cash - long-term 1,200 1,200 Total cash, cash equivalents and restricted cash $ 201,798 $ 75,711

Property and Equipment (Tables)

Property and Equipment (Tables)	12 Months Ended
	Dec. 31, 2020
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, 2020 2019 (in thousands) Leasehold improvements $ 50,132 $ 49,028 Lab equipment 8,033 6,815 Machinery and equipment 5,023 3,832 Computer equipment and software 4,060 3,299 Furniture and fixtures 2,066 1,764 Construction in progress 879 1,116 Property and equipment, gross 70,193 65,854 Less: accumulated depreciation and amortization (19,676 ) (11,678 ) Property and equipment, net $ 50,517 $ 54,176

Leases (Tables)

Leases (Tables)	12 Months Ended
	Dec. 31, 2020
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, 2020 were as follows: Operating Leases Finance Leases Years Ending December 31, (in thousands) 2021 $ 3,177 $ 282 2022 2,835 130 2023 2,696 29 2024 2,593 — 2025 2,587 — Thereafter 9,300 — Total lease payments $ 23,188 $ 441 Less: amount representing interest (8,417 ) (37 ) Present value of lease liabilities $ 14,771 $ 404 Balance as of December 31, 2020 Other current liabilities $ 1,730 $ 255 Operating lease liabilities - long-term 13,041 — Other long-term liabilities — 149 Total $ 14,771 $ 404
Components of Lease Cost	The components of lease cost were as follows: Year Ended Year Ended December 31, 2020 December 31, 2019 (in thousands) Operating lease cost: Operating lease cost $ 3,020 $ 2,578 Short-term lease cost 987 770 Total operating lease cost $ 4,007 $ 3,348 Finance lease cost: Amortization expense $ 389 $ 324 Interest on lease liabilities 60 56 Total finance lease cost $ 449 $ 380
Summary of Other Information Related to Leases	Other information related to leases was as follows: Year Ended Year Ended December 31, 2020 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 $ 2,878 $ 2,346 Operating cash flows for finance leases 62 50 Financing cash flows for finance leases 389 486 Operating lease assets obtained in exchange for lease obligations: $ — $ 838 Finance lease assets obtained in exchange for lease obligations: 281 323 Non-cash increase to operating lease assets due to remeasurement of lease liabilities: 639 — Weighted Average Remaining Lease Term Operating leases 9.4 years 10.3 years Finance leases 1.7 years 2.5 years Weighted Average Discount Rate Operating leases 10.3 % 10.4 % Finance leases 9.7 % 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, 2019 $ 788 Accretion expense 78 Balance as of December 31, 2020 $ 866

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	12 Months Ended
	Dec. 31, 2020
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 Balance as of December 31, 2019 1,910,764 $ 26.93 Granted 3,973,991 $ 12.19 Forfeited (836,190 ) $ 18.39 Vested (1,218,945 ) $ 19.34 Balance as of December 31, 2020 3,829,620 $ 15.91
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) Balance as of December 31, 2019 6,934,262 $ 28.25 5.9 $ 4,103 Granted 2,641,125 12.01 Exercised (268,938 ) 14.75 Forfeited or expired (1,454,563 ) 30.17 Balance as of December 31, 2020 7,851,886 $ 22.89 6.4 $ 26,834 Vested and expected to vest as of December 31, 2020 7,851,886 $ 22.89 6.4 $ 26,834 Exercisable as of December 31, 2020 3,747,718 $ 26.74 4.5 $ 6,214
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 during the periods indicated: Year ended December 31, 2020 2019 2018 Assumptions: Expected term (years) 6.0 5.9 4.6 Expected volatility 76.8 % 76.1 % 73.5 % Risk-free interest rate 0.8 % 2.1 % 2.7 % Expected dividend yield 0.0 % 0.0 % 0.0 % Fair Value: Weighted-average estimated grant date fair value per share $ 7.96 $ 18.06 $ 22.96 Options granted 2,641,125 2,535,425 2,998,650 Total estimated grant date fair value $ 21,023,000 $ 45,790,000 $ 68,849,000
Schedule of Common Stock Reserved for Future Issuance Under Equity Incentive Plans	The following shares of common stock were reserved for future issuance under our equity incentive plans as of December 31, 2020 Total Shares Reserved 2014 Equity Incentive Plan 13,545,106 2018 Inducement Plan 2,634,836 2014 Employee Stock Purchase Plan 950,803 Total reserved shares of common stock 17,130,745
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, 2020 2019 2018 (in thousands) Research and development $ 31,527 $ 26,773 $ 16,211 General and administrative 19,824 24,923 17,606 Total stock-based compensation expense $ 51,351 $ 51,696 $ 33,817

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2020
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, 2020 2019 2018 (in thousands) United States $ (306,758 ) $ (291,049 ) $ (230,765 ) Foreign 153 85 22 Total loss before provision for income taxes $ (306,605 ) $ (290,964 ) $ (230,743 )
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, 2020 2019 2018 (in thousands) Current provision for (benefit from) income taxes: Federal $ — $ — $ (31 ) State 2 — (15 ) Foreign 13 12 2 Total current provision for (benefit from) income taxes $ 15 $ 12 $ (44 )
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, 2020 2019 2018 Federal income taxes at statutory rate 21.0 % 21.0 % 21.0 % Impact of stock compensation (2.4 %) 0.1 % — Non-deductible executive compensation (0.1 %) (0.7 %) (0.7 %) Capitalized research — — 7.8 % Other 0.3 % (0.2 %) (0.6 %) Change in valuation allowance (18.8 %) (20.2 %) (27.5 %) 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, 2020 2019 (in thousands) Deferred tax assets: Net operating losses $ 237,010 $ 162,436 Stock-based compensation 20,672 17,573 Capitalized expenses 12,590 14,129 License fees 8,159 6,870 Operating lease liabilities 4,251 4,325 Legal fees 2,136 1,683 Tax credits 1,580 — Other 5,360 3,329 Total deferred tax assets 291,758 210,345 Valuation allowance (287,349 ) (205,249 ) Total deferred tax assets 4,409 5,096 Deferred tax liabilities: Operating lease assets (3,541 ) (3,921 ) Other (868 ) (1,175 ) Total deferred tax liabilities (4,409 ) (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, 2018, 2019 and 2020 are as follows: (In thousands) Balance as of January 1, 2018 $ 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 Gross increases for tax positions related to current year 24,648 Gross increases for tax positions related to prior year — Gross decreases for tax positions related to prior year (47 ) Balance as of December 31, 2020 $ 110,601

Description of Business (Detail

Description of Business (Detail)	12 Months Ended
	Dec. 31, 2020
Description Of Business [Abstract]
Entity incorporation state	DE
Entity incorporation date	Aug. 1, 2012

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2020USD ($)Segment	Dec. 31, 2019USD ($)	Dec. 31, 2018	Jan. 01, 2019USD ($)
Significant Accounting Policies [Line Items]
Number of reportable segments | Segment	1
Number of operating segments | Segment	1
Operating lease assets	$ 12,303,000	$ 14,007,000		$ 14,300,000
Operating lease liabilities	14,771,000			$ 15,300,000
Lease obligation	$ 10,300,000
Cash and cash equivalents maturity period	90 days
Investment maturity period	90 days
Impairment of long-lived assets	$ 0
Expected dividend yield	0.00%	0.00%	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	$ 2,100,000	$ 1,600,000
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
ASU 2016-02
Significant Accounting Policies [Line Items]
Change in Accounting Principle, Accounting Standards Update, Adopted [true false]	true
Change in Accounting Principle, Accounting Standards Update, Adoption Date	Jan. 1, 2019
Change in Accounting Principle, Accounting Standards Update, Immaterial Effect [true false]	true
ASU 2016-13
Significant Accounting Policies [Line Items]
Change in Accounting Principle, Accounting Standards Update, Adopted [true false]	true
Change in Accounting Principle, Accounting Standards Update, Adoption Date	Jan. 1, 2020
Change in Accounting Principle, Accounting Standards Update, Immaterial Effect [true false]	true
ASU 2018-15
Significant Accounting Policies [Line Items]
Change in Accounting Principle, Accounting Standards Update, Adopted [true false]	true
Change in Accounting Principle, Accounting Standards Update, Adoption Date	Jan. 1, 2020
Change in Accounting Principle, Accounting Standards Update, Immaterial Effect [true false]	true
ASU 2019-12
Significant Accounting Policies [Line Items]
Change in Accounting Principle, Accounting Standards Update, Adoption Date	Jan. 1, 2020
Change in Accounting Principle, Accounting Standards Update, Immaterial Effect [true false]	true
Change in Accounting Principle, Accounting Standards Update, Early Adoption [true false]	true

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Summary of Other Current Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Accounting Policies [Abstract]
Accrued operating expenses	$ 3,016	$ 3,900
Current portion of operating lease liabilities	1,730	1,312
Current portion of finance lease liabilities	255	269
Other accrued liabilities	1,056	252
Total other current liabilities	$ 6,057	$ 5,733

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, 2020	Dec. 31, 2019	Dec. 31, 2018
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	10,727,142	8,865,464	7,690,433
Unvested RSUs
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	2,868,407	1,910,764	1,405,460
Vested and Unvested Options
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	7,832,386	6,934,262	6,276,999
ESPP Share Purchase Rights
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	26,349	20,438	7,974

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, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Total Fair Value	$ 499,345	$ 257,299
Fair Value, Measurements, Recurring
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	499,049	257,078
Total Unrealized Gain	311	233
Total Unrealized Loss	(15)	(12)
Total Fair Value	499,345	257,299
Fair Value, Measurements, Recurring | Money Market Funds | Level 1
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	168,343	63,554
Total Unrealized Gain	0	0
Total Unrealized Loss	0	0
Total Fair Value	168,343	63,554
Fair Value, Measurements, Recurring | U.S. Treasury Obligations | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	230,239	52,805
Total Unrealized Gain	113	46
Total Unrealized Loss	(6)	(1)
Total Fair Value	230,346	52,850
Fair Value, Measurements, Recurring | Government Agency Obligations | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	22,537	6,151
Total Unrealized Gain	22	1
Total Unrealized Loss	(3)	(1)
Total Fair Value	22,556	6,151
Fair Value, Measurements, Recurring | Corporate Debt Obligations | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	50,080	100,512
Total Unrealized Gain	166	180
Total Unrealized Loss	(1)	(10)
Total Fair Value	50,245	100,682
Fair Value, Measurements, Recurring | Commercial Paper | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	17,990	26,290
Total Unrealized Gain	0	0
Total Unrealized Loss	0	0
Total Fair Value	17,990	26,290
Fair Value, Measurements, Recurring | Asset-Backed Securities | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	9,860	7,266
Total Unrealized Gain	10	6
Total Unrealized Loss	(5)	0
Total Fair Value	9,865	7,272
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	199,090	72,507
Total Unrealized Gain	0	0
Total Unrealized Loss	0	0
Total Fair Value	199,090	72,507
Fair Value, Measurements, Recurring | Amounts Classified As Short-Term Investments
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	299,959	184,571
Total Unrealized Gain	311	233
Total Unrealized Loss	(15)	(12)
Total Fair Value	$ 300,255	$ 184,792

Financial Instruments - Amortiz

Financial Instruments - Amortized Cost and Fair Value of Available-for-Sale Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Amortized cost
Maturing within one year, Amortized cost	$ 434,828	$ 214,085
Maturing in one to five years, Amortized cost	64,221	42,993
Total available-for-sale securities, Amortized cost	499,049	257,078
Estimated Fair value
Maturing within one year, Estimated fair value	435,023	214,199
Maturing in one to five years, Estimated fair value	64,322	43,100
Total available-for-sale securities, Estimated fair value	$ 499,345	$ 257,299

Financial Instruments - Additio

Financial Instruments - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Financial Instruments Disclosure [Abstract]
Accrued interest receivable	$ 700,000	$ 900,000
Write off, of accrued interest receivable	0	0	$ 0
Restricted Cash	$ 1,400,000	$ 1,400,000

Financial Instruments - Reconci

Financial Instruments - Reconciliation of Cash, Cash Equivalents and Restricted Cash (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Financial Instruments Disclosure [Abstract]
Cash and cash equivalents	$ 200,404	$ 74,317
Restricted cash - short-term	194	194
Restricted cash - long-term	1,200	1,200
Total cash, cash equivalents and restricted cash	$ 201,798	$ 75,711	$ 62,092	$ 80,617

Property and Equipment - Schedu

Property and Equipment - Schedule of Property and Equipment (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 70,193	$ 65,854
Less: accumulated depreciation and amortization	(19,676)	(11,678)
Property and equipment, net	50,517	54,176
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	50,132	49,028
Lab Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	8,033	6,815
Machinery and Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	5,023	3,832
Computer Equipment and Software
Property Plant And Equipment [Line Items]
Property and equipment, gross	4,060	3,299
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and equipment, gross	2,066	1,764
Construction in Progress
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 879	$ 1,116

Property and Equipment - Additi

Property and Equipment - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Property Plant And Equipment Net By Type [Abstract]
Depreciation and amortization expense	$ 8,332	$ 7,070	$ 3,732

License, Collaboration and Ma_2

License, Collaboration and Manufacturing Agreements - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019
License, Collaboration and Manufacturing Agreements [Line Items]
Contractual obligations due to Bayer, MSK and QIMR		$ 0	$ 0
MSK Agreements
License, Collaboration and Manufacturing Agreements [Line Items]
Potential milestone payments payable		$ 33,000,000
Payments for research and development	$ 12,500,000

Research, Development and Lic_2

Research, Development and License Agreement - Additional Information (Detail) - Bayer License Agreement	1 Months Ended	12 Months Ended
	Dec. 31, 2020USD ($)	Dec. 31, 2020USD ($)
License, Collaboration and Manufacturing Agreements [Line Items]
Upfront cash payment received	$ 45,000,000
Additional upfront reimbursement payment for research and process development	15,000,000
Additional upfront cash payment entitle to receive for translational activities, invoiced amount	$ 1,300,000	$ 1,300,000
Royalty eligible to receive term after first commercial sale	12 years
Determined upfront payment for license in order to evaluate transaction price	$ 45,000,000
Determined research and process development activities in order to evaluate transaction price	15,000,000
Determined additional specified translational activities in order to evaluate transaction price	5,000,000
Revenue		0
Deferred revenue	61,300,000	61,300,000
Development or sales-based milestone payments received		0
Current Liabilities
License, Collaboration and Manufacturing Agreements [Line Items]
Deferred revenue	33,500,000	33,500,000
Long-Term Liabilities
License, Collaboration and Manufacturing Agreements [Line Items]
Deferred revenue	27,800,000	27,800,000
Maximum
License, Collaboration and Manufacturing Agreements [Line Items]
Additional upfront cash payment entitle to receive for translational activities	5,000,000	5,000,000
Aggregate milestone payments entitle to receive upon achieving certain development, regulatory and commercial milestones	$ 610,000,000	$ 610,000,000

Research, Development and Lic_3

Research, Development and License Agreement - Additional Information (Detail 1)	Dec. 31, 2020
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2021-01-01
License, Collaboration and Manufacturing Agreements [Line Items]
Revenue expected to be recognized over period	3 years

Leases - Additional Information

Leases - Additional Information (Detail)	Feb. 15, 2018USD ($)ft²	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)
Lessee Lease Description [Line Items]
Rent expenses under operating leases				$ 2,200,000
Build To Suit Lease Arrangement
Lessee Lease Description [Line Items]
Ground lease expense				400,000
ASU 2016-02 | Build To Suit Lease Arrangement
Lessee Lease Description [Line Items]
Ground lease expense		$ 0	$ 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		May 31, 2022
Lease option to extend additional term		5 years
South San Francisco California | Letter of Credit
Lessee Lease Description [Line Items]
Letter of credit maintained		$ 200,000
Letter of credit renewal term		12 months
Thousand Oaks California
Lessee Lease Description [Line Items]
Lease option to extend additional term		5 years
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, 2020	Dec. 31, 2019	Jan. 01, 2019
Leases [Abstract]
Operating leases, 2021	$ 3,177
Operating leases, 2022	2,835
Operating leases, 2023	2,696
Operating leases, 2024	2,593
Operating leases, 2025	2,587
Operating leases, Thereafter	9,300
Operating leases, Total minimum payments	23,188
Less: amount representing interest	(8,417)
Present value of lease liabilities	14,771		$ 15,300
Other current liabilities	$ 1,730	$ 1,312
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesCurrent
Operating lease liabilities - long-term	$ 13,041	14,136
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesNoncurrent
Total	$ 14,771		$ 15,300
Finance leases, 2021	282
Finance leases, 2022	130
Finance leases, 2023	29
Finance leases, 2024	0
Finance leases, 2025	0
Finance leases, Thereafter	0
Finance leases, Total minimum payments	441
Less: amount representing interest	(37)
Present value of lease liabilities	404
Other current liabilities	$ 255	$ 269
Finance Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesCurrent
Other long-term liabilities	$ 149
Finance Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesNoncurrent
Total	$ 404

Leases - Components of Lease Co

Leases - Components of Lease Cost (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Operating lease cost:
Operating lease cost	$ 3,020	$ 2,578
Short-term lease cost	987	770
Total operating lease cost	4,007	3,348
Finance lease cost:
Amortization expense	389	324
Interest on lease liabilities	60	56
Total finance lease cost	$ 449	$ 380

Leases - Summary of Other Infor

Leases - Summary of Other Information Related to Leases (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flows for operating leases	$ 2,878	$ 2,346
Operating cash flows for finance leases	62	50
Financing cash flows for finance leases	389	486	$ 528
Operating lease assets obtained in exchange for lease obligations:	0	838
Finance lease assets obtained in exchange for lease obligations:	281	323
Non-cash increase to operating lease assets due to remeasurement of lease liabilities:	$ 639	$ 0
Weighted Average Remaining Lease Term
Operating leases	9 years 4 months 24 days	10 years 3 months 18 days
Finance leases	1 year 8 months 12 days	2 years 6 months
Weighted Average Discount Rate
Operating leases	10.30%	10.40%
Finance leases	9.70%	10.00%

Leases - Summary of Activity fo

Leases - Summary of Activity for ARO Liabilities (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2020USD ($)
Leases [Abstract]
Balance as of December 31, 2019	$ 788
Accretion expense	78
Balance as of December 31, 2020	$ 866

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - USD ($)	Dec. 31, 2020	Dec. 31, 2019
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. 29, 2020	Jan. 03, 2020	Jul. 31, 2019	Jul. 18, 2019	Feb. 25, 2019	Mar. 31, 2018	Jan. 31, 2018	Mar. 31, 2017	Jun. 30, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Sep. 30, 2020
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										17,130,745
Common stock, shares exercised										268,938	347,716	1,051,180
Option intrinsic value, exercised										$ 1,000,000	$ 3,800,000	$ 19,200,000
Net tax benefits related to exercised options										0
Estimated fair value of stock option, vested										29,400,000	31,600,000	16,200,000
Stock-based compensation expense										$ 51,351,000	$ 51,696,000	$ 33,817,000
Restricted Stock Units (RSUs)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted average grant date fair value per share										$ 12.19	$ 27.04	$ 36.83
Estimated fair value, vested in period										$ 23,600,000	$ 13,800,000	$ 10,800,000
Unrecognized stock-based compensation expense										$ 39,400,000
Unrecognized stock-based compensation weighted average recognition period										2 years 3 months 18 days
Aggregate intrinsic value										$ 75,200,000
Restricted stock units, settled										1,218,945	574,168
RSU settlements, net of shares withheld, shares										276,822	488,964
Restricted stock units withheld for tax obligations										106,459	212,879
Restricted stock units withheld for tax obligations, value										$ 1,500,000	$ 6,700,000
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 7 months 6 days
Stock options, issued													275,000
Unrecognized stock-based compensation										$ 48,800,000
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										13,545,106
Aggregate number of awards available for grant to be issued										2,764,225
Outstanding options and RSUs										10,780,881
Inducement Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares of common stock, reserved for issuance										2,634,836
Aggregate number of awards available for grant to be issued										1,734,211
Outstanding options and RSUs										900,625
Additional shares of common stock, reserved for issuance														1,500,000
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										950,803
Unrecognized stock-based compensation expense										$ 600,000
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										$ 1,800,000	$ 1,300,000	$ 1,200,000
Shares purchased										282,514	139,466	109,193
Aggregate number of shares authorized										1,586,742
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		12,633,039	5,102,041
Shares issued, price per share			$ 15.28			$ 35	$ 18.25		$ 11.32	$ 24.50
Net proceeds from sale of common stock			$ 140,700,000			$ 161,900,000	$ 131,400,000		$ 189,300,000	$ 164,300,000
Proceeds from sale of common stock, gross			$ 150,000,000						$ 201,800,000	$ 175,000,000
Warrants outstanding										2,888,526
Option price to purchase shares									$ 11.32
Underwritten Public Offering | Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Additional shares of our common stock									2,325,000
Underwritten Public Offering | Warrant
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of warrants issued			2,945,026						2,866,961	2,040,816
Warrant, price per share			$ 15.2799						$ 11.3199	$ 24.4999
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										4,785,514	3,135,347
Net proceeds from sale of common stock		$ 1,200,000								$ 68,000,000	$ 48,900,000
Proceeds from sale of common stock, gross										$ 69,900,000	$ 50,500,000
Common stock average price										$ 14.60	$ 16.09
Common stock value available to be sold										$ 79,700,000
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				$ 100,000,000			$ 75,000,000
Percentage of commission to be paid on gross sales proceeds of common stock sold	3.00%				3.00%			3.00%

Stockholders' Equity - Summary

Stockholders' Equity - Summary of RSU Activity (Detail) - Unvested RSUs - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares, Unvested as of December 31, 2019	1,910,764
Shares, Granted	3,973,991
Shares, Forfeited	(836,190)
Shares, Vested	(1,218,945)
Shares, Outstanding as of December 31, 2020	3,829,620	1,910,764
Weighted Average Grant Date Fair Value, Unvested as of December 31, 2019	$ 26.93
Weighted Average Grant Date Fair Value, Granted	12.19	$ 27.04	$ 36.83
Weighted Average Grant Date Fair Value, Forfeited	18.39
Weighted Average Grant Date Fair Value, Vested	19.34
Weighted Average Grant Date Fair Value, Outstanding as of December 31, 2020	$ 15.91	$ 26.93

Stockholders' Equity - Summar_2

Stockholders' Equity - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Granted, Shares	2,641,125	2,535,425	2,998,650
Exercised, Shares	(268,938)	(347,716)	(1,051,180)
2014 EIP and Inducement Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Outstanding, Shares, beginning balance	6,934,262
Granted, Shares	2,641,125
Exercised, Shares	(268,938)
Forfeited or expired, Shares	(1,454,563)
Outstanding, Shares, ending balance	7,851,886	6,934,262
Vested and expected to vest, Shares	7,851,886
Exercisable, Shares	3,747,718
Outstanding, Weighted Average Exercise Price, beginning balance	$ 28.25
Granted, Weighted Average Exercise Price	12.01
Exercised, Weighted Average Exercise Price	14.75
Forfeited or expired, Weighted Average Exercise price	30.17
Outstanding, Weighted Average Exercise Price, ending balance	22.89	$ 28.25
Stock options vested and expected to vest, Weighted Average Exercise Price	22.89
Exercisable, Weighted Average Exercise Price	$ 26.74
Outstanding, Weighted Average Remaining Contractual Term	6 years 4 months 24 days	5 years 10 months 24 days
Stock options vested and expected to vest, Weighted Average Remaining Contractual Term	6 years 4 months 24 days
Exercisable, Weighted Average Remaining Contractual Term	4 years 6 months
Aggregate intrinsic value	$ 26,834	$ 4,103
Vested and expected to vest, Aggregate Intrinsic Value	26,834
Exercisable, Aggregate Intrinsic Value	$ 6,214

Stockholders' Equity - Summar_3

Stockholders' Equity - Summary of Estimated Weighted-Average Assumptions (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Expected term (years)	6 years	5 years 10 months 24 days	4 years 7 months 6 days
Expected volatility	76.80%	76.10%	73.50%
Risk-free interest rate	0.80%	2.10%	2.70%
Expected dividend yield	0.00%	0.00%	0.00%
Weighted-average estimated grant date fair value per share	$ 7.96	$ 18.06	$ 22.96
Options granted	2,641,125	2,535,425	2,998,650
Total estimated grant date fair value	$ 21,023,000	$ 45,790,000	$ 68,849,000

Stockholders' Equity - Schedule

Stockholders' Equity - Schedule of Common Stock Reserved for Future Issuance Under Equity Incentive Plans (Detail)	Dec. 31, 2020shares
Class Of Stock [Line Items]
Total reserved shares of common stock	17,130,745
2014 Equity Incentive Plan
Class Of Stock [Line Items]
Total reserved shares of common stock	13,545,106
2018 Inducement Plan
Class Of Stock [Line Items]
Total reserved shares of common stock	2,634,836
2014 Employee Stock Purchase Plan
Class Of Stock [Line Items]
Total reserved shares of common stock	950,803

Stockholders' Equity - Schedu_2

Stockholders' Equity - Schedule of Stock-based Compensation Related to Stock Awards (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 51,351	$ 51,696	$ 33,817
Research and development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	31,527	26,773	16,211
General and administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 19,824	$ 24,923	$ 17,606

Income Taxes - Losses Before Pr

Income Taxes - Losses Before Provision for Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
United States	$ (306,758)	$ (291,049)	$ (230,765)
Foreign	153	85	22
Loss before provision for income taxes	$ (306,605)	$ (290,964)	$ (230,743)

Income Taxes - Components of Pr

Income Taxes - Components of Provision for (Benefit from) Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Current provision for (benefit from) income taxes:
Federal	$ 0	$ 0	$ (31)
State	2	0	(15)
Foreign	13	12	2
Total current provision for (benefit from) income taxes	$ 15	$ 12	$ (44)

Income Taxes - Reconciliation o

Income Taxes - Reconciliation of Statutory Tax Rates to Effective Tax Rates (Detail)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Effective Income Tax Rate Continuing Operations Tax Rate Reconciliation [Abstract]
Federal income taxes at statutory rate	21.00%	21.00%	21.00%
Impact of stock compensation	(2.40%)	0.10%	0.00%
Non-deductible executive compensation	(0.10%)	(0.70%)	(0.70%)
Capitalized research	0.00%	0.00%	7.80%
Other	0.30%	(0.20%)	(0.60%)
Change in valuation allowance	(18.80%)	(20.20%)	(27.50%)
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, 2020	Dec. 31, 2019
Deferred tax assets:
Net operating losses	$ 237,010	$ 162,436
Stock-based compensation	20,672	17,573
Capitalized expenses	12,590	14,129
License fees	8,159	6,870
Operating lease liabilities	4,251	4,325
Legal fees	2,136	1,683
Tax credits	1,580	0
Other	5,360	3,329
Total deferred tax assets	291,758	210,345
Valuation allowance	(287,349)	(205,249)
Total deferred tax assets	4,409	5,096
Deferred tax liabilities:
Operating lease assets	(3,541)	(3,921)
Other	(868)	(1,175)
Total deferred tax liabilities	(4,409)	(5,096)
Net deferred tax assets (liabilities)	$ 0	$ 0

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Taxes [Line Items]
Increase in valuation allowance tax	$ 82,100	$ 78,100
Net operating loss carryforwards, federal	811,700
Net operating loss carryforwards, state	988,100
Research and development tax credit carryforwards	$ 75,100
Research and development tax credit carryforwards, expiration year	2035
Unrecognized tax benefits	$ 110,601	$ 86,000	$ 41,074	$ 30,051
Unrecognized tax benefits if recognized would impact effective tax rate	0
Domestic
Income Taxes [Line Items]
Net operating loss carryforwards subject to expiration	$ 65,300
Net tax operating losses, expiration	begin to expire in 2032
Net operating loss carryforwards not subject to expiration	$ 746,400
Research and development tax credit carryforwards	$ 12,900
Research and development tax credit carryforwards, expiration year	2032
State
Income Taxes [Line Items]
Net operating loss carryforwards subject to expiration	$ 986,400
Net tax operating losses, expiration	begin to expire in 2030
Research and development tax credit carryforwards	$ 21,600

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, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Beginning Balance	$ 86,000	$ 41,074	$ 30,051
Gross increases for tax positions related to current year	24,648	22,800	12,927
Gross increases for tax positions related to prior year	0	22,126	704
Gross decreases for tax positions related to prior year	(47)	0	(2,608)
Ending Balance	$ 110,601	$ 86,000	$ 41,074