Atara Biotherapeutics (ATRA) 10-K 27 Feb 18 2017 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2017	Feb. 15, 2018	Jun. 30, 2017
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	FY
Trading Symbol	ATRA
Entity Registrant Name	Atara Biotherapeutics, Inc.
Entity Central Index Key	1,604,464
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	Accelerated Filer
Entity Common Stock, Shares Outstanding		38,825,835
Entity Public Float			$ 271,251,806

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 79,223	$ 47,968
Short-term investments	86,873	207,714
Restricted cash	194	194
Prepaid expenses and other current assets	5,900	4,677
Total current assets	172,190	260,553
Property and equipment, net	44,129	3,259
Restricted cash, long term	1,200	0
Other assets	260	102
Total assets	217,779	263,914
Current liabilities:
Accounts payable	14,711	2,778
Accrued compensation	5,664	3,745
Accrued research and development expenses	4,006	2,408
Other current liabilities	3,265	744
Total current liabilities	27,646	9,675
Long-term liabilities	12,269	503
Total liabilities	39,915	10,178
Commitments and contingencies (Note 7)
Stockholders’ equity:
Common stock—$0.0001 par value, 500,000 shares authorized as of December 31, 2017 and December 31, 2016; 30,730 and 28,933 shares issued and outstanding as of December 31, 2017 and December 31, 2016, respectively	3	3
Additional paid-in capital	474,662	431,075
Accumulated other comprehensive loss	(151)	(183)
Accumulated deficit	(296,650)	(177,159)
Total stockholders’ equity	177,864	253,736
Total liabilities and stockholders’ equity	$ 217,779	$ 263,914

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
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	30,730,000	28,933,000
Common stock, shares outstanding	30,730,000	28,933,000

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating expenses:
Research and development	$ 81,206	$ 56,514	$ 41,618
General and administrative	40,326	24,728	16,830
Total operating expenses	121,532	81,242	58,448
Loss from operations	(121,532)	(81,242)	(58,448)
Interest and other income, net	2,027	2,203	1,218
Loss before provision (benefit) for income taxes	(119,505)	(79,039)	(57,230)
Provision (benefit) for income taxes	(14)	10	(9)
Net loss	(119,491)	(79,049)	(57,221)
Other comprehensive gain (loss):
Unrealized gain (loss) on available-for-sale securities	32	335	(418)
Comprehensive loss	$ (119,459)	$ (78,714)	$ (57,639)
Net loss per common share:
Basic and diluted net loss per common share	$ (4)	$ (2.75)	$ (2.24)
Weighted-average common shares outstanding used to calculate basic and diluted net loss per common share	29,863	28,732	25,583

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	February 2,015	July 2,015	Common Stock	Common StockFebruary 2015	Common StockJuly 2015	Additional Paid-in Capital	Additional Paid-in CapitalFebruary 2015	Additional Paid-in CapitalJuly 2015	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Beginning balance at Dec. 31, 2014	$ 103,182			$ 2			$ 144,169			$ (100)	$ (40,889)
Beginning balance (in shares) at Dec. 31, 2014				19,693,000
Issuance of common stock, net of discounts and offering costs, Value		$ 69,487	$ 193,947		$ 1			$ 69,486	$ 193,947
Issuance of common stock, net of discounts and offering costs, shares					4,147,000	3,981,000
Issuance of common stock upon vesting of restricted stock awards	80						80
Issuance of common stock upon vesting of restricted stock awards, shares				287,000
RSU settlements, net of shares withheld	(4,647)						(4,647)
RSU settlements, net of shares withheld, shares				327,000
Issuance of common stock pursuant to stock option exercises	$ 439						439
Issuance of common stock pursuant to stock option exercises, shares	23,822			24,000
Stock-based compensation expense	$ 10,251						10,251
Net loss	(57,221)										(57,221)
Unrealized gain (loss) on available-for-sale securities	(418)									(418)
Ending balance at Dec. 31, 2015	315,100			$ 3			413,725			(518)	(98,110)
Ending balance (in shares) at Dec. 31, 2015				28,459,000
Issuance of common stock upon vesting of restricted stock awards	60						60
Issuance of common stock upon vesting of restricted stock awards, shares				233,000
RSU settlements, net of shares withheld	(94)						(94)
RSU settlements, net of shares withheld, shares				199,000
Issuance of common stock pursuant to employee stock awards	$ 600						600
Issuance of common stock pursuant to employee stock awards, shares				42,000
Issuance of common stock pursuant to stock option exercises, shares	18,947
Stock-based compensation expense	$ 16,784						16,784
Net loss	(79,049)										(79,049)
Unrealized gain (loss) on available-for-sale securities	335									335
Ending balance at Dec. 31, 2016	253,736			$ 3			431,075			(183)	(177,159)
Ending balance (in shares) at Dec. 31, 2016				28,933,000
Issuance of common stock through ATM facility, net of commissions and offering costs, Value	19,156						19,156
Issuance of common stock through ATM facility, net of commissions and offering costs, Shares				1,350,000
RSU settlements, net of shares withheld	(357)						(357)
RSU settlements, net of shares withheld, shares				305,000
Issuance of common stock pursuant to employee stock awards	$ 1,688						1,688
Issuance of common stock pursuant to employee stock awards, shares				142,000
Issuance of common stock pursuant to stock option exercises, shares	60,125
Stock-based compensation expense	$ 23,100						23,100
Net loss	(119,491)										(119,491)
Unrealized gain (loss) on available-for-sale securities	32									32
Ending balance at Dec. 31, 2017	$ 177,864			$ 3			$ 474,662			$ (151)	$ (296,650)
Ending balance (in shares) at Dec. 31, 2017				30,730,000

Consolidated Statements of Sto6

Consolidated Statements of Stockholders' Equity (Parenthetical) - Common Stock - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2015
Stock issuance, discounts, commissions and offering costs	$ 844
February 2,015
Stock issuance, discounts, commissions and offering costs		$ 5,166
July 2,015
Stock issuance, discounts, commissions and offering costs		$ 13,053

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating activities
Net loss	$ (119,491,000)	$ (79,049,000)	$ (57,221,000)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	23,100,000	16,784,000	10,251,000
Amortization of investment premiums and discounts	732,000	2,582,000	3,465,000
Depreciation and amortization expense	956,000	383,000	48,000
Loss on foreign exchange	0	0	94,000
Write-off of property and equipment	0	0	21,000
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(784,000)	(742,000)	(767,000)
Other assets	2,000	6,000	(61,000)
Accounts payable	2,163,000	981,000	1,005,000
Accrued compensation	1,919,000	1,121,000	1,399,000
Accrued research and development expenses	1,598,000	(2,704,000)	4,288,000
Other current liabilities	1,896,000	215,000	293,000
Long-term liabilities	407,000	398,000	29,000
Net cash used in operating activities	(87,502,000)	(60,025,000)	(37,156,000)
Investing activities
Purchases of short-term investments	(176,459,000)	(304,928,000)	(379,776,000)
Sales of short-term investments	107,627,000	242,643,000	64,020,000
Maturities of short-term investments	188,973,000	149,046,000	96,113,000
Purchases of property and equipment	(20,232,000)	(3,020,000)	(290,000)
Restricted cash	(1,200,000)	0	(194,000)
Net cash provided by (used in) investing activities	98,709,000	83,741,000	(220,127,000)
Financing activities
Taxes paid related to net share settlement of restricted stock units	(357,000)	(94,000)	(4,647,000)
Proceeds from employee stock awards	1,249,000	600,000	439,000
Net cash provided by financing activities	20,048,000	506,000	259,226,000
Effect of exchange rates on cash	0	0	(94,000)
Increase in cash and cash equivalents	31,255,000	24,222,000	1,849,000
Cash and cash equivalents at beginning of period	47,968,000	23,746,000	21,897,000
Cash and cash equivalents at end of period	79,223,000	47,968,000	23,746,000
Non-cash investing and financing activities
Property and equipment purchases included in accounts payable and other accrued liabilities	10,122,000	352,000	0
Capitalized lease obligations	9,904,000	0	0
Property & equipment acquired under capital leases	1,076,000	0	0
Asset retirement cost	580,000	0	0
Interest capitalized during construction period for build-to-suit lease transaction	264,000	0	0
Proceeds from options exercised not yet received	439,000	0	0
Accrued costs related to underwritten public offering	160,000	0	0
Issuance of common stock upon vesting of stock awards	0	60,000	80,000
Change in long-term liabilities related to non-vested stock awards	0	(60,000)	(80,000)
Supplemental cash flow disclosure
Cash paid for taxes	0	10,000	3,000
Underwritten Offerings
Financing activities
Proceeds from sale of common stock, net	0	0	263,434,000
At The Market Offering
Financing activities
Proceeds from sale of common stock, net	$ 19,156,000	$ 0	$ 0

Description of Business

Description of Business	12 Months Ended
	Dec. 31, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Description of Business	1. Description of Business Atara Biotherapeutics, Inc. (“Atara”, “we”, “our” or “the Company”) was incorporated in August 2012 in Delaware. Atara is a cell therapy company developing novel treatments for patients with cancer and multiple sclerosis (“MS”). The Company’s off-the-shelf, or allogeneic, T-cells are engineered from donors with healthy immune function and allow for rapid delivery from inventory to patients without a requirement for pretreatment. Atara’s T-cell immunotherapies are designed to precisely recognize and eliminate cancerous or diseased cells without affecting normal, healthy cells. We licensed rights to T-cell product candidates from Memorial Sloan Kettering Cancer Center (“MSK”) in June 2015 and to know-how and technology from QIMR Berghofer Medical Research Institute (“QIMR Berghofer”) in October 2015 and September 2016. See Note 6 for further information. In 2017, we received net proceeds of $19.2 million from the aggregate sale of 1,349,865 shares of our common stock through our ATM facility with Cowen (see Note 8). Further, 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 (see Note 10).

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The accompanying consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”) and the rules and regulations of the U.S. Securities and Exchange Commission (the “SEC”). Principles of Consolidation The consolidated financial statements include the accounts of Atara and its wholly owned subsidiaries Nina Biotherapeutics, Inc., Santa Maria Biotherapeutics, Inc., Pinta Biotherapeutics, Inc., Islands Segment and Geographic Information We operate and manage our business as one reporting and one operating 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 United States. Liquidity Risk We have incurred significant operating losses since inception and have relied on public and private equity financings to fund our operations. As of December 31, 2017, we had an accumulated deficit of $296.7 million. As we continue to incur losses, our transition to profitability will depend on the successful development, approval and commercialization of product candidates and on the achievement of sufficient revenues to support our cost structure. We may never achieve profitability, and unless and until we do, we will need to continue to raise additional capital. Management expects that our cash, cash equivalents and short-term investments as of December 31, 2017, along with the proceeds from the public offering completed in January 2018, will be sufficient to fund our planned operations into the first half of 2020. 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 have short-term investments in money market funds, U.S. Treasury, government agency and corporate debt obligations, commercial paper and asset-backed securities, which can be subject to certain credit risk. However, we mitigate the risks by investing in high-grade instruments, limiting our exposure to any one issuer, and monitoring the ongoing creditworthiness of the financial institutions and issuers. We are subject to certain risks and uncertainties and believe that changes in any of the following areas could have a material adverse effect on future financial position or results of operations: our ability to obtain future financing; regulatory approval and market acceptance of, and reimbursement for, our product candidates, if approved; 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 GAAP requires management to make estimates and assumptions and judgments that affect the amounts reported in the financial statements and accompanying notes. Significant estimates relied upon in preparing these financial statements include estimates related to clinical trial and other accruals, stock-based compensation expense, fair value of investments and income taxes. Actual results could differ materially from those estimates. Leases We lease office space in multiple locations. In addition, we are constructing a manufacturing facility in Thousand Oaks, California under a non-cancelable lease agreement. The leases are reviewed for classification as operating or capital leases. For operating leases, rent is recognized on a straight-line basis over the lease period. For capital leases, we record the leased asset with a corresponding liability for principal and interest. Payments are recorded as reductions to these liabilities with interest being charged to interest expense in our 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 are the deemed “owner” of the construction project during the construction period. As a result, we are 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). Upon occupancy for build-to-suit leases, we are also required to assess whether the circumstances qualify for sale recognition under “sale-leaseback” accounting guidance. 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 foreign currency-denominated 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. We held no foreign currency as of December 31, 2017 and 2016. 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 statements of operations only when such securities are sold or if an other-than-temporary impairment is recognized. Realized gains and losses on the sale of securities are determined by specific identification of each security’s cost basis. We regularly review our investment portfolio to determine if any security is other-than-temporarily impaired, which would require us to record an impairment charge in the period any such determination is made. In making this judgment, we evaluate, among other things, the duration and extent to which the fair value of a security is less than its cost, the financial condition of the issuer and any changes thereto, our intent to sell, or whether it is more likely than not that we will be required to sell the security before recovery of its amortized cost basis. Our assessment on whether a security is other-than-temporarily impaired could change in the future due to new developments or changes in assumptions related to any particular security. Realized gains and losses and declines in value judged to be other-than-temporary on available-for-sale securities, if any, are recorded to interest and other income (expense), net in the statements of operations and comprehensive loss. Fair Value Measurement The carrying amounts of certain of our financial instruments including cash equivalents, prepaid expenses, 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 and liabilities 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, and 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. Equipment leased under capital leases is amortized over the shorter of the lease term or the asset’s estimated useful life. 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”) and 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 for employee awards and the date when the service performance is completed for non-employees. 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 employees’ awards with performance-based vesting criteria, we assess the probability of the achievement of the performance conditions at the end of each reporting period and recognize the share-based compensation costs when it becomes probable that the performance conditions will be met. For non-employees’ awards with performance-based vesting criteria, we assess all possible outcomes at the end of each reporting period and recognize the lowest aggregate fair value in the range of possible outcomes. The lowest value in the range of possible outcomes may be zero. 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 non-employee stock options is estimated using the Black-Scholes valuation model with assumptions generally consistent with those used for employee stock options, with the exception of the expected term, which is the remaining contractual life at each measurement date. Prior to our IPO in October 2014, due to the absence of an active market for our common stock, we estimated the fair value of our common stock in accordance with the framework of the American Institute of Certified Public Accountants Technical Practice Aid, Valuation of Privately-Held Company Equity Securities Issued as Compensation. Each valuation included estimates and assumptions that required management’s judgment, including assumptions regarding the probability and estimated time to completion of our IPO. Subsequent to the completion of our IPO, the fair value of our common stock is based on observable market prices. 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 trials and preclinical studies, the costs of acquiring and manufacturing clinical trial materials and other supplies; payments under licensing and research and development agreements; other outside services and consulting costs, and an allocation of facility, information technology and overhead expenses. Research and development costs are expensed as incurred. Clinical Trial Accruals Costs for preclinical study and clinical trial 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 trials, 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. Other Current Liabilities As of December 31, 2017, other current liabilities included $2.6 million of accrued operating expenses, $0.5 million of current portion of capital lease obligations and $0.2 million of other accrued liabilities. As of December 31, 2016, other current liabilities included $0.6 million of accrued operating expenses and $0.1 million of other accrued liabilities. Income Taxes We use the assets and liabilities 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, 2017 and 2016. 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 Loss Comprehensive 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 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 loss to net loss during any period presented. Recent Accounting Pronouncements In February 2016, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2016-02, Leases (Topic 842) In March 2016, the FASB issued ASU No. 2016-09, Improvements to Employee Share-Based Payment Accounting (Topic 718) In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments - Credit Losses: Measurement of Credit Losses on Financial Instruments In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments In November 2016, the FASB issued ASU No. 2016-18 Statement of Cash Flows (Topic 230): Restricted Cash

Net Loss per Common Share

Net Loss per Common Share	12 Months Ended
	Dec. 31, 2017
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 the net loss by the weighted-average number of shares of common stock outstanding during the period, without consideration of common stock equivalents. Diluted net loss per common share is computed by dividing the net loss by the weighted-average number of shares of common stock and common share equivalents outstanding for the period. 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 RSAs, unvested RSUs, vested and unvested options and ESPP share purchase rights, have been excluded from the computation of diluted net loss per share as their 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, 2017 2016 2015 Unvested RSAs — — 233,413 Unvested RSUs 1,685,000 1,286,262 427,605 Vested and unvested options 5,229,648 3,733,847 3,137,529 ESPP share purchase rights 14,905 7,037 — Total 6,929,553 5,027,146 3,798,547

Financial Instruments

Financial Instruments	12 Months Ended
	Dec. 31, 2017
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, 2017: Input Level Cost Gain Loss Fair Value (in thousands) Money market funds Level 1 $ 68,730 $ — $ — $ 68,730 U.S. Treasury obligations Level 2 39,068 — (28 ) 39,040 Government agency obligations Level 2 4,749 — (21 ) 4,728 Corporate debt obligations Level 2 46,532 2 (98 ) 46,436 Commercial paper Level 2 1,592 — — 1,592 Asset-backed securities Level 2 4,122 — (6 ) 4,116 Total available-for-sale securities 164,793 2 (153 ) 164,642 Less amounts classified as cash equivalents (77,769 ) — — (77,769 ) Amounts classified as short-term investments $ 87,024 $ 2 $ (153 ) $ 86,873 Total Total Total Total Amortized Unrealized Unrealized Estimated As of December 31, 2016: Input Level Cost Gain Loss Fair Value (in thousands) Money market funds Level 1 $ 28,816 $ — $ — $ 28,816 U.S. Treasury obligations Level 2 65,403 3 (21 ) 65,385 Government agency obligations Level 2 23,860 5 (5 ) 23,860 Corporate debt obligations Level 2 113,649 8 (172 ) 113,485 Commercial paper Level 2 699 — — 699 Asset-backed securities Level 2 13,414 4 (6 ) 13,412 Total available-for-sale securities 245,841 20 (204 ) 245,657 Less amounts classified as cash equivalents (37,944 ) — 1 (37,943 ) Amounts classified as short-term investments $ 207,897 $ 20 $ (203 ) $ 207,714 The amortized cost and fair value of our available-for-sale securities by contractual maturity were as follows: As of December 31, 2017 As of December 31, 2016 Amortized Estimated Amortized Estimated Cost Fair Value Cost Fair Value (in thousands) (in thousands) Maturing within one year $ 151,938 $ 151,852 $ 198,022 $ 197,956 Maturing in one to five years 12,855 12,790 47,819 47,701 Total available-for-sale securities $ 164,793 $ 164,642 $ 245,841 $ 245,657 As of December 31, 2017, certain available-for-sale securities had been in a continuous unrealized loss position, each for less than twelve months. As of this date, no significant facts or circumstances were present to indicate a deterioration in the creditworthiness of the respective issuers, and the Company had no requirement or intention to sell these securities before maturity or recovery of their amortized cost basis. During the years ended December 31, 2017, 2016 and 2015, we did not recognize any other-than-temporary impairment loss. 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.

Property and Equipment

Property and Equipment	12 Months Ended
	Dec. 31, 2017
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, 2017 2016 (in thousands) Construction in progress $ 40,797 $ 970 Lab equipment 2,156 1,506 Manufacturing equipment 885 - Leasehold improvements 623 580 Furniture and fixtures 536 526 Computer equipment and software 477 66 45,474 3,648 Less accumulated depreciation and amortization (1,345 ) (389 ) Property and equipment, net $ 44,129 $ 3,259 Construction in progress represents capitalized costs for our manufacturing facility in Thousand Oaks, California and capitalizable costs incurred for development of internal use software. Depreciation and amortization expense was $1.0 million, $0.4 million and $48,000 for the years ended December 31, 2017, 2016 and 2015, respectively.

License and Collaboration Agree

License and Collaboration Agreements	12 Months Ended
	Dec. 31, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
License and Collaboration Agreements	6. License and Collaboration Agreements MSK Agreements – In September 2014, the Company entered into an exclusive option agreement with MSK under which it had the right to acquire the exclusive worldwide license rights to three clinical stage T-cell therapies from MSK. In June 2015, the Company exercised the option and entered into an exclusive license agreement with MSK. In connection with the execution of the license agreement, the Company paid was recorded as research and development expense in our consolidated statements of operations and comprehensive loss. We are required to make as well as 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 later 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. QIMR Berghofer Agreements – In October 2015, the Company entered into an exclusive license agreement and a research and development collaboration agreement with QIMR Berghofer. Under the terms of the license agreement, the Company obtained an exclusive, worldwide license to develop and commercialize allogeneic cytotoxic T-lymphocyte (“CTL”) therapy programs utilizing technology and know-how developed by QIMR Berghofer. In consideration for the exclusive license, we paid $3.0 million in cash to QIMR Berghofer, which was recorded as research and development expense in our consolidated statements of operations and comprehensive loss in the fourth quarter of 2015. 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 CTL programs as well as the option to license additional technology in exchange for $3.3 million in cash, which was recorded as research and development expense in our consolidated statement of operations and comprehensive loss in the third quarter of 2016 and paid in October 2016. The amended and restated license agreement also provides for various milestone and royalty payments to QIMR Berghofer based on future product sales, if any. Under the terms of the amended and restated 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 as incurred over the related development periods and resulted in research and development expense of $2.9 million, $1.6 million and $0.2 million for the years ended December 31, 2017, 2016 and 2015, respectively. The agreement also provides for various milestone payments to QIMR Berghofer based on achievement of certain developmental and regulatory milestones. 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, 2017 and 2016, there were no outstanding obligations for milestones and royalties to MSK and QIMR Berghofer. Amgen License Agreements – In September 2012, we entered into license agreements with Amgen, Inc., for several molecular programs, including PINTA745, ATA842 and STM434. In December 2015, we announced the suspension of further development of PINTA745 and, in June 2016, we returned the rights related to this and the ATA842 program to Amgen. In October 2017, we returned all remaining rights under the license agreements to Amgen.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2017
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	7. 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 also 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 pre-clinical studies, supplies and other services and products for operating purposes. These contracts generally provide for termination on Leases We lease our corporate headquarters in South San Francisco, California under a non-cancellable lease agreement that expires in April 2021. In connection with the lease, we are required to maintain a letter of credit in the amount of $0.2 million to the landlord, which expires and is renewed every 12 months, and is classified as restricted cash in our consolidated balance sheet. We also lease office space in Westlake Village, California under a lease agreement that expires in April 2019. Future minimum payments under our operating and capital leases as of December 31, 2017 were as follows: Operating Leases Capital Leases Years Ending December 31, (in thousands) 2018 $ 1,979 $ 546 2019 823 498 2020 613 160 2021 259 — 2022 — — Thereafter — — Total minimum payments $ 3,674 $ 1,204 Less: amount representing interest 128 Present value of capital lease obligations 1,076 Less: current portion 463 Capital lease obligation, net of current portion $ 613 Rent expenses under operating leases for the years ended December 31, 2017, 2016 and 2015 were $1.4 million, $1.2 million and $0.4 million, respectively. Financing Obligation—Build-to-Suit Lease 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 commences 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 due to our involvement in certain aspects of the construction, we have been deemed the owner of the building (for accounting purposes only) during the construction period in accordance with GAAP. Under this build-to-suit lease arrangement, we recognize construction in progress based on all construction costs incurred by both us and the landlord. We also recognize a financing obligation equal to all costs funded by the landlord. As of December 31, 2017, we have recorded $9.9 million of construction in progress relating to landlord’s costs of the building incurred through that date, and have recorded a corresponding long-term financing obligation for the same amount included in the long-term liabilities in our consolidated balance sheets. In addition, we have recorded $25.0 million of construction in progress for construction costs incurred by us and $0.3 million of capitalized interest during the construction period through December 31, 2017. Further, we recorded ground lease expense of $0.3 million for the year ended December 31, 2017, in our consolidated statement of operations and comprehensive loss, representing the estimated cost of renting the land during the construction period. Ground lease expense for the years ended December 31, 2016 and 2015 was zero. Future minimum lease payments, under the Company’s facility lease financing obligations as of December 31, 2017 were as follows: Years Ending December 31, (in thousands) 2018 $ 372 2019 911 2020 938 2021 966 2022 996 Thereafter 12,204 Total minimum payments $ 16,387 Asset Retirement Obligation The Company recognizes its 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 as construction in progress. 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 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 following table presents the activity for our ARO liabilities: (in thousands) Balance as of December 31, 2016 $ — Liabilities incurred during the year 580 Balance as of December 31, 2017 $ 580 Indemnification Agreements In the normal course of business, we enter into contracts and agreements that contain a variety of representations and warranties and provide for indemnification for certain liabilities. The exposure under these agreements is unknown because it involves claims that may be made against us in the future but have not yet been made. To date, we have not paid any claims or been required to defend any action related to our indemnification obligations. However, we may record charges in the future as a result of these indemnification obligations. We also have indemnification obligations to our directors and executive officers for specified events or occurrences, subject to some limits, while they are serving at our request in such capacities. There have been no claims to date and we believe the fair value of these indemnification agreements is minimal. Accordingly, we have not recorded any liabilities for these agreements as of December 31, 2017 and 2016. 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, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stockholders' Equity	8. 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 December 31, 2017 and 2016. ATM Facility In March 2017, we entered into a sales agreement (the “ATM facility”) with Cowen under which we may offer and sell, in our sole discretion, shares of our common stock, having an aggregate offering price of up to $75.0 million through Cowen, as our sales agent. We will pay Cowen a commission of up to 3.0% of the gross sales proceeds of any common stock sold under the ATM facility. The issuance and sale of these shares by us pursuant to the ATM facility are deemed “at the market” offerings and are available under the Securities Act of 1933, as amended. During the fiscal year ended December 31, 2017, we sold an aggregate of 1,349,865 shares of common stock, under the ATM facility, at an average price of approximately $14.82 per share, for gross proceeds of $20.0 million, and net proceeds of $19.2 million, after deducting commissions and offering expenses. As of December 31, 2017, $55.0 million of common stock remained available to be sold under this facility, subject to certain conditions as specified in the agreement. Restricted Stock Awards In August 2012, in connection with our formation, our CEO purchased 1,066,154 post-recap, post-split shares of restricted common stock at a nominal per share purchase price. The shares were issued subject to certain vesting conditions, restrictions on transfer and a Company right of repurchase of any unvested share at their original purchase price. The combined grant date intrinsic value for this award was $1.7 million. In March 2013, an Atara employee purchased 269,230 post-recap, post-split shares of restricted common stock for $0.3 million. The shares were issued under our 2012 Equity Incentive Plan and were subject to certain vesting conditions, restrictions on transfer and a Company right of repurchase of any unvested shares at their original purchase price. The amounts paid for both RSAs were initially recorded as other long-term liabilities. As the shares vested, we reclassified liabilities to equity. As of December 31, 2016, all of these shares had vested and are reported as common stock shares outstanding in the consolidated financial statements. There were no grants of RSAs in the years ended December 31, 2017, 2016 and 2015. Stock-based compensation expense related to the RSAs is recorded using the accelerated graded vesting model and was none, $0.2 million and $0.8 million for the years ended December 31, 2017, 2016 and 2015, respectively. Equity Incentive Plan In March 2014, we adopted the 2014 Equity Incentive Plan (the “2014 EIP”), which was amended and restated on October 15, 2014 upon the pricing of our 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 options, RSAs and RSUs to employees, directors, consultants and other service providers. RSUs typically require settlement by the earlier of seven years from the date of grant or the service termination (or, for RSUs granted prior to February 2014, two years following the service termination date). 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 to employees and non-employees generally vest over four years and expire in seven years. As of December 31, 2017, a total of 10,214,174 shares of common stock were reserved for issuance under the 2014 Plan, of which 3,557,041 shares were available for future grant and 6,657,133 were subject to outstanding options and RSUs. Restricted Stock Units and Awards The weighted average grant date fair value of RSUs granted during the years ended December 31, 2017, 2016 and 2015 was $15.07, $17.83 and $25.15, respectively. As of December 31, 2017, there was $20.6 million of unrecognized stock-based compensation expense related to RSUs that is expected to be recognized over a weighted average period of 2.6 years. The aggregate intrinsic value of the RSUs outstanding as of December 31, 2017 was $30.8 million. The following is a summary of RSU activity under our 2014 EIP: RSUs Shares Weighted Average Grant Date Fair Value Unvested as of December 31, 2016 1,286,262 $ 16.61 Granted 782,413 15.07 Forfeited (64,313 ) 15.33 Vested (319,362 ) 11.57 Unvested as of December 31, 2017 1,685,000 $ 16.90 Vested and unreleased 17,485 Outstanding as of December 31, 2017 1,702,485 Under our RSU net settlement procedures, we withhold shares at settlement to cover the minimum payroll withholding tax obligations. During 2017, we settled 327,282 RSUs, of which 52,624 RSUs were net settled by withholding 22,274 shares. The value of the RSUs withheld was $0.4 million, based on the closing price of our common stock on the settlement date. During 2016, we settled 204,611 RSUs, of which 13,573 RSUs were net settled by withholding 5,222 shares. The value of the RSUs withheld was $0.1 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 option activity under our 2014 EIP: Shares Weighted Weighted Average Remaining Contractual Term (Years) Aggregate Intrinsic Value (in thousands) Outstanding as of December 31, 2016 3,733,847 $ 24.14 Granted 1,694,000 15.79 Exercised (60,125 ) 12.37 Forfeited or expired (413,074 ) 23.77 Outstanding as of December 31, 2017 4,954,648 $ 21.46 5.21 $ 7,433 Vested and expected to vest as of December 31, 2017 4,954,648 $ 21.46 5.21 $ 7,433 Exercisable as of December 31, 2017 2,030,454 $ 24.06 4.39 $ 2,465 Aggregate intrinsic value represents the difference between the closing stock price of our common stock on December 31, 2017 and the exercise price of outstanding, in-the-money options. As of December 31, 2017, there was $30.3 million of unrecognized stock-based compensation expense related to stock options that is expected to be recognized over a weighted average period of 2.6 years. Options for 60,125, 18,947 and 23,822 shares of our common stock were exercised during the years ended December 31, 2017, 2016 and 2015, with an intrinsic value of $0.2 million, $0.2 million and $0.6 million, respectively. As we believe it is more likely than not that no stock option related tax benefits will be realized, we do not record any net tax benefits related to exercised options. The fair value of each option issued was estimated at the date of grant using the Black-Scholes valuation model. The following table summarizes the weighted-average assumptions used as inputs to the Black-Scholes model, and resulting weighted-average grant date fair values of stock options granted to employees during the periods indicated: Year ended December 31, 2017 2016 2015 Assumptions: Expected term (years) 4.5 4.5 4.5 Expected volatility 71.3 % 69.0 % 72.4 % Risk-free interest rate 1.9 % 1.3 % 1.6 % Expected dividend yield 0.0 % 0.0 % 0.0 % Fair Value: Weighted-average estimated grant date fair value per share $ 9.01 $ 11.02 $ 16.63 Options granted 1,694,000 966,250 2,601,174 Total estimated grant date fair value $ 15,263,000 $ 10,648,000 $ 43,258,000 The estimated fair value of stock options that vested in the years ended December 31, 2017, 2016 and 2015 was $14.0 million, $14.0 million and $2.9 million, respectively. 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 be issued or transferred shares of common stock valued at more than $25,000 per calendar year. On June 1, 2016, the first offering under the 2014 ESPP commenced, and the Company recorded $0.4 million of expense in the year ended December 31, 2016. A total of 22,844 shares were purchased at the end of the first purchase period on November 30, 2016. The 2017 offering period commenced on June 1, 2017 and will end on May 31, 2018. The Company recorded $0.6 million of expense related to the 2014 ESPP in the year ended December 31, 2017. A total of 81,922 shares were purchased under the ESPP during the year ended December 31, 2017. As of December 31, 2017, there was $0.2 million of unrecognized stock-based compensation expense related to ESPP that is expected to be recognized by the end of second quarter of 2018. 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, 2017, there were 789,669 shares available for purchase under the 2014 ESPP. Options issued outside the 2014 EIP During the year ended December 31, 2017, we granted 275,000 options, at a weighted average exercise price of $13.96 per share, outside of our 2014 EIP. These options have terms similar to the options granted under the 2014 EIP. The weighted average grant date fair value of such grants was $2.2 million. No options were granted outside the 2014 EIP during the years ended December 31, 2016 and 2015. As of December 31, 2017, there was $2.0 million of unrecognized stock-based compensation expense related to options issued outside the 2014 EIP that is expected to be recognized over a weighted average period of 3.5 years. The aggregate intrinsic value of such options as of December 31, 2017 was $1.1 million. The following shares of common stock were reserved for future issuance as of December 31, 2017: Total Shares Reserved 2014 Equity Incentive Plan 10,214,174 2014 Employee Stock Purchase Plan 789,669 Options issued outside the 2014 EIP 275,000 Total reserved shares of common stock 11,278,843 Stock-based Compensation Expense Total stock-based compensation expense related to all employee and non-employee awards was as follows: Year Ended December 31, 2017 2016 2015 (in thousands) Research and development $ 8,778 $ 7,612 $ 4,822 General and administrative 14,322 9,172 5,429 Total stock-based compensation expense $ 23,100 $ 16,784 $ 10,251

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income Taxes	9. Income Taxes Losses before provision for income taxes were as follows in each period presented: Year Ended December 31, 2017 2016 2015 (in thousands) United States $ (12,894 ) $ (48,795 ) $ (57,230 ) Foreign (106,611 ) (30,244 ) — Total loss before provision for income taxes $ (119,505 ) $ (79,039 ) $ (57,230 ) The components of income tax provision (benefit) were as follows in each period presented: Year Ended December 31, 2017 2016 2015 Current provision (benefit) for income taxes: (in thousands) Federal $ (14 ) $ — $ (1 ) State — 10 (8 ) Total current provision (benefit) for income taxes $ (14 ) $ 10 $ (9 ) A reconciliation of statutory tax rates to effective tax rates were as follows in each of the periods presented: Year Ended December 31, 2017 2016 2015 Federal income taxes at statutory rate 34.0 % 34.0 % 34.0 % Impact of stock compensation (1.5 %) (1.3 %) (0.6 %) Foreign income tax at different rate (30.3 %) (13.0 %) — Impact of US tax reform (11.3 %) — — Other (3.8 %) (0.9 %) — Change in valuation allowance 12.9 % (18.8 %) (33.4 %) Effective tax rate 0.0 % 0.0 % 0.0 % In the first quarter of 2017, the Company adopted ASU 2016-09, which requires excess tax benefits and tax deficiencies generated by the settlement of share-based awards to be recognized as part of the income tax provision. The adoption of ASU 2016-09 did not have an impact on our balance sheet, results of operations, cash flows or statement of stockholders’ equity because we have a full valuation allowance on our deferred tax assets. Upon adoption, the Company recognized the previously unrecognized excess tax benefits. The previously unrecognized excess tax effects were recorded as a deferred tax asset, which was fully offset by a valuation allowance. Upon adoption, the Company recorded additional federal and state net operating losses of $10.5 million. These net operating losses are offset with a valuation allowance. Beginning in 2017, the impact of excess tax benefits and tax deficiencies are included in the Impact of Stock Compensation tax rate line. 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 as of the dates indicated: As of December 31, 2017 2016 Deferred tax assets: (in thousands) Net operating losses $ 31,110 $ 36,911 License fees 2,940 5,800 Stock-based compensation 10,489 9,600 Legal fees 1,490 1,933 Other 1,268 1,643 Total deferred tax assets 47,297 55,887 Valuation allowance (47,297 ) (55,887 ) Net deferred tax assets $ — $ — 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, 2017 and 2016. We intend to maintain a full valuation allowance on our deferred tax assets until sufficient positive evidence exists to support reversal of the valuation allowance. The valuation allowance decreased by $8.6 million and increased by $18.9 million for the years ended December 31, 2017 and 2016, respectively. As of December 31, 2017, we had federal and state net operating loss carryforwards for tax return purposes of $76.0 million and $231.4 million, respectively. The federal and state net operating loss carryforwards begin to expire in 2032 in various amounts if not utilized. Under Section 382 of the Internal Revenue Code of 1986, as amended (the “Code”), 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% 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, 2015. The study concluded that we have experienced at least one ownership change since inception and that our utilization of net operating loss carryforwards will be subject to annual limitations. However, it is not expected that these limitations will result in the expiration of tax attribute carryforwards prior to utilization. The Company has also completed an updated analysis since the previous ownership change through December 31, 2017. As a result, no new ownership changes have occurred that would limit the current generated NOLs. On December 22, 2017, the Tax Act was enacted into law. The Tax Act, among other things, contains significant changes to corporate taxation, including reduction of the corporate tax rate from a top marginal rate of 35% to a flat rate of ; limitation of the tax deduction for interest expense to 30% of adjusted earnings (except for certain small businesses); limitation of the deduction of net operating losses generated in tax years beginning after December 31, 2017 to 80% of taxable income, indefinite carryforward of net operating losses generated in tax years after 2018 and elimination of net operating loss carrybacks; changes in the treatment of offshore earnings regardless of whether they are repatriated; current inclusion in U.S. federal taxable income of certain earnings of controlled foreign corporations, mandatory capitalization of research and development expenses beginning in 2022; immediate deductions for certain new investments instead of deductions for depreciation expense over time; further deduction limits on executive compensation; and modifying, repealing and creating many other business deductions and credits, including the reduction in the orphan drug credit from 50% to 25% of qualifying expenditures. current inclusion in U.S. federal taxable income for the transition tax on earnings of controlled foreign corporations Additionally, the SEC staff has issued SAB 118, which allows us to record provisional amounts during a measurement period not to extend beyond one year of the enactment date. On December 22, 2017, Staff Accounting Bulletin No. 118 ("SAB 118") was issued to address the application of U.S. GAAP in situations when a registrant does not have the necessary information available, prepared, or analyzed (including computations) in reasonable detail to complete the accounting for certain income tax effects of the Act. Because the Company is still in the process of analyzing certain provisions of the Act including the application of new executive compensation limitation provisions under Internal Revenue Section 162(m) in accordance with SAB 118, the Company has determined that the adjustment to its deferred taxes was a provisional amount and a reasonable estimate at December 31, 2017. A reconciliation of the beginning and ending amount of gross unrecognized tax benefits is as follows: (In thousands) Balance as of December 31, 2014 $ 1,643 Gross increases for tax positions related to current year 2,671 Balance as of December 31, 2015 4,314 Gross increases for tax positions related to current year 4,971 Balance as of December 31, 2016 9,285 Gross increases for tax positions related to current year 16,371 Gross increases for tax positions related to prior year 9,534 Gross decreases for tax positions related to prior year (4,643 ) Impact of change in tax rate (496 ) Balance as of December 31, 2017 $ 30,051 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 $30.1 million total unrecognized tax benefits as of December 31, 2017, $0.1 million, if recognized, would affect the Company’s effective tax rate. During July 2016, the Company licensed certain intellectual property rights to a wholly-owned subsidiary outside the United States. Although the license of intellectual property rights between consolidated entities did not result in any gain in the consolidated statements of operations and comprehensive loss, the transaction generated a taxable gain in the United States. However, as this gain is offset by current and existing tax losses, there was no cash tax impact from the transaction in the periods presented. As a result of the transaction, there was an increase of $0.4 million and $0.8 million in unrecognized tax benefits during the year ended December 31, 2016 and December 31, 2017, respectively. The remaining increases and decreases in unrecognized tax benefits related to changes to federal and state research and development and orphan drug tax credit carryforwards. The Company expects to record an uncertain tax benefit of $0.8 million during the next 12 months related to the licensed intellectual property rights. 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 amount of accrued interest and penalties as of December 31, 2017 and for the years ended December 31, 2017, 2016 and 2015 was immaterial. Our significant jurisdictions are the Cayman Islands, the U.S. federal, and the California state jurisdiction. All of our tax years remain open to examination by the U.S. federal and California tax authorities.

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2017
Subsequent Events [Abstract]
Subsequent Events	10. Subsequent events In January 2018, we completed an underwritten public offering of 7,675,072 shares of common stock, including 675,072 from the exercise by the underwriters of their option to purchase additional shares, at an offering price of $18.25 per share. We received net proceeds of approximately $131.4 million, after deducting underwriting discounts and commissions and offering expenses.

Summary of Significant Accoun18

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”) and the rules and regulations of the U.S. Securities and Exchange Commission (the “SEC”).
Principles of Consolidation	Principles of Consolidation The consolidated financial statements include the accounts of Atara and its wholly owned subsidiaries Nina Biotherapeutics, Inc., Santa Maria Biotherapeutics, Inc., Pinta Biotherapeutics, Inc., Islands
Segment and Geographic Information	Segment and Geographic Information We operate and manage our business as one reporting and one operating 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 United States.
Liquidity Risk	Liquidity Risk We have incurred significant operating losses since inception and have relied on public and private equity financings to fund our operations. As of December 31, 2017, we had an accumulated deficit of $296.7 million. As we continue to incur losses, our transition to profitability will depend on the successful development, approval and commercialization of product candidates and on the achievement of sufficient revenues to support our cost structure. We may never achieve profitability, and unless and until we do, we will need to continue to raise additional capital. Management expects that our cash, cash equivalents and short-term investments as of December 31, 2017, along with the proceeds from the public offering completed in January 2018, will be sufficient to fund our planned operations into the first half of 2020.
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 have short-term investments in money market funds, U.S. Treasury, government agency and corporate debt obligations, commercial paper and asset-backed securities, which can be subject to certain credit risk. However, we mitigate the risks by investing in high-grade instruments, limiting our exposure to any one issuer, and monitoring the ongoing creditworthiness of the financial institutions and issuers. We are subject to certain risks and uncertainties and believe that changes in any of the following areas could have a material adverse effect on future financial position or results of operations: our ability to obtain future financing; regulatory approval and market acceptance of, and reimbursement for, our product candidates, if approved; 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 GAAP requires management to make estimates and assumptions and judgments that affect the amounts reported in the financial statements and accompanying notes. Significant estimates relied upon in preparing these financial statements include estimates related to clinical trial and other accruals, stock-based compensation expense, fair value of investments and income taxes. Actual results could differ materially from those estimates.
Leases	Leases We lease office space in multiple locations. In addition, we are constructing a manufacturing facility in Thousand Oaks, California under a non-cancelable lease agreement. The leases are reviewed for classification as operating or capital leases. For operating leases, rent is recognized on a straight-line basis over the lease period. For capital leases, we record the leased asset with a corresponding liability for principal and interest. Payments are recorded as reductions to these liabilities with interest being charged to interest expense in our 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 are the deemed “owner” of the construction project during the construction period. As a result, we are 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). Upon occupancy for build-to-suit leases, we are also required to assess whether the circumstances qualify for sale recognition under “sale-leaseback” accounting guidance.
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 foreign currency-denominated 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. We held no foreign currency as of December 31, 2017 and 2016.
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 statements of operations only when such securities are sold or if an other-than-temporary impairment is recognized. Realized gains and losses on the sale of securities are determined by specific identification of each security’s cost basis. We regularly review our investment portfolio to determine if any security is other-than-temporarily impaired, which would require us to record an impairment charge in the period any such determination is made. In making this judgment, we evaluate, among other things, the duration and extent to which the fair value of a security is less than its cost, the financial condition of the issuer and any changes thereto, our intent to sell, or whether it is more likely than not that we will be required to sell the security before recovery of its amortized cost basis. Our assessment on whether a security is other-than-temporarily impaired could change in the future due to new developments or changes in assumptions related to any particular security. Realized gains and losses and declines in value judged to be other-than-temporary on available-for-sale securities, if any, are recorded to interest and other income (expense), net in the statements of operations and comprehensive loss.
Fair Value Measurement	Fair Value Measurement The carrying amounts of certain of our financial instruments including cash equivalents, prepaid expenses, 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 and liabilities 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, and 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. Equipment leased under capital leases is amortized over the shorter of the lease term or the asset’s estimated useful life. 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”) and 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 for employee awards and the date when the service performance is completed for non-employees. 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 employees’ awards with performance-based vesting criteria, we assess the probability of the achievement of the performance conditions at the end of each reporting period and recognize the share-based compensation costs when it becomes probable that the performance conditions will be met. For non-employees’ awards with performance-based vesting criteria, we assess all possible outcomes at the end of each reporting period and recognize the lowest aggregate fair value in the range of possible outcomes. The lowest value in the range of possible outcomes may be zero. 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 non-employee stock options is estimated using the Black-Scholes valuation model with assumptions generally consistent with those used for employee stock options, with the exception of the expected term, which is the remaining contractual life at each measurement date. Prior to our IPO in October 2014, due to the absence of an active market for our common stock, we estimated the fair value of our common stock in accordance with the framework of the American Institute of Certified Public Accountants Technical Practice Aid, Valuation of Privately-Held Company Equity Securities Issued as Compensation. Each valuation included estimates and assumptions that required management’s judgment, including assumptions regarding the probability and estimated time to completion of our IPO. Subsequent to the completion of our IPO, the fair value of our common stock is based on observable market prices.
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 trials and preclinical studies, the costs of acquiring and manufacturing clinical trial materials and other supplies; payments under licensing and research and development agreements; other outside services and consulting costs, and an allocation of facility, information technology and overhead expenses. Research and development costs are expensed as incurred.
Clinical Trial Accruals	Clinical Trial Accruals Costs for preclinical study and clinical trial 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 trials, 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.
Other Current Liabilities	Other Current Liabilities As of December 31, 2017, other current liabilities included $2.6 million of accrued operating expenses, $0.5 million of current portion of capital lease obligations and $0.2 million of other accrued liabilities. As of December 31, 2016, other current liabilities included $0.6 million of accrued operating expenses and $0.1 million of other accrued liabilities.
Income Taxes	Income Taxes We use the assets and liabilities 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, 2017 and 2016. 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 Loss	Comprehensive Loss Comprehensive 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 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 loss to net loss during any period presented.
Recent Accounting Pronouncements	Recent Accounting Pronouncements In February 2016, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2016-02, Leases (Topic 842) In March 2016, the FASB issued ASU No. 2016-09, Improvements to Employee Share-Based Payment Accounting (Topic 718) In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments - Credit Losses: Measurement of Credit Losses on Financial Instruments In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments In November 2016, the FASB issued ASU No. 2016-18 Statement of Cash Flows (Topic 230): Restricted Cash

Net Loss per Common Share (Tabl

Net Loss per Common Share (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017 2016 2015 Unvested RSAs — — 233,413 Unvested RSUs 1,685,000 1,286,262 427,605 Vested and unvested options 5,229,648 3,733,847 3,137,529 ESPP share purchase rights 14,905 7,037 — Total 6,929,553 5,027,146 3,798,547

Financial Instruments (Tables)

Financial Instruments (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017: Input Level Cost Gain Loss Fair Value (in thousands) Money market funds Level 1 $ 68,730 $ — $ — $ 68,730 U.S. Treasury obligations Level 2 39,068 — (28 ) 39,040 Government agency obligations Level 2 4,749 — (21 ) 4,728 Corporate debt obligations Level 2 46,532 2 (98 ) 46,436 Commercial paper Level 2 1,592 — — 1,592 Asset-backed securities Level 2 4,122 — (6 ) 4,116 Total available-for-sale securities 164,793 2 (153 ) 164,642 Less amounts classified as cash equivalents (77,769 ) — — (77,769 ) Amounts classified as short-term investments $ 87,024 $ 2 $ (153 ) $ 86,873 Total Total Total Total Amortized Unrealized Unrealized Estimated As of December 31, 2016: Input Level Cost Gain Loss Fair Value (in thousands) Money market funds Level 1 $ 28,816 $ — $ — $ 28,816 U.S. Treasury obligations Level 2 65,403 3 (21 ) 65,385 Government agency obligations Level 2 23,860 5 (5 ) 23,860 Corporate debt obligations Level 2 113,649 8 (172 ) 113,485 Commercial paper Level 2 699 — — 699 Asset-backed securities Level 2 13,414 4 (6 ) 13,412 Total available-for-sale securities 245,841 20 (204 ) 245,657 Less amounts classified as cash equivalents (37,944 ) — 1 (37,943 ) Amounts classified as short-term investments $ 207,897 $ 20 $ (203 ) $ 207,714
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, 2017 As of December 31, 2016 Amortized Estimated Amortized Estimated Cost Fair Value Cost Fair Value (in thousands) (in thousands) Maturing within one year $ 151,938 $ 151,852 $ 198,022 $ 197,956 Maturing in one to five years 12,855 12,790 47,819 47,701 Total available-for-sale securities $ 164,793 $ 164,642 $ 245,841 $ 245,657

Property and Equipment (Tables)

Property and Equipment (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017 2016 (in thousands) Construction in progress $ 40,797 $ 970 Lab equipment 2,156 1,506 Manufacturing equipment 885 - Leasehold improvements 623 580 Furniture and fixtures 536 526 Computer equipment and software 477 66 45,474 3,648 Less accumulated depreciation and amortization (1,345 ) (389 ) Property and equipment, net $ 44,129 $ 3,259

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2017
Commitments And Contingencies Disclosure [Abstract]
Future Payment Under Our Operating and Capital Leases	Future minimum payments under our operating and capital leases as of December 31, 2017 were as follows: Operating Leases Capital Leases Years Ending December 31, (in thousands) 2018 $ 1,979 $ 546 2019 823 498 2020 613 160 2021 259 — 2022 — — Thereafter — — Total minimum payments $ 3,674 $ 1,204 Less: amount representing interest 128 Present value of capital lease obligations 1,076 Less: current portion 463 Capital lease obligation, net of current portion $ 613
Future Minimum Lease Payments Under Lease Financing Obligations	Future minimum lease payments, under the Company’s facility lease financing obligations as of December 31, 2017 were as follows: Years Ending December 31, (in thousands) 2018 $ 372 2019 911 2020 938 2021 966 2022 996 Thereafter 12,204 Total minimum payments $ 16,387
Summary of Activity for ARO Liabilities	The following table presents the activity for our ARO liabilities: (in thousands) Balance as of December 31, 2016 $ — Liabilities incurred during the year 580 Balance as of December 31, 2017 $ 580

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	12 Months Ended
	Dec. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Summary of RSU Activity	The following is a summary of RSU activity under our 2014 EIP: RSUs Shares Weighted Average Grant Date Fair Value Unvested as of December 31, 2016 1,286,262 $ 16.61 Granted 782,413 15.07 Forfeited (64,313 ) 15.33 Vested (319,362 ) 11.57 Unvested as of December 31, 2017 1,685,000 $ 16.90 Vested and unreleased 17,485 Outstanding as of December 31, 2017 1,702,485
Summary of Stock Option Activity	The following is a summary of option activity under our 2014 EIP: Shares Weighted Weighted Average Remaining Contractual Term (Years) Aggregate Intrinsic Value (in thousands) Outstanding as of December 31, 2016 3,733,847 $ 24.14 Granted 1,694,000 15.79 Exercised (60,125 ) 12.37 Forfeited or expired (413,074 ) 23.77 Outstanding as of December 31, 2017 4,954,648 $ 21.46 5.21 $ 7,433 Vested and expected to vest as of December 31, 2017 4,954,648 $ 21.46 5.21 $ 7,433 Exercisable as of December 31, 2017 2,030,454 $ 24.06 4.39 $ 2,465
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 Year ended December 31, 2017 2016 2015 Assumptions: Expected term (years) 4.5 4.5 4.5 Expected volatility 71.3 % 69.0 % 72.4 % Risk-free interest rate 1.9 % 1.3 % 1.6 % Expected dividend yield 0.0 % 0.0 % 0.0 % Fair Value: Weighted-average estimated grant date fair value per share $ 9.01 $ 11.02 $ 16.63 Options granted 1,694,000 966,250 2,601,174 Total estimated grant date fair value $ 15,263,000 $ 10,648,000 $ 43,258,000
Schedule of Common Stock Reserved for Future Issuance	The following shares of common stock were reserved for future issuance as of December 31, 2017: Total Shares Reserved 2014 Equity Incentive Plan 10,214,174 2014 Employee Stock Purchase Plan 789,669 Options issued outside the 2014 EIP 275,000 Total reserved shares of common stock 11,278,843
Schedule of Stock-based Compensation, Related to Employee and Nonemployee Awards	Total stock-based compensation expense related to all employee and non-employee awards was as follows: Year Ended December 31, 2017 2016 2015 (in thousands) Research and development $ 8,778 $ 7,612 $ 4,822 General and administrative 14,322 9,172 5,429 Total stock-based compensation expense $ 23,100 $ 16,784 $ 10,251

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017 2016 2015 (in thousands) United States $ (12,894 ) $ (48,795 ) $ (57,230 ) Foreign (106,611 ) (30,244 ) — Total loss before provision for income taxes $ (119,505 ) $ (79,039 ) $ (57,230 )
Components of Income Tax Provision (Benefit)	The components of income tax provision (benefit) were as follows in each period presented: Year Ended December 31, 2017 2016 2015 Current provision (benefit) for income taxes: (in thousands) Federal $ (14 ) $ — $ (1 ) State — 10 (8 ) Total current provision (benefit) for income taxes $ (14 ) $ 10 $ (9 )
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, 2017 2016 2015 Federal income taxes at statutory rate 34.0 % 34.0 % 34.0 % Impact of stock compensation (1.5 %) (1.3 %) (0.6 %) Foreign income tax at different rate (30.3 %) (13.0 %) — Impact of US tax reform (11.3 %) — — Other (3.8 %) (0.9 %) — Change in valuation allowance 12.9 % (18.8 %) (33.4 %) 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 as of the dates indicated: As of December 31, 2017 2016 Deferred tax assets: (in thousands) Net operating losses $ 31,110 $ 36,911 License fees 2,940 5,800 Stock-based compensation 10,489 9,600 Legal fees 1,490 1,933 Other 1,268 1,643 Total deferred tax assets 47,297 55,887 Valuation allowance (47,297 ) (55,887 ) Net deferred tax assets $ — $ —
Reconciliation of Beginning and Ending Amount of Gross Unrecognized Tax Benefits	A reconciliation of the beginning and ending amount of gross unrecognized tax benefits is as follows: (In thousands) Balance as of December 31, 2014 $ 1,643 Gross increases for tax positions related to current year 2,671 Balance as of December 31, 2015 4,314 Gross increases for tax positions related to current year 4,971 Balance as of December 31, 2016 9,285 Gross increases for tax positions related to current year 16,371 Gross increases for tax positions related to prior year 9,534 Gross decreases for tax positions related to prior year (4,643 ) Impact of change in tax rate (496 ) Balance as of December 31, 2017 $ 30,051

Description of Business (Detail

Description of Business (Detail) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	12 Months Ended
	Jan. 31, 2018	Dec. 31, 2017
Organization And Description Of Business [Line Items]
Entity incorporation state		Delaware
Entity incorporation date		Aug. 1, 2012
At The Market Offering | Cowen
Organization And Description Of Business [Line Items]
Net proceeds from sale of common stock		$ 19.2
Common stock, shares issued		1,349,865
Underwritten Public Offering | Subsequent Events
Organization And Description Of Business [Line Items]
Net proceeds from sale of common stock	$ 131.4
Common stock, shares issued	7,675,072
Shares issued, price per share	$ 18.25

Summary of Significant Accoun26

Summary of Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017USD ($)Segment	Dec. 31, 2017GBP (£)	Dec. 31, 2016USD ($)	Dec. 31, 2016GBP (£)
Significant Accounting Policies [Line Items]
Number of reportable segments | Segment	1
Number of operating segments | Segment	1
Accumulated deficit	$ 296,650,000		$ 177,159,000
Foreign currency value held | £		£ 0		£ 0
Cash and cash equivalents maturity period	90 days
Investment maturity period	90 days
Impairment of long-lived assets	$ 0
Expected dividend yield	0.00%
Capital lease obligations, current	$ 463,000
Other Current Liabilities
Significant Accounting Policies [Line Items]
Accrued operating expenses, current	2,600,000		600,000
Capital lease obligations, current	500,000		0
Other accrued liabilities, current	$ 200,000		$ 100,000
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

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, 2017	Dec. 31, 2016	Dec. 31, 2015
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	6,929,553	5,027,146	3,798,547
Unvested RSAs
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	0	0	233,413
Unvested RSUs
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	1,685,000	1,286,262	427,605
Vested and Unvested Options
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	5,229,648	3,733,847	3,137,529
ESPP Share Purchase Rights
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	14,905	7,037	0

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, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	$ 164,793	$ 245,841
Total Unrealized Gain	2	20
Total Unrealized Loss	(153)	(204)
Total Fair Value	164,642	245,657
Money Market Funds | Level 1
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	68,730	28,816
Total Unrealized Gain	0	0
Total Unrealized Loss	0	0
Total Fair Value	68,730	28,816
U.S. Treasury Obligations | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	39,068	65,403
Total Unrealized Gain	0	3
Total Unrealized Loss	(28)	(21)
Total Fair Value	39,040	65,385
Government Agency Obligations | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	4,749	23,860
Total Unrealized Gain	0	5
Total Unrealized Loss	(21)	(5)
Total Fair Value	4,728	23,860
Corporate Debt Obligations | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	46,532	113,649
Total Unrealized Gain	2	8
Total Unrealized Loss	(98)	(172)
Total Fair Value	46,436	113,485
Commercial Paper | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	1,592	699
Total Unrealized Gain	0	0
Total Unrealized Loss	0	0
Total Fair Value	1,592	699
Asset-Backed Securities | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	4,122	13,414
Total Unrealized Gain	0	4
Total Unrealized Loss	(6)	(6)
Total Fair Value	4,116	13,412
Amounts Classified As Cash Equivalents
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	77,769	37,944
Total Unrealized Gain	0	0
Total Unrealized Loss	0	(1)
Total Fair Value	77,769	37,943
Amounts Classified As Short-Term Investments
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	87,024	207,897
Total Unrealized Gain	2	20
Total Unrealized Loss	(153)	(203)
Total Fair Value	$ 86,873	$ 207,714

Financial Instruments - Amortiz

Financial Instruments - Amortized Cost and Fair Value of Available-for-Sale Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Amortized cost
Maturing within one year, Amortized cost	$ 151,938	$ 198,022
Maturing in one to five years, Amortized cost	12,855	47,819
Total available-for-sale securities, Amortized cost	164,793	245,841
Estimated Fair value
Maturing within one year, Estimated fair value	151,852	197,956
Maturing in one to five years, Estimated fair value	12,790	47,701
Total available-for-sale securities, Estimated fair value	$ 164,642	$ 245,657

Property and Equipment - Schedu

Property and Equipment - Schedule of Property and Equipment (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 45,474	$ 3,648
Less accumulated depreciation and amortization	(1,345)	(389)
Property and equipment, net	44,129	3,259
Construction in Progress
Property Plant And Equipment [Line Items]
Property and equipment, gross	40,797	970
Lab Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	2,156	1,506
Manufacturing Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	885
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	623	580
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and equipment, gross	536	526
Computer Equipment and Software
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 477	$ 66

Property and Equipment - Additi

Property and Equipment - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property Plant And Equipment Net By Type [Abstract]
Depreciation and amortization expense	$ 956,000	$ 383,000	$ 48,000

License and Collaboration Agr32

License and Collaboration Agreements - Additional Information (Detail) - USD ($)	3 Months Ended		12 Months Ended
	Sep. 30, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Related Party Transaction [Line Items]
Cash paid to exercise option agreement			$ 4,500,000
Potential milestone payments payable			33,000,000
Research and development expense			81,206,000	$ 56,514,000	$ 41,618,000
Contractual obligations due MSK and QIMR			0	0
QIMR Berghofer
Related Party Transaction [Line Items]
Research and development expense			$ 2,900,000	$ 1,600,000	$ 200,000
License Agreement | QIMR Berghofer
Related Party Transaction [Line Items]
Research and development expense	$ 3,300,000
Payments under license agreement		$ 3,000,000

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail)	1 Months Ended	12 Months Ended
	Feb. 28, 2017USD ($)ft²	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Loss Contingencies [Line Items]
Accrued termination charges		$ 0	$ 0
Letter of credit maintained		194,000	194,000
Rent expenses under operating leases		1,400,000	1,200,000	$ 400,000
Contractual obligations		16,387,000
Letter of credit issued, classified as long-term restricted cash		1,200,000	0
Ground lease expense		300,000	0	$ 0
Liabilities related to indemnification agreements		0	$ 0
Building
Loss Contingencies [Line Items]
Construction in progress		9,900,000
Additional construction in progress		25,000,000
Capitalized interest costs during the construction period		$ 300,000
Letter of Credit
Loss Contingencies [Line Items]
Letter of credit issued, classified as long-term restricted cash	$ 1,200,000
South San Francisco California Lease
Loss Contingencies [Line Items]
Lease expiration date		Apr. 30, 2021
Letter of credit renewal term		12 months
South San Francisco California Lease | Letter of Credit
Loss Contingencies [Line Items]
Letter of credit maintained		$ 200,000
Westlake Village California Lease
Loss Contingencies [Line Items]
Lease expiration date		Apr. 30, 2019
Thousand Oaks California Lease
Loss Contingencies [Line Items]
Lease agreement area of office, lab and cellular therapy manufacturing space | ft²	90,580
Build To Suit Lease Arrangement
Loss Contingencies [Line Items]
Lease initial term	15 years
Contractual obligations	$ 16,400,000
Lease extension term, option one	10 years
Lease extension term, option two	9 years

Commitments and Contingencies34

Commitments and Contingencies - Schedule of Future Minimum Payments Under Operating and Capital Leases (Detail) $ in Thousands	Dec. 31, 2017USD ($)
Commitments And Contingencies Disclosure [Abstract]
Operating leases, 2018	$ 1,979
Operating leases 2019	823
Operating leases, 2020	613
Operating leases, 2021	259
Operating leases, 2022	0
Operating leases, Thereafter	0
Operating leases, Total minimum payments	3,674
Capital leases, 2018	546
Capital leases, 2019	498
Capital leases, 2020	160
Capital leases, 2021	0
Capital leases, 2022	0
Capital leases, Thereafter	0
Capital leases, Total minimum payments	1,204
Less: amount representing interest	128
Present value of capital lease obligations	1,076
Less: current portion	463
Capital lease obligation, net of current portion	$ 613

Commitments and Contingencies35

Commitments and Contingencies - Schedule of Future Minimum Lease Payments Under Lease Financing Obligations (Detail) $ in Thousands	Dec. 31, 2017USD ($)
Commitments And Contingencies Disclosure [Abstract]
2,018	$ 372
2,019	911
2,020	938
2,021	966
2,022	996
Thereafter	12,204
Total minimum payments	$ 16,387

Commitments and Contingencies36

Commitments and Contingencies - Summary of Activity for ARO Liabilities (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2017USD ($)
Commitments And Contingencies Disclosure [Abstract]
Balance as of December 31, 2016	$ 0
Liabilities incurred during the year	580
Balance as of December 31, 2017	$ 580

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Detail) - USD ($)	Nov. 30, 2016	Mar. 31, 2017	Mar. 31, 2013	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Aug. 31, 2012
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
Common stock, shares issued				30,730,000	28,933,000
Stock-based compensation expense				$ 23,100,000	$ 16,784,000	$ 10,251,000
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 to employees and non-employees generally vest over four years and expire in seven years.
Ownership percent				10.00%
Shares of common stock ,reserved for issuance				11,278,843
Aggregate intrinsic value				$ 7,433,000
Common stock, shares exercised				60,125	18,947	23,822
Option intrinsic value, exercised				$ 200,000	$ 200,000	$ 600,000
Net tax benefits related to exercised options				0
Estimated fair value of stock option, vested				14,000,000	14,000,000	$ 2,900,000
Stock may be issued or transferred, value				$ 1,688,000	600,000
Granted, Number of shares				1,694,000
Weighted average exercise price of options granted				$ 15.79
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				10,214,174
Aggregate number of awards available for grant to be issued				3,557,041
Outstanding options and RSUs				6,657,133
2014 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense				$ 600,000	$ 400,000
Percentage of employees purchase price of common stock				85.00%
Shares of common stock ,reserved for issuance				789,669
Aggregate number of awards available for grant to be issued				789,669
Unrecognized stock-based compensation expense				$ 200,000
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
Shares purchased	22,844			81,922
Offering period commenced date				Jun. 1, 2017
Offering period end date				May 31, 2018
Outside 2014 EIP
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares of common stock ,reserved for issuance				275,000
Unrecognized stock-based compensation expense				$ 2,000,000
Aggregate intrinsic value				$ 1,100,000
Unrecognized stock-based compensation weighted average recognition period				3 years 6 months
Granted, Number of shares				275,000	0	0
Weighted average exercise price of options granted				$ 13.96
Weighted average grant date fair value of options granted				$ 2,200,000
Unvested RSAs
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Combined grant date intrinsic value for award							$ 1,700,000
Shares, Granted				0	0	0
Unvested RSAs | Service And Performance Conditions
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense				$ 0	$ 200,000	$ 800,000
Unvested RSAs | 2012 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Proceeds from sale of common stock, net			$ 300,000
Unvested RSAs | Chief Executive Officer
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, shares issued							1,066,154
Unvested RSAs | Atara Employee
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, shares issued			269,230
Unvested RSUs
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares, Granted				782,413
Weighted average grant date fair value per share				$ 15.07	$ 17.83	$ 25.15
Unrecognized stock-based compensation expense				$ 20,600,000
Aggregate intrinsic value				$ 30,800,000
Unrecognized stock-based compensation weighted average recognition period				2 years 7 months 6 days
Restricted stock units, settled				327,282	204,611
Restricted stock units, issued net of tax withholdings				52,624	13,573
Restricted stock units withheld for tax obligations				22,274	5,222
Restricted stock units withheld for tax obligations, value				$ 400,000	$ 100,000
Unvested RSUs | From Date Of Grant
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share based compensation award expiration period				7 years
Unvested RSUs | Following Service Termination Date
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share based compensation award expiration period				2 years
Employees And Non Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share based compensation award expiration period				7 years
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
Unrecognized stock-based compensation				$ 30,300,000
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%
Maximum increase in number of shares available for issuance				230,769
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 | 2014 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock may be issued or transferred, value				$ 25,000
At The Market Offering
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Proceeds from sale of common stock, net				$ 19,156,000	$ 0	$ 0
At The Market Offering | Cowen
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, shares issued				1,349,865
Common stock average price				$ 14.82
Proceeds from sale of common stock, gross				$ 20,000,000
Proceeds from sale of common stock, net				19,200,000
Common stock value remained available to be sold				$ 55,000,000
At The Market Offering | Cowen | Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock aggregate offering price		$ 75,000,000
Percentage of commission to be paid on gross sales proceeds of common stock sold		3.00%

Stockholders' Equity - Summary

Stockholders' Equity - Summary of RSU Activity (Detail) - Unvested RSUs - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares, Unvested as of December 31, 2016	1,286,262
Shares, Granted	782,413
Shares, Forfeited	(64,313)
Shares, Vested	(319,362)
Shares, Unvested as of December 31, 2017	1,685,000	1,286,262
Shares, Vested and unreleased	17,485
Shares, Outstanding as of December 31, 2017	1,702,485
Weighted Average Grant Date Fair Value, Unvested as of December 31, 2016	$ 16.61
Weighted Average Grant Date Fair Value, Granted	15.07	$ 17.83	$ 25.15
Weighted Average Grant Date Fair Value, Forfeited	15.33
Weighted Average Grant Date Fair Value, Vested	11.57
Weighted Average Grant Date Fair Value, Unvested as of December 31, 2017	$ 16.90	$ 16.61

Stockholders' Equity - Summar39

Stockholders' Equity - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Outstanding, Shares, beginning balance	3,733,847
Granted, Shares	1,694,000
Exercised, Shares	(60,125)	(18,947)	(23,822)
Forfeited or expired, Shares	(413,074)
Outstanding, Shares, ending balance	4,954,648	3,733,847
Vested and expected to vest, Shares	4,954,648
Exercisable, Shares	2,030,454
Outstanding, Weighted Average Exercise Price, beginning balance	$ 24.14
Granted, Weighted Average Exercise Price	15.79
Exercised, Weighted Average Exercise Price	12.37
Forfeited or expired, Weighted Average Exercise price	23.77
Outstanding, Weighted Average Exercise Price, ending balance	21.46	$ 24.14
Stock options vested and expected to vest, Weighted Average Exercise Price	21.46
Exercisable, Weighted Average Exercise Price	$ 24.06
Outstanding, Weighted Average Remaining Contractual Term	5 years 2 months 15 days
Stock options vested and expected to vest, Weighted Average Remaining Contractual Term	5 years 2 months 15 days
Exercisable, Weighted Average Remaining Contractual Term	4 years 4 months 20 days
Aggregate intrinsic value	$ 7,433
Vested and expected to vest, Aggregate Intrinsic Value	7,433
Exercisable, Aggregate Intrinsic Value	$ 2,465

Stockholders' Equity - Summar40

Stockholders' Equity - Summary of Estimated Weighted-Average Assumptions (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected dividend yield	0.00%
Options granted	1,694,000
Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (years)	4 years 6 months	4 years 6 months	4 years 6 months
Expected volatility	71.30%	69.00%	72.40%
Risk-free interest rate	1.90%	1.30%	1.60%
Expected dividend yield	0.00%	0.00%	0.00%
Weighted-average estimated grant date fair value per share	$ 9.01	$ 11.02	$ 16.63
Options granted	1,694,000	966,250	2,601,174
Total estimated grant date fair value	$ 15,263,000	$ 10,648,000	$ 43,258,000

Stockholders' Equity - Schedule

Stockholders' Equity - Schedule of Common Stock Reserved for Future Issuance (Detail)	Dec. 31, 2017shares
Class Of Stock [Line Items]
Total reserved shares of common stock	11,278,843
2014 Equity Incentive Plan
Class Of Stock [Line Items]
Total reserved shares of common stock	10,214,174
2014 Employee Stock Purchase Plan
Class Of Stock [Line Items]
Total reserved shares of common stock	789,669
Options Issued Outside the 2014 EIP
Class Of Stock [Line Items]
Total reserved shares of common stock	275,000

Stockholders' Equity - Schedu42

Stockholders' Equity - Schedule of Stock-based Compensation Related to All Employee And Non-employee Awards (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 23,100	$ 16,784	$ 10,251
Research and development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	8,778	7,612	4,822
General and administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 14,322	$ 9,172	$ 5,429

Income Taxes - Losses Before Pr

Income Taxes - Losses Before Provision For Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
United States	$ (12,894)	$ (48,795)	$ (57,230)
Foreign	(106,611)	(30,244)	0
Loss before provision (benefit) for income taxes	$ (119,505)	$ (79,039)	$ (57,230)

Income Taxes - Components of In

Income Taxes - Components of Income Tax Provision (Benefit) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current provision (benefit) for income taxes:
Federal	$ (14)	$ 0	$ (1)
State	0	10	(8)
Total current provision (benefit) for income taxes	$ (14)	$ 10	$ (9)

Income Taxes - Reconciliation o

Income Taxes - Reconciliation of Statutory Tax Rates to Effective Tax Rates (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Effective Income Tax Rate Continuing Operations Tax Rate Reconciliation [Abstract]
Federal income taxes at statutory rate	34.00%	34.00%	34.00%
Impact of stock compensation	(1.50%)	(1.30%)	(0.60%)
Foreign income tax at different rate	(30.30%)	(13.00%)	0.00%
Impact of US tax reform	(11.30%)	0.00%	0.00%
Other	(3.80%)	(0.90%)	0.00%
Change in valuation allowance	12.90%	(18.80%)	(33.40%)
Effective tax rate	0.00%	0.00%	0.00%

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	1 Months Ended	12 Months Ended
	Jul. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Taxes [Line Items]
Increased (decreased) in valuation allowance tax		$ (8,600,000)	$ 18,900,000
Net operating loss carryforwards, federal		76,000,000
Net operating loss carryforwards, state		$ 231,400,000
Net tax operating losses, expiration		Begin to expire in 2032 in various amounts if not utilized.
Reduction in expected deductibility of certain executive compensation due to application of new limitations		$ 600,000
Reduction in net federal and state deferred tax assets due to reduction in corporate tax rate		13,500,000
Unrecognized tax benefits		30,051,000	9,285,000	$ 4,314,000	$ 1,643,000
Unrecognized tax benefits if recognized would impact effective tax rate		100,000
Cash tax impact of license of intellectual property rights between consolidated entities	$ 0
Increase in unrecognized tax benefits		800,000	$ 400,000
Licensed Intellectual Property Rights
Income Taxes [Line Items]
Expected uncertain tax benefits during next 12 months		800,000
Federal and State | ASU 2016-09
Income Taxes [Line Items]
Net operating losses		$ 10,500,000

Income Taxes - Components of De

Income Taxes - Components of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Net operating losses	$ 31,110	$ 36,911
License fees	2,940	5,800
Stock-based compensation	10,489	9,600
Legal fees	1,490	1,933
Other	1,268	1,643
Total deferred tax assets	47,297	55,887
Valuation allowance	(47,297)	(55,887)
Net deferred tax assets	$ 0	$ 0

Income Taxes - Reconciliation48

Income Taxes - Reconciliation of Beginning and Ending Amount of Gross Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Beginning Balance	$ 9,285	$ 4,314	$ 1,643
Gross increases for tax positions related to current year	16,371	4,971	2,671
Gross increases for tax positions related to prior year	9,534
Gross decreases for tax positions related to prior year	(4,643)
Impact of change in tax rate	(496)
Ending Balance	$ 30,051	$ 9,285	$ 4,314

Subsequent Events - Additional

Subsequent Events - Additional Information (Detail) - Underwritten Public Offering - Subsequent Events $ / shares in Units, $ in Millions	1 Months Ended
	Jan. 31, 2018USD ($)$ / sharesshares
Subsequent Event [Line Items]
Common stock, shares issued	7,675,072
Number of securities called by underwriters from their option to purchase additional shares	675,072
Shares issued, price per share | $ / shares	$ 18.25
Net proceeds from sale of common stock | $	$ 131.4