Atara Biotherapeutics (ATRA) 10-Q 6 Nov 18 2018 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2018	Oct. 31, 2018
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q3
Trading Symbol	ATRA
Entity Registrant Name	Atara Biotherapeutics, Inc.
Entity Central Index Key	1,604,464
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		45,648,266

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 66,028	$ 79,223
Short-term investments	298,515	86,873
Restricted cash - short-term	194	194
Prepaid expenses and other current assets	8,239	5,900
Total current assets	372,976	172,190
Property and equipment, net	68,279	44,129
Restricted cash - long-term	1,200	1,200
Other assets	485	260
Total assets	442,940	217,779
Current liabilities:
Accounts payable	6,193	14,711
Accrued compensation	9,072	5,664
Accrued research and development expenses	3,803	4,006
Other current liabilities	7,932	3,265
Total current liabilities	27,000	27,646
Long-term liabilities	12,886	12,269
Total liabilities	39,886	39,915
Commitments and contingencies (Note 7)
Stockholders’ equity:
Common stock—$0.0001 par value, 500,000 shares authorized as of September 30, 2018 and December 31, 2017; 45,645 and 30,730 shares issued and outstanding as of September 30, 2018 and December 31, 2017, respectively	5	3
Additional paid-in capital	850,835	474,662
Accumulated other comprehensive loss	(449)	(151)
Accumulated deficit	(447,337)	(296,650)
Total stockholders’ equity	403,054	177,864
Total liabilities and stockholders’ equity	$ 442,940	$ 217,779

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 2018	Dec. 31, 2017
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	45,645,000	30,730,000
Common stock, shares outstanding	45,645,000	30,730,000

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Operating expenses:
Research and development	$ 43,355	$ 20,598	$ 105,202	$ 56,435
General and administrative	16,865	11,062	50,093	29,295
Total operating expenses	60,220	31,660	155,295	85,730
Loss from operations	(60,220)	(31,660)	(155,295)	(85,730)
Interest and other income, net	1,859	564	4,611	1,554
Loss before provision for income taxes	(58,361)	(31,096)	(150,684)	(84,176)
Provision for income taxes	0	0	3	2
Net loss	(58,361)	(31,096)	(150,687)	(84,178)
Other comprehensive loss:
Unrealized gain (loss) on available-for-sale securities	56	26	(298)	95
Comprehensive loss	$ (58,305)	$ (31,070)	$ (150,985)	$ (84,083)
Net loss per common share:
Basic and diluted net loss per common share	$ (1.29)	$ (1.02)	$ (3.49)	$ (2.84)
Weighted-average shares outstanding used to calculate basic and diluted net loss per common share	45,406	30,474	43,148	29,597

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Operating activities
Net loss	$ (150,687)	$ (84,178)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	24,264	17,003
Amortization (accretion) of investment premiums (discounts)	(1,339)	607
Depreciation and amortization expense	2,254	678
Non-cash interest expense	211	0
Asset retirement obligation accretion expense	32	0
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(2,339)	(1,125)
Other assets	(225)	2
Accounts payable	116	(1)
Accrued compensation	3,408	1,574
Accrued research and development expenses	(203)	1,402
Other current liabilities	2,063	1,104
Long-term liabilities	92	370
Net cash used in operating activities	(122,353)	(62,564)
Investing activities
Purchases of short-term investments	(402,621)	(152,837)
Maturities of short-term investments	131,983	162,094
Sales of short-term investments	60,037	58,217
Purchases of property and equipment	(31,756)	(10,535)
Net cash (used in) provided by investing activities	(242,357)	56,939
Financing activities
Proceeds from sale of common stock in underwritten offerings, net	293,290	0
Proceeds from issuance of common stock from "at-the-market" facility, net	47,586	19,206
Proceeds from employee stock awards	18,528	495
Taxes paid related to net share settlement of restricted stock units	(7,493)	(351)
Principal payments on capital lease obligations	(396)	0
Net cash provided by financing activities	351,515	19,350
(Decrease) increase in cash, cash equivalents and restricted cash	(13,195)	13,725
Cash, cash equivalents and restricted cash at beginning of period	80,617	48,162
Cash, cash equivalents and restricted cash at end of period	67,422	61,887
Non-cash investing and financing activities
Property and equipment purchases included in accounts payable and other accrued liabilities	3,973	3,064
Facility lease financing obligations	441	6,215
Property & equipment acquired under capital leases	191	0
Asset retirement cost	88	0
Interest capitalized during construction period for build-to-suit lease transaction	77	133
Supplemental cash flow disclosure
Cash paid for interest	$ 110	$ 0

Description of Business

Description of Business	9 Months Ended
	Sep. 30, 2018
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 T-cell immunotherapy company developing novel treatments for patients with cancer, autoimmune and viral diseases. We licensed rights to T-cell product candidates from Memorial Sloan Kettering Cancer Center (“MSK”) in June 2015 and entered into agreements related to our next-generation CAR T programs with MSK in May 2018 and with Moffitt Cancer Center in August 2018. Additionally, we licensed rights to know-how and technology from QIMR Berghofer Medical Research Institute (“QIMR Berghofer”) in October 2015, September 2016 and June 2018. See Note 6 for further information.

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The accompanying unaudited condensed consolidated financial statements have been prepared in accordance with U.S. generally accepted accounting principles (“U.S. GAAP”) and follow the requirements of the Securities and Exchange Commission (“SEC”) for interim reporting. As permitted under those rules, certain footnotes or other financial information that are normally required by U.S. GAAP can be condensed or omitted. These condensed consolidated financial statements have been prepared on the same basis as the Company’s annual consolidated financial statements included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2017, except that the presentation of total cash, cash equivalents and restricted cash has been conformed to current period presentation. In the opinion of management, the condensed consolidated financial statements reflect all adjustments, consisting only of normal recurring adjustments, which are necessary for a fair statement of the Company’s consolidated financial information. The results of operations for the nine-month period ended September 30, 2018 are not necessarily indicative of the results to be expected for the full year or any other future period. The condensed consolidated balance sheet as of December 31, 2017 has been derived from audited consolidated financial statements at that date but does not include all of the information required by U.S. GAAP for complete consolidated financial statements. 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 September 30, 2018, we had an accumulated deficit of $447.3 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 will be sufficient to fund our planned operations to mid-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 by applicable regulatory authorities; performance of third-party clinical research organizations and manufacturers upon which we rely; development of sales channels; protection of our intellectual property; litigation or claims against us based on intellectual property, patent, product, regulatory or other factors; and our ability to attract and retain employees necessary to support our growth. Use of Estimates The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates, assumptions, and judgments that affect the amounts reported in the financial statements and accompanying notes. Significant estimates relied upon in preparing these financial statements include estimates related to clinical trial and other accruals, stock-based compensation expense, construction costs and income taxes. Actual results could differ materially from those estimates. Leases We lease office space in multiple locations. In addition, we recently constructed 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 condensed consolidated balance sheet along with a corresponding financing liability for landlord-paid construction costs (i.e. “build-to-suit” accounting). Once construction is complete, the Company considers the requirements for sale-leaseback accounting treatment, including evaluating whether all risks of ownership have been transferred back to the landlord, as evidenced by a lack of continuing involvement in the leased property. If the arrangement does not qualify for sale-leaseback accounting treatment, the building asset remains on the Company’s consolidated balance sheets at its historical cost, and such asset is depreciated over its estimated useful life. The Company bifurcates its lease payments into a portion allocated to the building and a portion allocated to the parcel of land on which the building has been built. The portion of the lease payments allocated to the land is treated for accounting purposes as operating lease payments, and therefore is recorded as rent expense in the consolidated statements of operations. The portion of the lease payments allocated to the building is further bifurcated into a portion allocated to interest expense and a portion allocated to reduce the build-to-suit lease obligation. The initial recording of these assets and liabilities is classified as non-cash investing and financing items, respectively, for purposes of the consolidated statements of cash flows. 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. Recent Accounting Pronouncements In February 2016, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2016-02, Leases (Topic 842) In July 2018, the FASB issued ASU No. 2018-10, “Codification Improvements to Topic 842, Leases” to clarify the implementation guidance and ASU No. 2018-11, “Leases (Topic 842) Targeted Improvements.” This updated guidance provides an optional transition method, which allows for the initial application of the new accounting standard at the adoption date and the recognition of a cumulative-effect adjustment to the opening balance of retained earnings as of the beginning of the period of adoption. The Company will adopt the new standard on January 1, 2019 and intends to elect certain practical expedients, including the optional transition method that allows for the application of the new standard at its adoption date with no restatement of prior period amounts. We have identified our significant lease contracts and are in the process of finalizing our assessment of the impact of the new standard on our consolidated financial statements. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments - Credit Losses: Measurement of Credit Losses on Financial Instruments In February 2018, the FASB issued ASU 2018-02, Income Statement – Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income In March 2018, the FASB issued ASU 2018-05, Income Taxes (Topic 740) Amendments to SEC Paragraphs Pursuant to SEC Staff Accounting Bulletin No. 118 In August 2018, the FASB issued ASU 2018-15, Intangibles—Goodwill and Other—Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract , The new standard is effective for fiscal years and interim periods within those fiscal years beginning after December 15, 2019, with early adoption permitted. We have not yet determined the potential effect the new standard will have on our consolidated financial statements. Adoption of New Accounting Pronouncements On January 1, 2018, the Company adopted two new accounting standards issued by the FASB that clarify presentation and classification in the statement of cash flows on a retrospective basis. As a result of adoption, amounts generally described as restricted cash and restricted cash equivalents are now presented with cash and cash equivalents when reconciling the beginning-of-period and end-of-period total amounts shown on the statement of cash flows. As a result of adoption, cash, cash equivalents and restricted cash reported on the condensed consolidated statements of cash flows now includes restricted cash of $1.4 million, $1.4 million, and $0.2 million as of December 31, 2017, September 30, 2017, and December 31, 2016, respectively, as well as previously reported cash and cash equivalents.

Net Loss per Common Share

Net Loss per Common Share	9 Months Ended
	Sep. 30, 2018
Earnings Per Share [Abstract]
Net Loss per Common Share	3. Basic net loss per common share is calculated by dividing net loss by the weighted-average number of shares of common stock outstanding during the period, without consideration of common share equivalents. Diluted net loss per common share is computed by dividing net loss by the weighted-average number of shares of common stock 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 restricted stock units (“RSUs”), vested and unvested options to purchase common stock and shares to be issued under our employee stock purchase plan (“ESPP”) have been excluded from the computation of diluted net loss per share as the effect is antidilutive. Therefore, the denominator used to calculate both basic and diluted net loss per common share is the same in all periods presented. The following table represents the potential common shares issuable pursuant to outstanding securities as of the related period end dates that were excluded from the computation of diluted net loss per common share as their inclusion would have an antidilutive effect: As of September 30, 2018 2017 Unvested RSUs 1,431,891 1,735,999 Vested and unvested options 6,332,199 4,910,449 ESPP share purchase rights 23,509 32,205 Total 7,787,599 6,678,653

Financial Instruments

Financial Instruments	9 Months Ended
	Sep. 30, 2018
Financial Instruments Disclosure [Abstract]
Financial Instruments	4. Our financial assets are measured at fair value on a recurring basis using the following hierarchy to prioritize valuation inputs, Level 1: Quoted prices in active markets for identical assets or liabilities that we have the ability to access Level 2: Observable market-based inputs or unobservable inputs that are corroborated by market data such as quoted prices, interest rates and yield curves Level 3: Inputs that are unobservable data points that are not corroborated by market data We review the fair value hierarchy classification on a quarterly basis. Changes in the ability to observe valuation inputs may result in a reclassification of levels of certain securities within the fair value hierarchy. We recognize transfers into and out of levels within the fair value hierarchy in the period in which the actual event or change in circumstances that caused the transfer occurs. There have been no transfers between Level 1, Level 2 and Level 3 in any periods presented. Financial assets and liabilities are considered Level 2 when their fair values are determined using inputs that are observable in the market or can be derived principally from or corroborated by observable market data such as pricing for similar securities, recently executed transactions, cash flow models with yield curves, and benchmark securities. In addition, Level 2 financial instruments are valued using comparisons to like-kind financial instruments and models that use readily observable market data as their basis. U.S. Treasury, government agency and corporate debt obligations, commercial paper and asset-backed securities are valued primarily using market prices of comparable securities, bid/ask quotes, interest rate yields and prepayment spreads and are included in Level 2. Financial assets and liabilities are considered Level 3 when their fair values are determined using pricing models, discounted cash flow methodologies, or similar techniques, and at least one significant model assumption or input is unobservable. We have no Level 3 financial assets or liabilities. 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 September 30, 2018: Input Level Cost Gain Loss Fair Value (in thousands) Money market funds Level 1 $ 40,424 $ — $ — $ 40,424 U.S. Treasury obligations Level 2 138,332 — (180 ) 138,152 Government agency obligations Level 2 18,990 — (39 ) 18,951 Corporate debt obligations Level 2 137,678 5 (206 ) 137,477 Commercial paper Level 2 15,488 — — 15,488 Asset-backed securities Level 2 12,809 1 (31 ) 12,779 Total available-for-sale securities 363,721 6 (456 ) 363,271 Less amounts classified as cash equivalents (64,761 ) — 5 (64,756 ) Amounts classified as short-term investments $ 298,960 $ 6 $ (451 ) $ 298,515 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 The amortized cost and fair value of our available-for-sale securities by contractual maturity were as follows: As of September 30, 2018 As of December 31, 2017 Amortized Estimated Amortized Estimated Cost Fair Value Cost Fair Value (in thousands) (in thousands) Maturing within one year $ 324,519 $ 324,204 $ 151,938 $ 151,852 Maturing in one to five years 39,202 39,067 12,855 12,790 Total available-for-sale securities $ 363,721 $ 363,271 $ 164,793 $ 164,642 As of September 30, 2018, 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 has no requirement or intention to sell these securities before maturity or recovery of their amortized cost basis. During , we did not recognize any other-than-temporary impairment losses. In addition, restricted cash collateralized by money market funds is a financial asset measured at fair value and is a Level 1 financial instrument under the fair value hierarchy. As of September 30, 2018 and December 31, 2017, restricted cash totaled $1.4 million and $1.4 million, respectively. The following table provides a reconciliation of cash, cash equivalents and restricted cash within the condensed consolidated balance sheets that sum to the total of the same such amounts in the condensed consolidated statement of cash flows: September 30, December 31, 2018 2017 (in thousands) Cash and cash equivalents 66,028 79,223 Restricted cash - short term 194 194 Restricted cash - long term 1,200 1,200 Total cash, cash equivalents and restricted cash $ 67,422 $ 80,617

Property and Equipment

Property and Equipment	9 Months Ended
	Sep. 30, 2018
Property Plant And Equipment [Abstract]
Property and Equipment	5. Property and Equipment Property and equipment consisted of the following as of each period end: September 30, December 31, 2018 2017 (in thousands) Leasehold improvements $ 40,390 $ 623 Construction in progress 14,136 40,797 Build-to-suit asset (see Note 7) 10,686 — Lab equipment 2,830 2,156 Machinery equipment 1,410 885 Furniture and fixtures 1,628 536 Computer equipment and software 798 477 71,878 45,474 Less accumulated depreciation and amortization (3,599 ) (1,345 ) Property and equipment, net $ 68,279 $ 44,129 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 $1.2 million and $0.3 million for the three months ended September 30, 2018 and 2017, respectively and $2.3 million and $0.7 million for the nine months ended September 30, 2018 and 2017, respectively.

License, Collaboration and Manu

License, Collaboration and Manufacturing Agreements	9 Months Ended
	Sep. 30, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
License, Collaboration and Manufacturing Agreements	6. License, Collaboration and Manufacturing 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, we exercised an option to enter into an exclusive license agreement with MSK for three clinical stage T-cell therapies. In connection with the execution of the license agreement, the Company paid $4.5 million in cash to MSK, which was recorded as research and development expense in our condensed consolidated statement of operations and comprehensive loss. We are required to make additional payments of up to $33.0 million to MSK based on achievement of specified regulatory and sales-related milestones, mid-single-digit percentage tiered royalty payments based on In addition, under certain circumstances, we are required to make certain minimum annual royalty payments to MSK, which are creditable against earned royalties owed for the same annual period. We are also required to pay a low double-digit percentage of any consideration we receive for sublicensing the licensed rights. QIMR Berghofer Agreements – In October 2015, we entered into an exclusive license agreement and a research and development collaboration agreement with QIMR Berghofer. Under the terms of the license agreement, we obtained an exclusive, worldwide license to develop and commercialize allogeneic 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 statement 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 statement of operations and comprehensive loss in the third quarter of 2016. We exercised this option in June 2018. 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 on a straight-line basis over the related development periods. The agreement also provides for various milestone payments to QIMR Berghofer based on achievement of certain developmental and regulatory milestones. 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 September 30, 2018 and December 31, 2017, there were no outstanding obligations for milestones and royalties to MSK and QIMR Berghofer. From time to time, we have entered into other license and collaboration agreements with other parties. For example we entered into agreements related to our next-generation CAR T programs with MSK in May 2018 and with Moffitt Cancer Center in August 2018. Cognate Agreement - In August 2015, Atara entered into a Development and Manufacturing Services Agreement (the “Manufacturing Agreement”) with Cognate Bioservices, Inc. (“Cognate”). The Manufacturing Agreement was amended in December 2017 to provide for additional rights for Atara in relation to the conduct of the services and amended again in May 2018 to modify certain financial provisions with respect to manufacturing services. Pursuant to the Manufacturing Agreement, Cognate provides process development and manufacturing services for certain Atara product candidates. Atara may terminate the Manufacturing Agreement for convenience on six months written notice to Cognate, or immediately if Cognate is unable to perform the Services or fails to obtain or maintain certain necessary approvals. The Manufacturing Agreement includes standard mutual termination rights for uncured breach or insolvency, or a force majeure event preventing the performance of Services for at least ninety days. The Manufacturing Agreement also includes standard provisions in the case of termination or cancellation of any specific manufacturing services.

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2018
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 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, clinical trials, supplies and other services for our operating purposes. These contracts generally provide for termination on Operating and Capital 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 condensed consolidated balance sheet. We also lease office space in Westlake Village, California under a lease agreement that expires in April 2019. Also, in fourth quarter of 2017, we entered into multiple agreements to lease certain equipment that have been accounted for as capital leases. The terms of the lease agreements range between 2-3 years. Rent expense was $0.6 million and $0.4 million for the three months ended September 30, 2018 and 2017, respectively and $1.6 million and $1.0 million for the nine months ended September 30, 2018 and 2017, respectively. Facility Lease Financing Obligation In February 2017, we entered into a lease agreement for approximately 90,580 square feet of office, lab and cellular therapy manufacturing space in Thousand Oaks, California. The initial 15-year term of the lease commenced on February 15, 2018, upon the substantial completion of landlord’s work as defined under the agreement. The contractual obligations during the initial term are $16.4 million in aggregate. Based on the terms of the lease agreement and due to our involvement in certain aspects of the construction, we were deemed the owner of the building (for accounting purposes only) during the construction period in accordance with U.S. GAAP. Under this build-to-suit lease arrangement, we recognized construction in progress based on all construction costs incurred by both us and the landlord. We also recognized a financing obligation equal to all costs funded by the landlord. As of September 30, 2018, due to completion of the construction by the landlord and not having met the criteria for sale-lease back accounting, we transferred the $10.3 million of landlord’s construction costs previously capitalized as construction in progress to a build-to-suit asset, and have recognized a corresponding long-term financing obligation for the same amount in long-term liabilities in our consolidated balance sheets. A portion of the monthly lease payment is allocated to land rent and recorded as an operating lease expense and the non-interest portion of the amortized lease payments to the landlord related to rent of the building is applied to the lease financing liability. Asset Retirement Obligation The Company’s ARO consists of a contractual requirement to remove the tenant improvements at our manufacturing facility in Thousand Oaks, California and restore the facility to a condition specified in the lease agreement. The Company records an estimate of the fair value of its ARO in long-term liabilities in the period incurred. The fair value of the ARO is also capitalized 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 following table presents the activity for our ARO liabilities: ARO Liability (in thousands) Balance as of December 31, 2017 $ 580 Liabilities incurred during the period 88 Accretion expense 32 Balance as of September 30, 2018 $ 700 Indemnification Agreements In the normal course of business, we enter into contracts and agreements that contain a variety of representations and warranties and provide for indemnification for certain liabilities. The exposure under these agreements is unknown because it involves claims that may be made against us in the future but have not yet been made. To date, we have not paid any claims or been required to defend any action related to our indemnification obligations. However, we may record charges in the future as a result of these indemnification obligations. We also have indemnification obligations to our directors and executive officers for specified events or occurrences, subject to some limits, while they are serving at our request in such capacities. There have been no claims to date and we believe the fair value of these indemnification agreements is minimal. Accordingly, we did not record liabilities for these agreements as of September 30, 2018 and December 31, 2017. 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	9 Months Ended
	Sep. 30, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stockholders' Equity	8. Stockholders’ Equity Equity Offerings In January 2018, we completed an underwritten public offering of 7,675,072 shares of common stock at an offering price of $18.25 per share and received net proceeds of $131.4 million, after deducting underwriting discounts and commissions and offering expenses payable by us. Further, in March 2018, we completed an underwritten public offering of 4,928,571 shares of common stock at an offering price of $35.00 per share and received net proceeds of $161.9 million, after deducting underwriting discounts and commissions and offering expenses payable by us. ATM Facility In March 2017, we entered into a sales agreement (the “ATM Facility”) with Cowen and Company, LLC (“Cowen”) for the sale, in our sole discretion, of shares of our common stock, having an aggregate offering price of up to $75.0 million through Cowen, as our sales agent. We 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 registered under the Securities Act of 1933, as amended. During the nine months ended September 30, 2018, we sold an aggregate of 1,007,806 shares of common stock under the ATM Facility, at an average price of approximately $48.52 per share, for gross proceeds of $48.9 million and net proceeds of $47.6 million, after deducting commissions and other offering expenses. No shares were sold under the ATM Facility in the third quarter of 2018. As of September 30, 2018, $6.1 million of common stock remained available to be sold under this facility, subject to certain conditions as specified in the agreement. Equity Incentive Plans Under the terms of the 2014 Equity Incentive Plan (“2014 EIP”), we may grant stock options, restricted stock awards (“RSAs”) and RSUs to employees, directors, consultants and other service providers. As of September 30, 2018, a total of 10,540,933 shares of common stock were reserved for issuance under the 2014 EIP, of which 3,457,163 shares were available for future grant and 7,083,770 shares were subject to outstanding options and RSUs. In February 2018, we adopted the 2018 Inducement Plan (“2018 IP”), under which we may grant options, stock appreciation rights, RSAs and RSUs to new employees. As of September 30, 2018, 1,250,000 shares of common stock were reserved for issuance under the 2018 IP, of which 720,000 shares were available for future grant and 530,000 shares were subject to outstanding options and RSUs. Restricted Stock Units The following is a summary of RSU activity under our 2014 EIP and 2018 IP: RSUs Shares Weighted Average Grant Date Fair Value Unvested as of December 31, 2017 1,685,000 $ 16.90 Granted 836,487 $ 34.61 Forfeited (469,613 ) $ 20.93 Vested (619,983 ) $ 17.33 Unvested as of September 30, 2018 1,431,891 $ 31.86 Vested and unreleased 2,617 Outstanding as of September 30, 2018 1,434,508 The fair value of RSUs is determined as the closing stock price on the date of grant. The weighted average grant date fair value of RSUs granted was $34.61 and $15.07 for the nine months ended September 30, 2018 and 2017, respectively. As of September 30, 2018, there was $33.0 million of unrecognized stock-based compensation expense related to RSUs that is expected to be recognized over a weighted average period of 2.7 years. The aggregate intrinsic value of the RSUs outstanding as of September 30, 2018 was $59.3 million. Under our RSU net settlement procedures, for most of our employees, we withhold shares at settlement to cover the minimum payroll withholding tax obligations. During the nine months ended September 30, 2018, we settled 634,851 RSUs, of which 439,986 RSUs were net settled by withholding 189,951 shares. The value of the RSUs withheld was $7.5 million, based on the closing price of our common stock on the settlement date. During the nine months ended September 30, 2017, we settled 290,534 RSUs, of which 51,592 RSUs were net settled by withholding 21,895 shares. The value of the RSUs withheld was $0.4 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 condensed consolidated statements of cash flows. Stock Options The following is a summary of stock option activity under our 2014 EIP and 2018 IP. The table below also includes 275,000 stock options which were issued in 2017 outside of these plans: Shares Weighted Average Exercise Price Weighted Average Remaining Contractual Term (Years) Aggregate Intrinsic Value (in thousands) Outstanding as of December 31, 2017 5,229,648 $ 21.06 Granted 2,633,950 38.55 Exercised (781,191 ) 22.53 Forfeited or expired (750,208 ) 30.00 Outstanding as of September 30, 2018 6,332,199 $ 27.10 5.4 $ 92,326 Vested and expected to vest as of September 30, 2018 6,332,199 $ 27.10 5.4 $ 92,326 Exercisable as of September 30, 2018 2,258,774 $ 21.26 4.0 $ 45,458 Aggregate intrinsic value represents the difference between the closing stock price of our common stock on September 30, 2018 and the exercise price of outstanding, in-the-money options. As of September 30, 2018, there was $67.9 million of unrecognized stock-based compensation expense related to stock options that is expected to be recognized over a weighted average period of 3.1 years. Options for 781,191 shares of our common stock were exercised during the nine months ended September 30, 2018, with an intrinsic value of $13.9 million. No options were exercised during the nine months ended September 30, 2017. 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: Nine months ended September 30, 2018 Nine months ended September 30, 2017 Assumptions: Expected term (years) 4.6 4.5 Expected volatility 73.4 % 69.1 % Risk-free interest rate 2.7 % 1.8 % Expected dividend yield 0.0 % 0.0 % Fair Value: Weighted-average estimated grant date fair value per share $ 22.86 $ 8.88 Options granted 2,633,950 1,155,900 Total estimated grant date fair value $ 60,212,000 $ 10,264,000 The estimated fair value of stock options that vested in the nine months ended September 30, 2018 and 2017 was $12.7 million and $10.9 million, respectively. Employee Stock Purchase Plan As of September 30, 2018, there were 943,338 shares available for purchase under the 2014 Employee Stock Purchase Plan (“2014 ESPP”). The Company recorded $0.5 million and $0.4 million of expense related to the 2014 ESPP in the nine months ended September 30, 2018 and 2017, respectively. 77,100 and 43,962 shares were purchased under the 2014 ESPP during the nine months ended September 30, 2018 and 2017, respectively. Reserved Shares The following shares of common stock were reserved for future issuance as of September 30, 2018: Total Shares Reserved 2014 Equity Incentive Plan 10,540,933 2018 Inducement Plan 1,250,000 2014 Employee Stock Purchase Plan 943,338 Options granted outside the equity plans 258,666 Total reserved shares of common stock 12,992,937 Stock-based Compensation Expense Total stock-based compensation expense related to all employee and non-employee stock awards was as follows: Three Nine months ended September 30, 2018 2017 2018 2017 (in thousands) (in thousands) Research and development $ 4,682 $ 2,136 $ 10,997 $ 6,260 General and administrative 4,570 3,864 13,267 10,743 Total stock-based compensation expense $ 9,252 $ 6,000 $ 24,264 $ 17,003

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying unaudited condensed consolidated financial statements have been prepared in accordance with U.S. generally accepted accounting principles (“U.S. GAAP”) and follow the requirements of the Securities and Exchange Commission (“SEC”) for interim reporting. As permitted under those rules, certain footnotes or other financial information that are normally required by U.S. GAAP can be condensed or omitted. These condensed consolidated financial statements have been prepared on the same basis as the Company’s annual consolidated financial statements included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2017, except that the presentation of total cash, cash equivalents and restricted cash has been conformed to current period presentation. In the opinion of management, the condensed consolidated financial statements reflect all adjustments, consisting only of normal recurring adjustments, which are necessary for a fair statement of the Company’s consolidated financial information. The results of operations for the nine-month period ended September 30, 2018 are not necessarily indicative of the results to be expected for the full year or any other future period. The condensed consolidated balance sheet as of December 31, 2017 has been derived from audited consolidated financial statements at that date but does not include all of the information required by U.S. GAAP for complete consolidated financial statements.
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 September 30, 2018, we had an accumulated deficit of $447.3 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 will be sufficient to fund our planned operations to mid-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 by applicable regulatory authorities; performance of third-party clinical research organizations and manufacturers upon which we rely; development of sales channels; protection of our intellectual property; litigation or claims against us based on intellectual property, patent, product, regulatory or other factors; and our ability to attract and retain employees necessary to support our growth.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates, assumptions, and judgments that affect the amounts reported in the financial statements and accompanying notes. Significant estimates relied upon in preparing these financial statements include estimates related to clinical trial and other accruals, stock-based compensation expense, construction costs and income taxes. Actual results could differ materially from those estimates.
Leases	Leases We lease office space in multiple locations. In addition, we recently constructed 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 condensed consolidated balance sheet along with a corresponding financing liability for landlord-paid construction costs (i.e. “build-to-suit” accounting). Once construction is complete, the Company considers the requirements for sale-leaseback accounting treatment, including evaluating whether all risks of ownership have been transferred back to the landlord, as evidenced by a lack of continuing involvement in the leased property. If the arrangement does not qualify for sale-leaseback accounting treatment, the building asset remains on the Company’s consolidated balance sheets at its historical cost, and such asset is depreciated over its estimated useful life. The Company bifurcates its lease payments into a portion allocated to the building and a portion allocated to the parcel of land on which the building has been built. The portion of the lease payments allocated to the land is treated for accounting purposes as operating lease payments, and therefore is recorded as rent expense in the consolidated statements of operations. The portion of the lease payments allocated to the building is further bifurcated into a portion allocated to interest expense and a portion allocated to reduce the build-to-suit lease obligation. The initial recording of these assets and liabilities is classified as non-cash investing and financing items, respectively, for purposes of the consolidated statements of cash flows.
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.
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 July 2018, the FASB issued ASU No. 2018-10, “Codification Improvements to Topic 842, Leases” to clarify the implementation guidance and ASU No. 2018-11, “Leases (Topic 842) Targeted Improvements.” This updated guidance provides an optional transition method, which allows for the initial application of the new accounting standard at the adoption date and the recognition of a cumulative-effect adjustment to the opening balance of retained earnings as of the beginning of the period of adoption. The Company will adopt the new standard on January 1, 2019 and intends to elect certain practical expedients, including the optional transition method that allows for the application of the new standard at its adoption date with no restatement of prior period amounts. We have identified our significant lease contracts and are in the process of finalizing our assessment of the impact of the new standard on our consolidated financial statements. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments - Credit Losses: Measurement of Credit Losses on Financial Instruments In February 2018, the FASB issued ASU 2018-02, Income Statement – Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income In March 2018, the FASB issued ASU 2018-05, Income Taxes (Topic 740) Amendments to SEC Paragraphs Pursuant to SEC Staff Accounting Bulletin No. 118 In August 2018, the FASB issued ASU 2018-15, Intangibles—Goodwill and Other—Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract , The new standard is effective for fiscal years and interim periods within those fiscal years beginning after December 15, 2019, with early adoption permitted. We have not yet determined the potential effect the new standard will have on our consolidated financial statements. Adoption of New Accounting Pronouncements On January 1, 2018, the Company adopted two new accounting standards issued by the FASB that clarify presentation and classification in the statement of cash flows on a retrospective basis. As a result of adoption, amounts generally described as restricted cash and restricted cash equivalents are now presented with cash and cash equivalents when reconciling the beginning-of-period and end-of-period total amounts shown on the statement of cash flows. As a result of adoption, cash, cash equivalents and restricted cash reported on the condensed consolidated statements of cash flows now includes restricted cash of $1.4 million, $1.4 million, and $0.2 million as of December 31, 2017, September 30, 2017, and December 31, 2016, respectively, as well as previously reported cash and cash equivalents.

Net Loss per Common Share (Tabl

Net Loss per Common Share (Tables)	9 Months Ended
	Sep. 30, 2018
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 September 30, 2018 2017 Unvested RSUs 1,431,891 1,735,999 Vested and unvested options 6,332,199 4,910,449 ESPP share purchase rights 23,509 32,205 Total 7,787,599 6,678,653

Financial Instruments (Tables)

Financial Instruments (Tables)	9 Months Ended
	Sep. 30, 2018
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 September 30, 2018: Input Level Cost Gain Loss Fair Value (in thousands) Money market funds Level 1 $ 40,424 $ — $ — $ 40,424 U.S. Treasury obligations Level 2 138,332 — (180 ) 138,152 Government agency obligations Level 2 18,990 — (39 ) 18,951 Corporate debt obligations Level 2 137,678 5 (206 ) 137,477 Commercial paper Level 2 15,488 — — 15,488 Asset-backed securities Level 2 12,809 1 (31 ) 12,779 Total available-for-sale securities 363,721 6 (456 ) 363,271 Less amounts classified as cash equivalents (64,761 ) — 5 (64,756 ) Amounts classified as short-term investments $ 298,960 $ 6 $ (451 ) $ 298,515 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
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 September 30, 2018 As of December 31, 2017 Amortized Estimated Amortized Estimated Cost Fair Value Cost Fair Value (in thousands) (in thousands) Maturing within one year $ 324,519 $ 324,204 $ 151,938 $ 151,852 Maturing in one to five years 39,202 39,067 12,855 12,790 Total available-for-sale securities $ 363,721 $ 363,271 $ 164,793 $ 164,642
Reconciliation of Cash, Cash Equivalents and Restricted Cash	The following table provides a reconciliation of cash, cash equivalents and restricted cash within the condensed consolidated balance sheets that sum to the total of the same such amounts in the condensed consolidated statement of cash flows: September 30, December 31, 2018 2017 (in thousands) Cash and cash equivalents 66,028 79,223 Restricted cash - short term 194 194 Restricted cash - long term 1,200 1,200 Total cash, cash equivalents and restricted cash $ 67,422 $ 80,617

Property and Equipment (Tables)

Property and Equipment (Tables)	9 Months Ended
	Sep. 30, 2018
Property Plant And Equipment [Abstract]
Schedule of Property and Equipment	Property and equipment consisted of the following as of each period end: September 30, December 31, 2018 2017 (in thousands) Leasehold improvements $ 40,390 $ 623 Construction in progress 14,136 40,797 Build-to-suit asset (see Note 7) 10,686 — Lab equipment 2,830 2,156 Machinery equipment 1,410 885 Furniture and fixtures 1,628 536 Computer equipment and software 798 477 71,878 45,474 Less accumulated depreciation and amortization (3,599 ) (1,345 ) Property and equipment, net $ 68,279 $ 44,129

Commitments and Contingencies (

Commitments and Contingencies (Tables)	9 Months Ended
	Sep. 30, 2018
Commitments And Contingencies Disclosure [Abstract]
Summary of Activity for ARO Liabilities	The following table presents the activity for our ARO liabilities: ARO Liability (in thousands) Balance as of December 31, 2017 $ 580 Liabilities incurred during the period 88 Accretion expense 32 Balance as of September 30, 2018 $ 700

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	9 Months Ended
	Sep. 30, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Summary of RSU Activity	The following is a summary of RSU activity under our 2014 EIP and 2018 IP: RSUs Shares Weighted Average Grant Date Fair Value Unvested as of December 31, 2017 1,685,000 $ 16.90 Granted 836,487 $ 34.61 Forfeited (469,613 ) $ 20.93 Vested (619,983 ) $ 17.33 Unvested as of September 30, 2018 1,431,891 $ 31.86 Vested and unreleased 2,617 Outstanding as of September 30, 2018 1,434,508
Summary of Stock Option Activity	The following is a summary of stock option activity under our 2014 EIP and 2018 IP. The table below also includes 275,000 stock options which were issued in 2017 outside of these plans: Shares Weighted Average Exercise Price Weighted Average Remaining Contractual Term (Years) Aggregate Intrinsic Value (in thousands) Outstanding as of December 31, 2017 5,229,648 $ 21.06 Granted 2,633,950 38.55 Exercised (781,191 ) 22.53 Forfeited or expired (750,208 ) 30.00 Outstanding as of September 30, 2018 6,332,199 $ 27.10 5.4 $ 92,326 Vested and expected to vest as of September 30, 2018 6,332,199 $ 27.10 5.4 $ 92,326 Exercisable as of September 30, 2018 2,258,774 $ 21.26 4.0 $ 45,458
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 Nine months ended September 30, 2018 Nine months ended September 30, 2017 Assumptions: Expected term (years) 4.6 4.5 Expected volatility 73.4 % 69.1 % Risk-free interest rate 2.7 % 1.8 % Expected dividend yield 0.0 % 0.0 % Fair Value: Weighted-average estimated grant date fair value per share $ 22.86 $ 8.88 Options granted 2,633,950 1,155,900 Total estimated grant date fair value $ 60,212,000 $ 10,264,000
Schedule of Common Stock Reserved for Future Issuance	The following shares of common stock were reserved for future issuance as of September 30, 2018: Total Shares Reserved 2014 Equity Incentive Plan 10,540,933 2018 Inducement Plan 1,250,000 2014 Employee Stock Purchase Plan 943,338 Options granted outside the equity plans 258,666 Total reserved shares of common stock 12,992,937
Schedule of Stock-based Compensation, Related to Employee and Nonemployee Stock Awards	Total stock-based compensation expense related to all employee and non-employee stock awards was as follows: Three Nine months ended September 30, 2018 2017 2018 2017 (in thousands) (in thousands) Research and development $ 4,682 $ 2,136 $ 10,997 $ 6,260 General and administrative 4,570 3,864 13,267 10,743 Total stock-based compensation expense $ 9,252 $ 6,000 $ 24,264 $ 17,003

Description of Business (Detail

Description of Business (Detail)	9 Months Ended
	Sep. 30, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Entity incorporation state	Delaware
Entity incorporation date	Aug. 1, 2012

Summary of Significant Accoun_3

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Significant Accounting Policies [Line Items]
Accumulated deficit	$ 447,337	$ 296,650
Restricted cash includes into cash, cash equivalents and restricted cash	$ 67,422	80,617	$ 61,887	$ 48,162
Accounting Standards Update 2016-18
Significant Accounting Policies [Line Items]
Restricted cash includes into cash, cash equivalents and restricted cash		$ 1,400	$ 1,400	$ 200
Adoption of new accounting pronouncements, Description	On January 1, 2018, the Company adopted two new accounting standards issued by the FASB that clarify presentation and classification in the statement of cash flows on a retrospective basis.

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	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	7,787,599	6,678,653
Unvested RSUs
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	1,431,891	1,735,999
Vested and Unvested Options
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	6,332,199	4,910,449
ESPP Share Purchase Rights
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	23,509	32,205

Financial Instruments - Summary

Financial Instruments - Summary of Estimated Fair Value and Related Valuation Input Hierarchy of Available-for-Sale Securities (Detail) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	$ 363,721	$ 164,793
Total Unrealized Gain	6	2
Total Unrealized Loss	(456)	(153)
Total Fair Value	363,271	164,642
Money Market Funds | Level 1
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	40,424	68,730
Total Unrealized Gain	0	0
Total Unrealized Loss	0	0
Total Fair Value	40,424	68,730
U.S. Treasury Obligations | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	138,332	39,068
Total Unrealized Gain		0
Total Unrealized Loss	(180)	(28)
Total Fair Value	138,152	39,040
Government Agency Obligations | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	18,990	4,749
Total Unrealized Gain	0	0
Total Unrealized Loss	(39)	(21)
Total Fair Value	18,951	4,728
Corporate Debt Obligations | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	137,678	46,532
Total Unrealized Gain	5	2
Total Unrealized Loss	(206)	(98)
Total Fair Value	137,477	46,436
Commercial Paper | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	15,488	1,592
Total Unrealized Gain	0	0
Total Unrealized Loss	0	0
Total Fair Value	15,488	1,592
Asset-Backed Securities | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	12,809	4,122
Total Unrealized Gain	1	0
Total Unrealized Loss	(31)	(6)
Total Fair Value	12,779	4,116
Amounts Classified As Cash Equivalents
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	64,761	77,769
Total Unrealized Gain	0	0
Total Unrealized Loss	(5)	0
Total Fair Value	64,756	77,769
Amounts Classified As Short-Term Investments
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	298,960	87,024
Total Unrealized Gain	6	2
Total Unrealized Loss	(451)	(153)
Total Fair Value	$ 298,515	$ 86,873

Financial Instruments - Amortiz

Financial Instruments - Amortized Cost and Fair Value of Available-for-Sale Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Amortized cost
Maturing within one year, Amortized cost	$ 324,519	$ 151,938
Maturing in one to five years, Amortized cost	39,202	12,855
Total available-for-sale securities, Amortized cost	363,721	164,793
Estimated Fair value
Maturing within one year, Estimated fair value	324,204	151,852
Maturing in one to five years, Estimated fair value	39,067	12,790
Total available-for-sale securities, Estimated fair value	$ 363,271	$ 164,642

Financial Instruments - Additio

Financial Instruments - Additional Information (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Financial Instruments Disclosure [Abstract]
Restricted cash	$ 1.4	$ 1.4

Financial Instruments - Reconci

Financial Instruments - Reconciliation of Cash, Cash Equivalents and Restricted Cash (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	$ 66,028	$ 79,223
Restricted cash - short-term	194	194
Restricted cash - long-term	1,200	1,200
Total cash, cash equivalents and restricted cash	$ 67,422	$ 80,617	$ 61,887	$ 48,162

Property and Equipment - Schedu

Property and Equipment - Schedule of Property and Equipment (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 71,878	$ 45,474
Less accumulated depreciation and amortization	(3,599)	(1,345)
Property and equipment, net	68,279	44,129
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	40,390	623
Construction in Progress
Property Plant And Equipment [Line Items]
Property and equipment, gross	14,136	40,797
Build-to-Suit Asset
Property Plant And Equipment [Line Items]
Property and equipment, gross	10,686	0
Lab Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	2,830	2,156
Machinery Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	1,410	885
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and equipment, gross	1,628	536
Computer Equipment and Software
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 798	$ 477

Property and Equipment - Additi

Property and Equipment - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Property Plant And Equipment Net By Type [Abstract]
Depreciation and amortization expense	$ 1,200	$ 300	$ 2,254	$ 678

License, Collaboration and Ma_2

License, Collaboration and Manufacturing Agreements - Additional Information (Detail) - USD ($)	1 Months Ended	3 Months Ended				9 Months Ended
	Jun. 30, 2015	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Related Party Transaction [Line Items]
Cash paid to exercise option agreement	$ 4,500,000
Potential milestone payments payable		$ 33,000,000				$ 33,000,000
Research and development expense		43,355,000	$ 20,598,000			105,202,000	$ 56,435,000
Contractual obligations due MSK and QIMR		$ 0				$ 0		$ 0
License Agreement | QIMR Berghofer
Related Party Transaction [Line Items]
Research and development expense				$ 3,300,000
Payments under license agreement					$ 3,000,000
Manufacturing Agreements | Cognate Agreement
Related Party Transaction [Line Items]
Written notice period for termination of agreement						6 months
Manufacturing Agreements | Cognate Agreement | Minimum
Related Party Transaction [Line Items]
Standard mutual termination rights period						90 days

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail)	Feb. 15, 2018USD ($)	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)	Dec. 31, 2017USD ($)	Feb. 28, 2017ft²
Loss Contingencies [Line Items]
Accrued termination charges		$ 0		$ 0		$ 0
Letter of credit issued		1,400,000		1,400,000		1,400,000
Rental expenses		600,000	$ 400,000	1,600,000	$ 1,000,000
Letter of credit issued, classified as long-term restricted cash		1,200,000		1,200,000		1,200,000
Liabilities related to indemnification agreements		0		$ 0		$ 0
Minimum
Loss Contingencies [Line Items]
Lease agreement term				2 years
Maximum
Loss Contingencies [Line Items]
Lease agreement term				3 years
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 issued		200,000		$ 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
Lease initial term	15 years
Lease commencement date				Feb. 15, 2018
Contractual obligations	$ 16,400,000
Lease extension term, option one	10 years
Lease extension term, option two	9 years
Thousand Oaks California Lease | Letter of Credit
Loss Contingencies [Line Items]
Letter of credit issued, classified as long-term restricted cash		1,200,000		$ 1,200,000
Build To Suit Lease Arrangement | Building
Loss Contingencies [Line Items]
Construction in progress		$ 10,300,000		$ 10,300,000

Commitments and Contingencies_2

Commitments and Contingencies - Summary of Activity for ARO Liabilities (Detail) $ in Thousands	9 Months Ended
	Sep. 30, 2018USD ($)
Commitments And Contingencies Disclosure [Abstract]
Balance as of December 31, 2017	$ 580
Liabilities incurred during the period	88
Accretion expense	32
Balance as of September 30, 2018	$ 700

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Detail) - USD ($)	1 Months Ended			3 Months Ended		9 Months Ended
	Mar. 31, 2018	Jan. 31, 2018	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Proceeds from sale of common stock, net						$ 47,586,000	$ 19,206,000
Shares of common stock, reserved for issuance				12,992,937		12,992,937
Common stock, shares exercised						781,191	0
Option intrinsic value, exercised						$ 13,900,000
Net tax benefits related to exercised options						0
Estimated fair value of stock option, vested						12,700,000	$ 10,900,000
Stock-based compensation expense				$ 9,252,000	$ 6,000,000	$ 24,264,000	$ 17,003,000
Restricted Stock Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted average grant date fair value per share						$ 34.61	$ 15.07
Unrecognized stock-based compensation expense				33,000,000		$ 33,000,000
Unrecognized stock-based compensation weighted average recognition period						2 years 8 months 12 days
Aggregate intrinsic value				59,300,000		$ 59,300,000
Restricted stock units, settled						634,851	290,534
Restricted stock units, issued net of tax withholdings						439,986	51,592
Restricted stock units withheld for tax obligations						189,951	21,895
Restricted stock units withheld for tax obligations, value						$ 7,500,000	$ 400,000
Vested and Unvested Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock-based compensation weighted average recognition period						3 years 1 month 6 days
Stock options, issued						275,000
Unrecognized stock-based compensation				$ 67,900,000		$ 67,900,000
2014 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares of common stock, reserved for issuance				10,540,933		10,540,933
Aggregate number of awards available for grant to be issued				3,457,163		3,457,163
Outstanding options and RSUs				7,083,770		7,083,770
2018 Inducement Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares of common stock, reserved for issuance				1,250,000		1,250,000
Aggregate number of awards available for grant to be issued				720,000		720,000
Outstanding options and RSUs				530,000		530,000
2014 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares of common stock, reserved for issuance				943,338		943,338
Aggregate number of awards available for grant to be issued				943,338		943,338
Stock-based compensation expense						$ 500,000	$ 400,000
Shares purchased						77,100	43,962
Options Granted Outside the Equity Plans
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares of common stock, reserved for issuance				258,666		258,666
Underwritten Public Offering
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, shares issued	4,928,571	7,675,072
Shares issued, price per share	$ 35	$ 18.25
Net proceeds from sale of common stock	$ 161,900,000	$ 131,400,000
At The Market Offering | Cowen
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, shares issued						1,007,806
Common stock average price						$ 48.52
Proceeds from sale of common stock, gross						$ 48,900,000
Proceeds from sale of common stock, net						47,600,000
Common stock value remained available to be sold				$ 6,100,000		$ 6,100,000
Shares sold under ATM Facility				0
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	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares, Unvested as of December 31, 2017	1,685,000
Shares, Granted	836,487
Shares, Forfeited	(469,613)
Shares, Vested	(619,983)
Shares, Unvested as of June 30, 2018	1,431,891
Shares, Vested and unreleased	2,617
Shares, Outstanding as of June 30, 2018	1,434,508
Weighted Average Grant Date Fair Value, Unvested as of December 31, 2017	$ 16.90
Weighted Average Grant Date Fair Value, Granted	34.61	$ 15.07
Weighted Average Grant Date Fair Value, Forfeited	20.93
Weighted Average Grant Date Fair Value, Vested	17.33
Weighted Average Grant Date Fair Value, Unvested as of June 30, 2018	$ 31.86
Outstanding as of September 30, 2018	1,434,508

Stockholders' Equity - Summar_2

Stockholders' Equity - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Exercised, Shares	(781,191)	0
2014 EIP and 2018 IP
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Outstanding, Shares, beginning balance	5,229,648
Granted, Shares	2,633,950
Exercised, Shares	(781,191)
Forfeited or expired, Shares	(750,208)
Outstanding, Shares, ending balance	6,332,199
Vested and expected to vest, Shares	6,332,199
Exercisable, Shares	2,258,774
Outstanding, Weighted Average Exercise Price, beginning balance	$ 21.06
Granted, Weighted Average Exercise Price	38.55
Exercised, Weighted Average Exercise Price	22.53
Forfeited or expired, Weighted Average Exercise price	30
Outstanding, Weighted Average Exercise Price, ending balance	27.10
Stock options vested and expected to vest, Weighted Average Exercise Price	27.10
Exercisable, Weighted Average Exercise Price	$ 21.26
Outstanding, Weighted Average Remaining Contractual Term	5 years 4 months 24 days
Stock options vested and expected to vest, Weighted Average Remaining Contractual Term	5 years 4 months 24 days
Exercisable, Weighted Average Remaining Contractual Term	4 years
Aggregate intrinsic value	$ 92,326
Vested and expected to vest, Aggregate Intrinsic Value	92,326
Exercisable, Aggregate Intrinsic Value	$ 45,458

Stockholders' Equity - Summar_3

Stockholders' Equity - Summary of Estimated Weighted-Average Assumptions (Detail) - Employees - USD ($)	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (years)	4 years 7 months 6 days	4 years 6 months
Expected volatility	73.40%	69.10%
Risk-free interest rate	2.70%	1.80%
Expected dividend yield	0.00%	0.00%
Weighted-average estimated grant date fair value per share	$ 22.86	$ 8.88
Options granted	2,633,950	1,155,900
Total estimated grant date fair value	$ 60,212,000	$ 10,264,000

Stockholders' Equity - Schedule

Stockholders' Equity - Schedule of Common Stock Reserved for Future Issuance (Detail)	Sep. 30, 2018shares
Class Of Stock [Line Items]
Total reserved shares of common stock	12,992,937
2014 Equity Incentive Plan
Class Of Stock [Line Items]
Total reserved shares of common stock	10,540,933
2018 Inducement Plan
Class Of Stock [Line Items]
Total reserved shares of common stock	1,250,000
2014 Employee Stock Purchase Plan
Class Of Stock [Line Items]
Total reserved shares of common stock	943,338
Options Granted Outside the Equity Plans
Class Of Stock [Line Items]
Total reserved shares of common stock	258,666

Stockholders' Equity - Schedu_2

Stockholders' Equity - Schedule of Stock-based Compensation Related to All Employee And Non-employee Stock Awards (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 9,252	$ 6,000	$ 24,264	$ 17,003
Research and development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	4,682	2,136	10,997	6,260
General and administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 4,570	$ 3,864	$ 13,267	$ 10,743