Atara Biotherapeutics (ATRA) 10-Q 8 Aug 19 2019 Q2 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2019	Jul. 31, 2019
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q2
Trading Symbol	ATRA
Entity Registrant Name	Atara Biotherapeutics, Inc.
Entity Central Index Key	0001604464
Entity Current Reporting Status	Yes
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Shell Company	false
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Common Stock, Shares Outstanding		53,757,962
Entity File Number	001-36548
Entity Tax Identification Number	460920988
Entity Address, Address Line One	611 Gateway Blvd.
Entity Address, Address Line Two	Suite 900
Entity Address, City or Town	South San Francisco
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94080
City Area Code	650
Local Phone Number	278-8930

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 59,159	$ 60,698
Short-term investments	130,976	248,933
Restricted cash - short-term	194	194
Prepaid expenses and other current assets	10,810	11,664
Total current assets	201,139	321,489
Property and equipment, net	57,090	68,576
Operating lease assets	14,396	0
Restricted cash - long-term	1,200	1,200
Other assets	319	574
Total assets	274,144	391,839
Current liabilities:
Accounts payable	6,420	3,719
Accrued compensation	7,822	10,636
Accrued research and development expenses	5,139	19,210
Other current liabilities	6,422	6,414
Total current liabilities	25,803	39,979
Operating lease liabilities - long-term	14,919	0
Other long-term liabilities	1,143	13,003
Total liabilities	41,865	52,982
Commitments and contingencies (Note 8)
Stockholders’ equity:
Common stock—$0.0001 par value, 500,000 shares authorized as of June 30, 2019 and December 31, 2018, respectively; 46,883 and 45,951 shares issued and outstanding as of June 30, 2019 and December 31, 2018, respectively	5	5
Additional paid-in capital	899,671	866,541
Accumulated other comprehensive income (loss)	173	(340)
Accumulated deficit	(667,570)	(527,349)
Total stockholders’ equity	232,279	338,857
Total liabilities and stockholders’ equity	$ 274,144	$ 391,839

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Jun. 30, 2019	Dec. 31, 2018
Statement Of Financial Position [Abstract]
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	500,000,000	500,000,000
Common stock, shares issued	46,883,000	45,951,000
Common stock, shares outstanding	46,883,000	45,951,000

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Operating expenses:
Research and development	$ 52,251	$ 33,387	$ 100,919	$ 61,847
General and administrative	23,284	19,236	42,507	33,228
Total operating expenses	75,535	52,623	143,426	95,075
Loss from operations	(75,535)	(52,623)	(143,426)	(95,075)
Interest and other income, net	1,207	1,743	2,841	2,752
Loss before provision for income taxes	(74,328)	(50,880)	(140,585)	(92,323)
Provision for income taxes	0	3	0	3
Net loss	(74,328)	(50,883)	(140,585)	(92,326)
Other comprehensive loss:
Unrealized gain (loss) on available-for-sale securities	135	19	513	(354)
Comprehensive loss	$ (74,193)	$ (50,864)	$ (140,072)	$ (92,680)
Net loss per common share:
Basic and diluted net loss per common share	$ (1.60)	$ (1.15)	$ (3.04)	$ (2.20)
Weighted-average shares outstanding used to calculate basic and diluted net loss per common share	46,426	44,379	46,276	42,001

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Changes in Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Underwritten Public Offering	At The Market Offering	Common Stock	Common StockUnderwritten Public Offering	Common StockAt The Market Offering	Additional Paid-in Capital	Additional Paid-in CapitalUnderwritten Public Offering	Additional Paid-in CapitalAt The Market Offering	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Beginning balance at Dec. 31, 2017	$ 177,864			$ 3			$ 474,662			$ (151)	$ (296,650)
Beginning balance (in shares) at Dec. 31, 2017				30,730
Issuance of common stock, net of commissions and offering costs, value		$ 293,289			$ 1			$ 293,288
Issuance of common stock, net of commissions and offering costs, shares					12,604
RSU settlements, net of shares withheld	(3,363)						(3,363)
RSU settlements, net of shares withheld, shares				250
Issuance of common stock pursuant to employee stock awards	6,196						6,196
Issuance of common stock pursuant to employee stock awards, shares				309
Stock-based compensation expense	7,014						7,014
Net loss	(41,443)										(41,443)
Unrealized gain (loss) on available-for-sale securities	(373)									(373)
Ending balance at Mar. 31, 2018	439,184			$ 4			777,797			(524)	(338,093)
Ending balance (in shares) at Mar. 31, 2018				43,893
Beginning balance at Dec. 31, 2017	177,864			$ 3			474,662			(151)	(296,650)
Beginning balance (in shares) at Dec. 31, 2017				30,730
Net loss	(92,326)
Unrealized gain (loss) on available-for-sale securities	(354)
Ending balance at Jun. 30, 2018	452,499			$ 5			841,975			(505)	(388,976)
Ending balance (in shares) at Jun. 30, 2018				45,334
Beginning balance at Mar. 31, 2018	439,184			$ 4			777,797			(524)	(338,093)
Beginning balance (in shares) at Mar. 31, 2018				43,893
Issuance of common stock, net of commissions and offering costs, value			$ 47,587			$ 1			$ 47,586
Issuance of common stock, net of commissions and offering costs, shares						1,008
RSU settlements, net of shares withheld	(67)						(67)
RSU settlements, net of shares withheld, shares				42
Issuance of common stock pursuant to employee stock awards	8,661						8,661
Issuance of common stock pursuant to employee stock awards, shares				391
Stock-based compensation expense	7,998						7,998
Net loss	(50,883)										(50,883)
Unrealized gain (loss) on available-for-sale securities	19									19
Ending balance at Jun. 30, 2018	452,499			$ 5			841,975			(505)	(388,976)
Ending balance (in shares) at Jun. 30, 2018				45,334
Beginning balance at Dec. 31, 2018	338,857			$ 5			866,541			(340)	(527,349)
Beginning balance (in shares) at Dec. 31, 2018				45,951
Effect of the adoption of ASC topic 842 (Leases) at Dec. 31, 2018	364										364
Balance at Dec. 31, 2018	339,221			$ 5			866,541			(340)	(526,985)
RSU settlements, net of shares withheld	(4,575)						(4,575)
RSU settlements, net of shares withheld, shares				197
Issuance of common stock pursuant to employee stock awards	2,898						2,898
Issuance of common stock pursuant to employee stock awards, shares				159
Stock-based compensation expense	12,269						12,269
Net loss	(66,257)										(66,257)
Unrealized gain (loss) on available-for-sale securities	378									378
Ending balance at Mar. 31, 2019	283,934			$ 5			877,133			38	(593,242)
Ending balance (in shares) at Mar. 31, 2019				46,307
Beginning balance at Dec. 31, 2018	338,857			$ 5			866,541			(340)	(527,349)
Beginning balance (in shares) at Dec. 31, 2018				45,951
Effect of the adoption of ASC topic 842 (Leases) at Dec. 31, 2018	364										364
Balance at Dec. 31, 2018	339,221			$ 5			866,541			(340)	(526,985)
Net loss	(140,585)
Unrealized gain (loss) on available-for-sale securities	513
Ending balance at Jun. 30, 2019	232,279			$ 5			899,671			173	(667,570)
Ending balance (in shares) at Jun. 30, 2019				46,883
Beginning balance at Mar. 31, 2019	283,934			$ 5			877,133			38	(593,242)
Beginning balance (in shares) at Mar. 31, 2019				46,307
Issuance of common stock, net of commissions and offering costs, value			$ 7,630						$ 7,630
Issuance of common stock, net of commissions and offering costs, shares						359
RSU settlements, net of shares withheld	(2,095)						(2,095)
RSU settlements, net of shares withheld, shares				120
Issuance of common stock pursuant to employee stock awards	1,802						1,802
Issuance of common stock pursuant to employee stock awards, shares				97
Stock-based compensation expense	15,201						15,201
Net loss	(74,328)										(74,328)
Unrealized gain (loss) on available-for-sale securities	135									135
Ending balance at Jun. 30, 2019	$ 232,279			$ 5			$ 899,671			$ 173	$ (667,570)
Ending balance (in shares) at Jun. 30, 2019				46,883

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Changes in Stockholders' Equity (Parenthetical) - Common Stock - USD ($) $ in Thousands	3 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Mar. 31, 2018
At The Market Offering
Stock issuance, discounts, commissions and offering costs	$ 338	$ 1,310
Underwritten Public Offering
Stock issuance, discounts, commissions and offering costs			$ 526

Condensed Consolidated Statem_4

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Operating activities
Net loss	$ (140,585)	$ (92,326)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	27,470	15,013
Accretion of investment discounts	(817)	(756)
Depreciation and amortization expense	3,385	1,064
Loss on disposals of property and equipment	127	0
Non-cash interest expense	0	125
Asset retirement obligation accretion expense	35	16
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	720	(1,961)
Operating lease assets	814	0
Other assets	255	(102)
Accounts payable	2,649	(207)
Accrued compensation	(2,814)	(388)
Accrued research and development expenses	(14,071)	2,655
Other current liabilities	(1,658)	2,545
Operating lease liabilities	(299)	0
Other long-term liabilities	0	66
Net cash used in operating activities	(124,789)	(74,256)
Investing activities
Purchases of short-term investments	(17,589)	(357,647)
Proceeds from maturities and sales of short-term investments	136,876	131,110
Purchases of property and equipment	(1,534)	(27,257)
Proceeds from sale of property and equipment	96	0
Net cash provided by (used in) investing activities	117,849	(253,794)
Financing activities
Proceeds from sale of common stock in underwritten offerings, net	0	293,290
Proceeds from issuance of common stock through ATM facility, net	7,630	47,586
Proceeds from employee stock awards	4,676	14,857
Taxes paid related to net share settlement of restricted stock units	(6,670)	(3,431)
Principal payments on finance and capital lease obligations	(235)	(272)
Net cash provided by financing activities	5,401	352,030
(Decrease) increase in cash, cash equivalents and restricted cash	(1,539)	23,980
Cash, cash equivalents and restricted cash at beginning of period	62,092	80,617
Cash, cash equivalents and restricted cash at end of period	60,553	104,597
Non-cash investing and financing activities
Property and equipment purchases included in accounts payable and other accrued liabilities	473	5,078
Capitalized lease obligations	0	441
Property & equipment acquired under capital leases	0	191
Asset retirement cost	0	88
Interest capitalized during construction period for build-to-suit lease transaction	0	77
Receivable for options exercised	24	0
Supplemental cash flow disclosure
Cash paid for interest	$ 31	$ 67

Description of Business

Description of Business	6 Months Ended
	Jun. 30, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Description of Business	1. Description of Business Atara Biotherapeutics, Inc. (“Atara”, “we”, “our” or “the Company”) was incorporated in August 2012 in Delaware. Atara is a leading off-the-shelf, allogeneic T-cell immunotherapy company that is cancer, autoimmune and viral diseases. We have several T-cell immunotherapies in clinical development and are progressing a next-generation allogeneic chimeric antigen receptor T-cell, or CAR T, program. We licensed rights to T-cell product candidates from Memorial Sloan Kettering Cancer Center (“MSK”) in June 2015 and licensed rights related to our next-generation CAR T programs from MSK in May 2018 and December 2018 and from Moffitt Cancer Center in August 2018. Additionally, we licensed rights to know-how and technology from the Council of the Queensland Institute of Medical Research We have incurred significant operating losses since inception and have relied on public and private equity financings to fund our operations. As of June 30, 2019, we had an accumulated deficit of $667.6 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. In July 2019, we completed an underwritten public offering of shares of common stock and pre-funded warrants and received aggregate net proceeds of approximately $140.6 million (see Note 10). Management expects that our cash, cash equivalents and short-term investments, together with the net proceeds from the underwritten public offering in July 2019, will be sufficient to fund our planned operations into 2021.

Summary of Significant Accounti

Summary of Significant Accounting Policies	6 Months Ended
	Jun. 30, 2019
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The accompanying unaudited condensed consolidated financial statements include the accounts of Atara and its wholly-owned subsidiaries and have been prepared in accordance with U.S. generally accepted accounting principles (“U.S. GAAP”) and 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, 2018, except for the recognition of operating lease assets and operating lease liabilities effective January 1, 2019, in accordance with newly-adopted accounting pronouncements relating to leases as discussed below. 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 and income taxes. Actual results could differ materially from those estimates. Leases We lease office space in multiple locations. We determine if an arrangement is a lease at inception. Operating leases are included in operating lease assets, other current liabilities, and operating lease liabilities on our condensed consolidated balance sheets. Leases with an initial term of 12 months or less are not recorded on the balance sheet; we recognize lease expense for these leases on a straight-line basis over the lease term. Finance leases are included in property and equipment, other current liabilities, and other long-term liabilities on our condensed consolidated balance sheets. Operating lease assets and operating lease liabilities are recognized based on the present value of the future minimum lease payments over the lease term at commencement date. As most of our leases do not provide an implicit rate, we use our incremental borrowing rate based on the information available at commencement date in determining the present value of future payments. The incremental borrowing rate for our leases is determined based on lease term and currency in which lease payments are made, adjusted for impacts of collateral. The operating lease asset also includes any lease payments made and excludes lease incentives and initial direct costs incurred. Lease expense for minimum lease payments is recognized on a straight-line basis over the lease term. Our facilities and equipment operating leases have lease and non-lease components and we have made a policy election to account for the lease and non-lease components as a single lease component. Through December 31, 2018, the leases were reviewed for classification as operating, capital or build-to-suit leases. For operating leases, rent was recognized on a straight-line basis over the lease period. For capital leases, we recorded the leased asset with a corresponding liability for principal and interest. Payments were recorded as reductions to these liabilities with interest being charged to interest expense in our condensed consolidated statements of operations and comprehensive loss. We analyzed the nature of the renovations and our involvement during the construction period of our manufacturing facility and determined that we were the deemed “owner” of the construction project during the construction period. As a result, we were required to capitalize the fair value of the building as well as the construction costs incurred on our condensed consolidated balance sheet along with a corresponding financing liability for landlord-paid construction costs (i.e. “build-to-suit” accounting). Once construction was complete, the Company considered the requirements for sale-leaseback accounting treatment, including evaluating whether all risks of ownership have been transferred back to the landlord, as evidenced by a lack of continuing involvement in the leased property. Since the arrangement did not qualify for sale-leaseback accounting treatment, the building asset remained on the Company’s condensed consolidated balance sheets at its historical cost, and such asset was depreciated over its estimated useful life. The Company bifurcated its lease payments into a portion allocated to the building and a portion allocated to the parcel of land on which the building has been built. The portion of the lease payments allocated to the land was treated for accounting purposes as operating lease payments, and therefore was recorded as rent expense in the condensed consolidated statements of operations and comprehensive loss. The portion of the lease payments allocated to the building was further bifurcated into a portion allocated to interest expense and a portion allocated to reduce the build-to-suit lease obligation. The initial recording of these assets and liabilities were classified as non-cash investing and financing items, respectively, for purposes of the condensed consolidated statements of cash flows. The build-to-suit asset and corresponding lease obligation was derecognized upon adoption of the new lease standard as we did not control the building during the construction period. Recent Accounting Pronouncements In June 2016, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2016-13, Financial Instruments - Credit Losses: Measurement of Credit Losses on Financial Instruments In August 2018, the FASB issued ASU No. 2018-15, Intangibles—Goodwill and Other—Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract Adoption of New Accounting Pronouncements We adopted ASU No. 2016-02, Leases (Topic 842), as of January 1, 2019, using the optional transition method, which allows for the initial application of the new accounting standard at the adoption date and the recognition of a cumulative-effect adjustment to the opening balance of retained earnings as of the beginning of the period of adoption. In addition, we elected the package of practical expedients permitted under the transition guidance within the new standard, which among other things, allowed us to carry forward the historical lease classification. In addition, we elected the hindsight practical expedient to determine the lease term for existing leases. Adoption of the new standard resulted in the recording of additional operating lease assets and operating lease liabilities of $14.3 million and $15.3 million, respectively, as of January 1, 2019. This was partially offset by de-recognition of the build-to-suit asset and corresponding lease obligation of $10.3 million for our Thousand Oaks manufacturing facility lease, as we did not control the building during the construction period (see Note 7). The cumulative effect adjustment to the opening balance of accumulated deficit was a decrease of $0.4 million. The standard did not have a significant impact on our condensed consolidated statements of operations and comprehensive loss, changes in stockholders’ equity, and cash flow for the three and six months ended June 30, 2019 and 2018.

Net Loss per Common Share

Net Loss per Common Share	6 Months Ended
	Jun. 30, 2019
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 June 30, 2019 2018 Unvested RSUs 1,531,211 1,797,702 Vested and unvested options 7,291,729 5,718,914 ESPP share purchase rights 15,296 9,366 Total 8,838,236 7,525,982

Financial Instruments

Financial Instruments	6 Months Ended
	Jun. 30, 2019
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 June 30, 2019: Input Level Cost Gain Loss Fair Value (in thousands) Money market funds Level 1 $ 36,621 $ — $ — $ 36,621 U.S. Treasury obligations Level 2 69,324 49 (1 ) 69,372 Government agency obligations Level 2 9,079 1 (4 ) 9,076 Corporate debt obligations Level 2 63,788 135 (6 ) 63,917 Commercial paper Level 2 2,846 — — 2,846 Asset-backed securities Level 2 5,270 — (1 ) 5,269 Total available-for-sale securities 186,928 185 (12 ) 187,101 Less: amounts classified as cash equivalents (56,124 ) (1 ) — (56,125 ) Amounts classified as short-term investments $ 130,804 $ 184 $ (12 ) $ 130,976 Total Total Total Total Amortized Unrealized Unrealized Estimated As of December 31, 2018: Input Level Cost Gain Loss Fair Value (in thousands) Money market funds Level 1 $ 38,708 $ — $ — $ 38,708 U.S. Treasury obligations Level 2 111,164 4 (80 ) 111,088 Government agency obligations Level 2 15,206 1 (32 ) 15,175 Corporate debt obligations Level 2 121,017 15 (217 ) 120,815 Commercial paper Level 2 12,935 — — 12,935 Asset-backed securities Level 2 11,894 — (31 ) 11,863 Total available-for-sale securities 310,924 20 (360 ) 310,584 Less: amounts classified as cash equivalents (61,651 ) — — (61,651 ) Amounts classified as short-term investments $ 249,273 $ 20 $ (360 ) $ 248,933 The amortized cost and fair value of our available-for-sale securities by contractual maturity were as follows: As of June 30, 2019 As of December 31, 2018 Amortized Estimated Amortized Estimated Cost Fair Value Cost Fair Value (in thousands) (in thousands) Maturing within one year $ 178,095 $ 178,197 $ 287,755 $ 287,469 Maturing in one to five years 8,833 8,904 23,169 23,115 Total available-for-sale securities $ 186,928 $ 187,101 $ 310,924 $ 310,584 As of June 30, 2019, certain available-for-sale securities had been in a continuous unrealized loss position, each for less than twelve months. As of this date, no significant facts or circumstances were present to indicate a deterioration in the creditworthiness of the respective issuers, 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 June 30, 2019 and December 31, 2018, restricted cash was $1.4 million. 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: June 30, December 31, 2019 2018 (in thousands) Cash and cash equivalents $ 59,159 $ 60,698 Restricted cash - short term 194 194 Restricted cash - long term 1,200 1,200 Total cash, cash equivalents and restricted cash $ 60,553 $ 62,092

Property and Equipment

Property and Equipment	6 Months Ended
	Jun. 30, 2019
Property Plant And Equipment [Abstract]
Property and Equipment	5. Property and Equipment Property and equipment consisted of the following as of each period end: June 30, December 31, 2019 2018 (in thousands) Leasehold improvements $ 48,993 $ 47,609 Build-to-suit asset (see Note 7) — 10,686 Construction in progress 3,533 4,682 Computer equipment and software 3,235 3,049 Lab equipment 4,624 3,019 Machinery and equipment 3,193 2,980 Furniture and fixtures 1,674 1,628 Property and equipment, gross 65,252 73,653 Accumulated depreciation and amortization (8,162 ) (5,077 ) Property and equipment, net $ 57,090 $ 68,576 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 $1.8 million and $0.7 million for the three months ended June 30, 2019 and 2018, respectively and $3.4 million and $1.1 million for the six months ended June 30, 2019 and 2018, respectively.

License and Collaboration Agree

License and Collaboration Agreements	6 Months Ended
	Jun. 30, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
License and Collaboration Agreements	6. License and Collaboration Agreements MSK Agreements – In June 2015, we entered into an exclusive license agreement with MSK for three clinical stage T-cell therapies. In connection with the execution of the agreement, the Company paid $4.5 million in cash to MSK . We are required to make additional payments of up to $33.0 million to MSK based on achievement of specified regulatory and sales-related milestones, mid-single-digit percentage tiered royalty payments based on In addition, under certain circumstances, we are required to make certain minimum annual royalty payments to MSK, which are creditable against earned royalties owed for the same annual period. We are also required to pay a low double-digit percentage of any consideration we receive for sublicensing the licensed rights. In December 2018, we licensed additional technology from MSK. In connection with the effectiveness of this license agreement, we made upfront cash payments of $12.5 million in the first quarter of 2019, which were recorded as research and development expense in our consolidated statement of operations and comprehensive loss in the fourth quarter of 2018. We are obligated to make additional milestone payments based on achievement of specified development, regulatory and sales-related milestones as well as future sales of products resulting from the development of the licensed product candidates, if any QIMR Berghofer Agreements – In October 2015, we entered into an exclusive license agreement and a research and development collaboration agreement with QIMR Berghofer. In consideration for the exclusive license, the Company paid $3.0 million in cash to QIMR Berghofer. Under the terms of the license agreement, we obtained an exclusive, worldwide license to develop and commercialize allogeneic T-cell therapy programs utilizing technology and know-how developed by QIMR Berghofer. In September 2016, the exclusive license agreement and research and development collaboration agreement were amended and restated. Under the amended and restated agreements, we obtained an exclusive, worldwide license to develop and commercialize additional T-cell programs as well as the option to license additional technology in exchange for $3.3 million in cash, which was recorded as research and development expense in our condensed consolidated 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. From time to time, we have entered into other license and collaboration agreements with other parties. For example, we licensed additional rights related to our next-generation CAR T programs from MSK in May 2018 and from Moffitt Cancer Center in August 2018 and agreed to collaborate in connection with each of these licenses. Milestones and royalties under each of the above agreements are contingent upon future events and will be recorded as expense when it is probable that the milestones will be achieved or royalties are due. As of June 30, 2019 and December 31, 2018, there were no outstanding obligations for milestones and royalties under our license and collaboration agreements. 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.

Leases

Leases	6 Months Ended
	Jun. 30, 2019
Leases [Abstract]
Leases	7. Leases We lease our corporate headquarters in South San Francisco, California under a non-cancellable lease agreement that expires in April 2021. 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. In November 2018, we entered into a lease agreement for additional office space in Thousand Oaks, California that expires in February 2026. Additionally, we entered into a new lease for our office and lab space in Aurora, Colorado, effective May 2019, that expires in April 2024. In February 2017, we entered into a lease agreement for approximately 90,580 square feet of office, lab and cellular therapy manufacturing space in Thousand Oaks, California. The initial 15-year term of the lease commenced on February 15, 2018, upon the substantial completion of landlord’s work as defined under the agreement. The contractual obligations during the initial term are $16.4 million in aggregate. Based on the terms of the lease agreement and on our involvement in certain aspects of the construction, we were deemed the owner of the building during the construction period in accordance with U.S. GAAP in effect prior to January 1, 2019. Under this build-to-suit lease arrangement, we recognized construction in progress based on all construction costs incurred by both us and the landlord. We also recognized a financing obligation equal to all costs funded by the landlord. Due to completion of the construction by the landlord and not having met the criteria for sale-lease back accounting, we transferred the $10.3 million of landlord’s construction costs previously capitalized as construction in progress to a build-to-suit asset, and have recognized a corresponding long-term financing obligation for the same amount in long-term liabilities in our condensed consolidated balance sheets. In addition, we recorded $0.3 million of capitalized interest during the construction period through December 31, 2018. A portion of the monthly lease payment was allocated to land rent and recorded as an operating lease expense and the non-interest portion of the amortized lease payments to the landlord related to rent of the building was applied to the lease financing liability. Future minimum payments under our operating, finance and capital leases as of December 31, 2018 were as follows: Operating Leases Finance Leases Capital Leases Years Ending December 31, (in thousands) 2019 $ 1,107 $ 934 $ 540 2020 1,666 962 234 2021 1,555 991 29 2022 1,337 1,020 — 2023 1,375 1,051 — Thereafter 3,122 11,458 — Total minimum payments $ 10,162 $ 16,416 $ 803 Less: amount representing interest 65 Present value of capital lease obligations 738 Less: current portion 490 Capital lease obligation, net of current portion $ 248 The maturities of lease liabilities under our operating and finance leases as of June 30, 2019 were as follows: Operating Leases Finance Leases Periods Ending December 31, (in thousands) Remaining 2019 $ 1,248 $ 272 2020 2,867 236 2021 2,740 30 2022 2,611 — 2023 2,685 — Thereafter 14,477 — Total lease payments $ 26,628 $ 538 Less: amount representing interest (10,748 ) (35 ) Present value of lease liabilities $ 15,880 $ 503 Balance as of June 30, 2019 Other current liabilities $ 961 $ 362 Operating lease liabilities 14,919 — Other long-term liabilities — 141 Total $ 15,880 $ 503 The components of lease cost were as follows: June 30, 2019 Three Months Ended Six Months Ended (in thousands) Operating lease cost: Operating lease cost $ 649 $ 1,374 Short-term lease cost 200 395 Total operating lease cost $ 849 $ 1,769 Finance lease cost: Amortization expense $ 84 $ 167 Interest on lease liabilities 14 31 Total finance lease cost $ 98 $ 198 Rent expense under operating leases for the three and six months ended June 30, 2018 was $0.5 million and $1.0 million respectively. Other information related to leases was as follows: Six Months Ended June 30, 2019 (in thousands, except lease term and discount rate) Supplemental Cash Flows Information Cash paid for amounts included in the measurement of lease liabilities: Operating cash flows for operating leases $ 1,098 Operating cash flows for finance leases 31 Financing cash flows for finance leases 235 Operating lease assets obtained in exchange for lease obligations: $ 838 Weighted Average Remaining Lease Term Operating Leases 10.7 years Finance Leases 0.9 years Weighted Average Discount Rate Operating leases 10.4 % Finance leases 9.5 % Asset Retirement Obligation The Company’s Asset Retirement Obligation (“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, 2018 $ 717 Accretion expense 35 Balance as of June 30, 2019 $ 752

Commitments and Contingencies

Commitments and Contingencies	6 Months Ended
	Jun. 30, 2019
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	8. Commitments and Contingencies License and Collaboration Agreements Potential payments related to our license and collaboration agreements, including milestone and royalty payments, are detailed in Note 6. Other Research and Development Agreements We may enter into contracts in the normal course of business with clinical research organizations for clinical trials, with contract manufacturing clinical supplies, and with other vendors for pre-clinical studies, supplies and other services for our operating purposes. These contracts generally provide for termination on Indemnification Agreements In the normal course of business, we enter into contracts and agreements that contain a variety of representations and warranties and provide for indemnification for certain liabilities. The exposure under these agreements is unknown because it involves claims that may be made against us in the future but have not yet been made. To date, we have not paid any claims or been required to defend any action related to our indemnification obligations. However, we may record charges in the future as a result of these indemnification obligations. We also have indemnification obligations to our directors and executive officers for specified events or occurrences, subject to some limits, while they are serving at our request in such capacities. There have been no claims to date and we believe the fair value of these indemnification agreements is minimal. Accordingly, we did not record liabilities for these agreements as of June 30, 2019 and December 31, 2018. Contingencies From time to time, we may be involved in legal proceedings, as well as demands, claims and threatened litigation, which arise in the normal course of our business or otherwise. The ultimate outcome of any litigation is uncertain and unfavorable outcomes could have a negative impact on our results of operations and financial condition. Regardless of outcome, litigation can have an adverse impact on us because of the defense costs, diversion of management resources and other factors. We are not currently involved in any material legal proceedings.

Stockholders' Equity

Stockholders' Equity	6 Months Ended
	Jun. 30, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stockholders' Equity	9 . Stockholders’ Equity Our authorized capital stock consists of 520,000,000 shares, all with a par value of $0.0001 per share, of which 500,000,000 shares are designated as common stock and 20,000,000 shares are designated as preferred stock. There were no shares of preferred stock outstanding as of ATM Facility In February 2019, we entered into a sales agreement (the “ATM Facility”) with Cowen and Company, LLC (“Cowen”), which provides for the sale, in our sole discretion, of shares of our common stock, in the aggregate offering price of up to $100.0 million through Cowen, as our sales agent. The issuance and sale of these shares by us pursuant to the ATM Facility are deemed “at the market” offerings as defined in Rule 415 under the Securities Act of 1933, as amended (the “Securities Act”), and are registered under the Securities Act. During the three and six months ended June 30, 2019, we sold an aggregate of 359,234 shares of common stock under the ATM Facility, at an average price of $22.18 per share, for gross proceeds of $8.0 million and net proceeds of $7.6 million, after deducting commissions and other offering expenses payable by us. As of June 30, 2019, $92.0 million of common stock remained available to be sold under this facility, subject to certain conditions as specified in the agreement and the expiration of the 60-day lock-up period in relation to the underwritten public offering described in Note 10. Equity Incentive Plans Under the terms of the 2014 Equity Incentive Plan, as amended (“2014 EIP”), we may grant stock options, restricted stock awards (“RSAs”) and RSUs to employees, directors, consultants and other service providers. RSUs generally vest over four years and typically require settlement by the earlier of seven to ten 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 generally vest over four years and expire in seven to ten years. As of June 30, 2019, a total of 12,135,316 shares of common stock were reserved for issuance under the 2014 EIP, of which 4,241,491 shares were available for future grant and 7,893,825 shares were subject to outstanding options and RSUs. In February 2018, we adopted the 2018 Inducement Plan (“Inducement Plan”), under which we may grant options, stock appreciation rights, RSAs and RSUs to new employees. As of June 30, 2019, 1,223,621 shares of common stock were reserved for issuance under the Inducement Plan, of which 404,172 shares were available for future grant and 819,449 shares were subject to outstanding options and RSUs. Restricted Stock Units The fair value of RSUs is determined as the closing stock price on the date of grant. The weighted average grant date fair value of RSUs granted during the six months ended June 30, 2019 and 2018 was $35.17 and $36.69, respectively. As of June 30, 2019, there was $44.6 million of unrecognized stock-based compensation expense related to RSUs that is expected to be recognized over a weighted average period of 2.9 years. The aggregate intrinsic value of the RSUs outstanding as of June 30, 2019 was $30.8 million. The following is a summary of RSU activity under our 2014 EIP and Inducement Plan: RSUs Shares Weighted Average Grant Date Fair Value Unvested as of December 31, 2018 1,405,460 $ 26.94 Granted 998,567 $ 35.17 Forfeited (346,327 ) $ 29.37 Vested (526,489 ) $ 23.69 Unvested as of June 30, 2019 1,531,211 $ 32.87 Vested and unreleased — Outstanding as of June 30, 2019 1,531,211 $ 32.87 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 six months ended June 30, 2019, we settled 528,589 shares underlying RSUs, of which 482,714 shares underlying RSUs were net settled by withholding 210,865 shares. The value of the RSUs withheld was $6.7 million, based on the closing price of our common stock on the settlement date. During the six months ended June 30, 2018, we settled 380,034 shares underlying RSUs, of which 233,836 shares underlying RSUs were net settled by withholding 87,954 shares. The value of the RSUs withheld was $3.4 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 Inducement Plan. The table below also includes the activity relating to options for 275,000 shares of our common stock which were issued in 2017 outside of these plans: Shares Weighted Average Exercise Price Weighted Average Remaining Contractual Term (Years) Aggregate Intrinsic Value (in thousands) Outstanding as of December 31, 2018 6,276,999 $ 28.15 Granted 1,750,825 32.47 Exercised (181,664 ) 18.13 Forfeited or expired (554,431 ) 33.29 Outstanding as of June 30, 2019 7,291,729 $ 29.04 5.5 $ 8,693 Vested and expected to vest as of June 30, 2019 7,291,729 $ 29.04 5.5 $ 8,693 Exercisable as of June 30, 2019 2,972,192 $ 24.40 3.5 $ 5,697 Aggregate intrinsic value represents the difference between the closing stock price of our common stock on June 30, 2019 and the exercise price of outstanding, in-the-money options. As of June 30, 2019, there was $82.9 million of unrecognized stock-based compensation expense related to stock options that is expected to be recognized over a weighted average period of 2.9 years. Options for 181,664 and 622,982 shares of our common stock were exercised during the six months ended June 30, 2019 and 2018, with an intrinsic value of $3.3 million and $11.3 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: Six months ended June 30, 2019 Six months ended June 30, 2018 Assumptions: Expected term (years) 5.9 4.6 Expected volatility 77.0 % 73.5 % Risk-free interest rate 2.3 % 2.6 % Expected dividend yield 0.0 % 0.0 % Fair Value: Weighted-average estimated grant date fair value per share $ 21.82 $ 22.79 Options granted 1,750,825 1,578,750 Total estimated grant date fair value $ 38,203,000 $ 35,980,000 Employee Stock Purchase Plan Our 2014 Employee Stock Purchase Plan (“2014 ESPP”) a llows As of June 30, 2019, there was $1.4 million of unrecognized stock-based compensation expense related to the ESPP that is expected to be recognized by the end of second quarter of 2020. The 2014 ESPP provides for annual increases in the number of shares available for issuance thereunder on the first business day of each fiscal year, beginning with 2015 and ending in 2024, equal to the lower of (i) one percent of the number of shares of our common stock outstanding as of such date, (ii) 230,769 shares of our common stock, or (iii) a lesser number of shares as determined by our board of directors. As of June 30, 2019, there were 1,355,973 shares authorized under the 2014 ESPP. Reserved Shares The following shares of common stock were reserved for future issuance as of June 30, 2019: Total Shares Reserved 2014 Equity Incentive Plan 12,135,316 2018 Inducement Plan 1,223,621 2014 Employee Stock Purchase Plan 1,067,575 Options granted outside the equity plans 109,666 Total reserved shares of common stock 14,536,178 Stock-based Compensation Expense Total stock-based compensation expense related to all employee and non-employee stock awards was as follows: Three Six months ended June 30, 2019 2018 2019 2018 (in thousands) (in thousands) Research and development $ 6,672 $ 3,384 $ 12,737 $ 6,316 General and administrative 8,529 4,614 14,733 8,697 Total stock-based compensation expense $ 15,201 $ 7,998 $ 27,470 $ 15,013

Subsequent Event

Subsequent Event	6 Months Ended
	Jun. 30, 2019
Subsequent Events [Abstract]
Subsequent Events	10. Subsequent Events In July 2019, we issued and sold 6,871,727 shares of common stock at an offering price of $15.28 per share and pre-funded warrants to purchase 2,945,026 shares of common stock at a public offering price of $15.2799 per warrant in an underwritten public offering pursuant to a shelf registration statement on Form S-3. We have granted the underwriters an option to purchase up to 1,472,512 additional shares of our common stock at an offering price of $15.28, less underwriting discounts and commissions. This option has not been exercised by the underwriters and expires 30 days from July 18, 2019. Each pre-funded warrant entitles the holder to purchase one share of common stock at an exercise price of $0.0001 per share and expires in seven years from the date of issuance. The gross proceeds from this public offering were $150.0 million, resulting in aggregate net proceeds of approximately $140.6 million, after deducting underwriting discounts and commissions and estimated offering expenses payable by us.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	6 Months Ended
	Jun. 30, 2019
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying unaudited condensed consolidated financial statements include the accounts of Atara and its wholly-owned subsidiaries and have been prepared in accordance with U.S. generally accepted accounting principles (“U.S. GAAP”) and 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, 2018, except for the recognition of operating lease assets and operating lease liabilities effective January 1, 2019, in accordance with newly-adopted accounting pronouncements relating to leases as discussed below.
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 and income taxes. Actual results could differ materially from those estimates.
Leases	Leases We lease office space in multiple locations. We determine if an arrangement is a lease at inception. Operating leases are included in operating lease assets, other current liabilities, and operating lease liabilities on our condensed consolidated balance sheets. Leases with an initial term of 12 months or less are not recorded on the balance sheet; we recognize lease expense for these leases on a straight-line basis over the lease term. Finance leases are included in property and equipment, other current liabilities, and other long-term liabilities on our condensed consolidated balance sheets. Operating lease assets and operating lease liabilities are recognized based on the present value of the future minimum lease payments over the lease term at commencement date. As most of our leases do not provide an implicit rate, we use our incremental borrowing rate based on the information available at commencement date in determining the present value of future payments. The incremental borrowing rate for our leases is determined based on lease term and currency in which lease payments are made, adjusted for impacts of collateral. The operating lease asset also includes any lease payments made and excludes lease incentives and initial direct costs incurred. Lease expense for minimum lease payments is recognized on a straight-line basis over the lease term. Our facilities and equipment operating leases have lease and non-lease components and we have made a policy election to account for the lease and non-lease components as a single lease component. Through December 31, 2018, the leases were reviewed for classification as operating, capital or build-to-suit leases. For operating leases, rent was recognized on a straight-line basis over the lease period. For capital leases, we recorded the leased asset with a corresponding liability for principal and interest. Payments were recorded as reductions to these liabilities with interest being charged to interest expense in our condensed consolidated statements of operations and comprehensive loss. We analyzed the nature of the renovations and our involvement during the construction period of our manufacturing facility and determined that we were the deemed “owner” of the construction project during the construction period. As a result, we were required to capitalize the fair value of the building as well as the construction costs incurred on our condensed consolidated balance sheet along with a corresponding financing liability for landlord-paid construction costs (i.e. “build-to-suit” accounting). Once construction was complete, the Company considered the requirements for sale-leaseback accounting treatment, including evaluating whether all risks of ownership have been transferred back to the landlord, as evidenced by a lack of continuing involvement in the leased property. Since the arrangement did not qualify for sale-leaseback accounting treatment, the building asset remained on the Company’s condensed consolidated balance sheets at its historical cost, and such asset was depreciated over its estimated useful life. The Company bifurcated its lease payments into a portion allocated to the building and a portion allocated to the parcel of land on which the building has been built. The portion of the lease payments allocated to the land was treated for accounting purposes as operating lease payments, and therefore was recorded as rent expense in the condensed consolidated statements of operations and comprehensive loss. The portion of the lease payments allocated to the building was further bifurcated into a portion allocated to interest expense and a portion allocated to reduce the build-to-suit lease obligation. The initial recording of these assets and liabilities were classified as non-cash investing and financing items, respectively, for purposes of the condensed consolidated statements of cash flows. The build-to-suit asset and corresponding lease obligation was derecognized upon adoption of the new lease standard as we did not control the building during the construction period.
Recent Accounting Pronouncements	Recent Accounting Pronouncements In June 2016, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2016-13, Financial Instruments - Credit Losses: Measurement of Credit Losses on Financial Instruments In August 2018, the FASB issued ASU No. 2018-15, Intangibles—Goodwill and Other—Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract Adoption of New Accounting Pronouncements We adopted ASU No. 2016-02, Leases (Topic 842), as of January 1, 2019, using the optional transition method, which allows for the initial application of the new accounting standard at the adoption date and the recognition of a cumulative-effect adjustment to the opening balance of retained earnings as of the beginning of the period of adoption. In addition, we elected the package of practical expedients permitted under the transition guidance within the new standard, which among other things, allowed us to carry forward the historical lease classification. In addition, we elected the hindsight practical expedient to determine the lease term for existing leases. Adoption of the new standard resulted in the recording of additional operating lease assets and operating lease liabilities of $14.3 million and $15.3 million, respectively, as of January 1, 2019. This was partially offset by de-recognition of the build-to-suit asset and corresponding lease obligation of $10.3 million for our Thousand Oaks manufacturing facility lease, as we did not control the building during the construction period (see Note 7). The cumulative effect adjustment to the opening balance of accumulated deficit was a decrease of $0.4 million. The standard did not have a significant impact on our condensed consolidated statements of operations and comprehensive loss, changes in stockholders’ equity, and cash flow for the three and six months ended June 30, 2019 and 2018.

Net Loss per Common Share (Tabl

Net Loss per Common Share (Tables)	6 Months Ended
	Jun. 30, 2019
Earnings Per Share [Abstract]
Antidilutive Securities Excluded From Computation of Diluted Net Loss per Common Share	The following table represents the potential common shares issuable pursuant to outstanding securities as of the related period end dates that were excluded from the computation of diluted net loss per common share as their inclusion would have an antidilutive effect: As of June 30, 2019 2018 Unvested RSUs 1,531,211 1,797,702 Vested and unvested options 7,291,729 5,718,914 ESPP share purchase rights 15,296 9,366 Total 8,838,236 7,525,982

Financial Instruments (Tables)

Financial Instruments (Tables)	6 Months Ended
	Jun. 30, 2019
Financial Instruments Disclosure [Abstract]
Summary of Estimated Fair Value and Related Valuation Input Hierarchy of Available-for-Sale Securities	The following tables summarize the estimated fair value and related valuation input hierarchy of our available-for-sale securities as of each period end: Total Total Total Total Amortized Unrealized Unrealized Estimated As of June 30, 2019: Input Level Cost Gain Loss Fair Value (in thousands) Money market funds Level 1 $ 36,621 $ — $ — $ 36,621 U.S. Treasury obligations Level 2 69,324 49 (1 ) 69,372 Government agency obligations Level 2 9,079 1 (4 ) 9,076 Corporate debt obligations Level 2 63,788 135 (6 ) 63,917 Commercial paper Level 2 2,846 — — 2,846 Asset-backed securities Level 2 5,270 — (1 ) 5,269 Total available-for-sale securities 186,928 185 (12 ) 187,101 Less: amounts classified as cash equivalents (56,124 ) (1 ) — (56,125 ) Amounts classified as short-term investments $ 130,804 $ 184 $ (12 ) $ 130,976 Total Total Total Total Amortized Unrealized Unrealized Estimated As of December 31, 2018: Input Level Cost Gain Loss Fair Value (in thousands) Money market funds Level 1 $ 38,708 $ — $ — $ 38,708 U.S. Treasury obligations Level 2 111,164 4 (80 ) 111,088 Government agency obligations Level 2 15,206 1 (32 ) 15,175 Corporate debt obligations Level 2 121,017 15 (217 ) 120,815 Commercial paper Level 2 12,935 — — 12,935 Asset-backed securities Level 2 11,894 — (31 ) 11,863 Total available-for-sale securities 310,924 20 (360 ) 310,584 Less: amounts classified as cash equivalents (61,651 ) — — (61,651 ) Amounts classified as short-term investments $ 249,273 $ 20 $ (360 ) $ 248,933
Amortized Cost and Fair Value of Available for Sale Securities by Contractual Maturity	The amortized cost and fair value of our available-for-sale securities by contractual maturity were as follows: As of June 30, 2019 As of December 31, 2018 Amortized Estimated Amortized Estimated Cost Fair Value Cost Fair Value (in thousands) (in thousands) Maturing within one year $ 178,095 $ 178,197 $ 287,755 $ 287,469 Maturing in one to five years 8,833 8,904 23,169 23,115 Total available-for-sale securities $ 186,928 $ 187,101 $ 310,924 $ 310,584
Reconciliation of Cash, Cash Equivalents and Restricted Cash	The following table provides a reconciliation of cash, cash equivalents and restricted cash within the condensed consolidated balance sheets that sum to the total of the same such amounts in the condensed consolidated statement of cash flows: June 30, December 31, 2019 2018 (in thousands) Cash and cash equivalents $ 59,159 $ 60,698 Restricted cash - short term 194 194 Restricted cash - long term 1,200 1,200 Total cash, cash equivalents and restricted cash $ 60,553 $ 62,092

Property and Equipment (Tables)

Property and Equipment (Tables)	6 Months Ended
	Jun. 30, 2019
Property Plant And Equipment [Abstract]
Schedule of Property and Equipment	Property and equipment consisted of the following as of each period end: June 30, December 31, 2019 2018 (in thousands) Leasehold improvements $ 48,993 $ 47,609 Build-to-suit asset (see Note 7) — 10,686 Construction in progress 3,533 4,682 Computer equipment and software 3,235 3,049 Lab equipment 4,624 3,019 Machinery and equipment 3,193 2,980 Furniture and fixtures 1,674 1,628 Property and equipment, gross 65,252 73,653 Accumulated depreciation and amortization (8,162 ) (5,077 ) Property and equipment, net $ 57,090 $ 68,576

Leases (Tables)

Leases (Tables)	6 Months Ended
	Jun. 30, 2019
Leases [Abstract]
Schedule of Future Minimum Payments Under Operating, Finance and Capital Leases	Future minimum payments under our operating, finance and capital leases as of December 31, 2018 were as follows: Operating Leases Finance Leases Capital Leases Years Ending December 31, (in thousands) 2019 $ 1,107 $ 934 $ 540 2020 1,666 962 234 2021 1,555 991 29 2022 1,337 1,020 — 2023 1,375 1,051 — Thereafter 3,122 11,458 — Total minimum payments $ 10,162 $ 16,416 $ 803 Less: amount representing interest 65 Present value of capital lease obligations 738 Less: current portion 490 Capital lease obligation, net of current portion $ 248
Schedule of Maturities of Lease Liabilities Under Operating and Finance Leases	The maturities of lease liabilities under our operating and finance leases as of June 30, 2019 were as follows: Operating Leases Finance Leases Periods Ending December 31, (in thousands) Remaining 2019 $ 1,248 $ 272 2020 2,867 236 2021 2,740 30 2022 2,611 — 2023 2,685 — Thereafter 14,477 — Total lease payments $ 26,628 $ 538 Less: amount representing interest (10,748 ) (35 ) Present value of lease liabilities $ 15,880 $ 503 Balance as of June 30, 2019 Other current liabilities $ 961 $ 362 Operating lease liabilities 14,919 — Other long-term liabilities — 141 Total $ 15,880 $ 503
Components of Lease Cost	The components of lease cost were as follows: June 30, 2019 Three Months Ended Six Months Ended (in thousands) Operating lease cost: Operating lease cost $ 649 $ 1,374 Short-term lease cost 200 395 Total operating lease cost $ 849 $ 1,769 Finance lease cost: Amortization expense $ 84 $ 167 Interest on lease liabilities 14 31 Total finance lease cost $ 98 $ 198
Summary of Other Information Related to Leases	Other information related to leases was as follows: Six Months Ended June 30, 2019 (in thousands, except lease term and discount rate) Supplemental Cash Flows Information Cash paid for amounts included in the measurement of lease liabilities: Operating cash flows for operating leases $ 1,098 Operating cash flows for finance leases 31 Financing cash flows for finance leases 235 Operating lease assets obtained in exchange for lease obligations: $ 838 Weighted Average Remaining Lease Term Operating Leases 10.7 years Finance Leases 0.9 years Weighted Average Discount Rate Operating leases 10.4 % Finance leases 9.5 %
Summary of Activity for ARO Liabilities	The following table presents the activity for our ARO liabilities: ARO Liability (in thousands) Balance as of December 31, 2018 $ 717 Accretion expense 35 Balance as of June 30, 2019 $ 752

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	6 Months Ended
	Jun. 30, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Summary of RSU Activity	The following is a summary of RSU activity under our 2014 EIP and Inducement Plan: RSUs Shares Weighted Average Grant Date Fair Value Unvested as of December 31, 2018 1,405,460 $ 26.94 Granted 998,567 $ 35.17 Forfeited (346,327 ) $ 29.37 Vested (526,489 ) $ 23.69 Unvested as of June 30, 2019 1,531,211 $ 32.87 Vested and unreleased — Outstanding as of June 30, 2019 1,531,211 $ 32.87
Summary of Stock Option Activity	The following is a summary of stock option activity under our 2014 EIP and Inducement Plan. The table below also includes the activity relating to options for 275,000 shares of our common stock which were issued in 2017 outside of these plans: Shares Weighted Average Exercise Price Weighted Average Remaining Contractual Term (Years) Aggregate Intrinsic Value (in thousands) Outstanding as of December 31, 2018 6,276,999 $ 28.15 Granted 1,750,825 32.47 Exercised (181,664 ) 18.13 Forfeited or expired (554,431 ) 33.29 Outstanding as of June 30, 2019 7,291,729 $ 29.04 5.5 $ 8,693 Vested and expected to vest as of June 30, 2019 7,291,729 $ 29.04 5.5 $ 8,693 Exercisable as of June 30, 2019 2,972,192 $ 24.40 3.5 $ 5,697
Summary of Options Estimated with Weighted Average	The fair value of each option issued was estimated at the date of grant using the Black-Scholes valuation model. The following table summarizes the weighted-average assumptions used as inputs to the Black-Scholes model, and resulting weighted-average grant date fair values of stock options granted to employees during the periods indicated: Six months ended June 30, 2019 Six months ended June 30, 2018 Assumptions: Expected term (years) 5.9 4.6 Expected volatility 77.0 % 73.5 % Risk-free interest rate 2.3 % 2.6 % Expected dividend yield 0.0 % 0.0 % Fair Value: Weighted-average estimated grant date fair value per share $ 21.82 $ 22.79 Options granted 1,750,825 1,578,750 Total estimated grant date fair value $ 38,203,000 $ 35,980,000
Schedule of Common Stock Reserved for Future Issuance	The following shares of common stock were reserved for future issuance as of June 30, 2019: Total Shares Reserved 2014 Equity Incentive Plan 12,135,316 2018 Inducement Plan 1,223,621 2014 Employee Stock Purchase Plan 1,067,575 Options granted outside the equity plans 109,666 Total reserved shares of common stock 14,536,178
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 Six months ended June 30, 2019 2018 2019 2018 (in thousands) (in thousands) Research and development $ 6,672 $ 3,384 $ 12,737 $ 6,316 General and administrative 8,529 4,614 14,733 8,697 Total stock-based compensation expense $ 15,201 $ 7,998 $ 27,470 $ 15,013

Description of Business (Detail

Description of Business (Detail) - USD ($) $ in Thousands	1 Months Ended	6 Months Ended
	Jul. 31, 2019	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Organization And Description Of Business [Line Items]
Entity incorporation state		Delaware
Entity incorporation date		Aug. 1, 2012
Accumulated Deficit		$ 667,570		$ 527,349
Proceeds from issuance of common stock through underwritten public offering, net		$ 7,630	$ 47,586
Subsequent Event | Underwritten Public Offering
Organization And Description Of Business [Line Items]
Proceeds from issuance of common stock through underwritten public offering, net	$ 140,600

Summary of Significant Accoun_3

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	Jan. 01, 2019	Jun. 30, 2019	Jun. 30, 2018
Significant Accounting Policies [Line Items]
Additional operating lease assets		$ (814)	$ 0
Additional operating lease liabilities		$ (299)	$ 0
Accounting Standards Update 2016-02
Significant Accounting Policies [Line Items]
Additional operating lease assets	$ 14,300
Additional operating lease liabilities	15,300
Corresponding lease obligation	10,300
Cumulative effect adjustment to opening balance of accumulated deficit, expected decrease	$ 400

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	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	8,838,236	7,525,982
Unvested RSUs
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	1,531,211	1,797,702
Vested and Unvested Options
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	7,291,729	5,718,914
ESPP Share Purchase Rights
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	15,296	9,366

Financial Instruments - Summary

Financial Instruments - Summary of Estimated Fair Value and Related Valuation Input Hierarchy of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Total Fair Value	$ 187,101	$ 310,584
Fair Value, Measurements, Recurring
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	186,928	310,924
Total Unrealized Gain	185	20
Total Unrealized Loss	(12)	(360)
Total Fair Value	187,101	310,584
Fair Value, Measurements, Recurring | Money Market Funds | Level 1
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	36,621	38,708
Total Unrealized Gain	0	0
Total Unrealized Loss	0	0
Total Fair Value	36,621	38,708
Fair Value, Measurements, Recurring | U.S. Treasury Obligations | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	69,324	111,164
Total Unrealized Gain	49	4
Total Unrealized Loss	(1)	(80)
Total Fair Value	69,372	111,088
Fair Value, Measurements, Recurring | Government Agency Obligations | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	9,079	15,206
Total Unrealized Gain	1	1
Total Unrealized Loss	(4)	(32)
Total Fair Value	9,076	15,175
Fair Value, Measurements, Recurring | Corporate Debt Obligations | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	63,788	121,017
Total Unrealized Gain	135	15
Total Unrealized Loss	(6)	(217)
Total Fair Value	63,917	120,815
Fair Value, Measurements, Recurring | Commercial Paper | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	2,846	12,935
Total Unrealized Gain	0	0
Total Unrealized Loss	0	0
Total Fair Value	2,846	12,935
Fair Value, Measurements, Recurring | Asset-Backed Securities | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	5,270	11,894
Total Unrealized Gain	0	0
Total Unrealized Loss	(1)	(31)
Total Fair Value	5,269	11,863
Fair Value, Measurements, Recurring | Amounts Classified As Cash Equivalents
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	(56,124)	(61,651)
Total Unrealized Gain	(1)	0
Total Unrealized Loss	0	0
Total Fair Value	(56,125)	(61,651)
Fair Value, Measurements, Recurring | Amounts Classified As Short-Term Investments
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	130,804	249,273
Total Unrealized Gain	184	20
Total Unrealized Loss	(12)	(360)
Total Fair Value	$ 130,976	$ 248,933

Financial Instruments - Amortiz

Financial Instruments - Amortized Cost and Fair Value of Available-for-Sale Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Amortized cost
Maturing within one year, Amortized cost	$ 178,095	$ 287,755
Maturing in one to five years, Amortized cost	8,833	23,169
Total available-for-sale securities, Amortized cost	186,928	310,924
Estimated Fair value
Maturing within one year, Estimated fair value	178,197	287,469
Maturing in one to five years, Estimated fair value	8,904	23,115
Total available-for-sale securities, Estimated fair value	$ 187,101	$ 310,584

Financial Instruments - Additio

Financial Instruments - Additional Information (Detail) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Financial Instruments Disclosure [Abstract]
Restricted Cash	$ 1.4	$ 1.4

Financial Instruments - Reconci

Financial Instruments - Reconciliation of Cash, Cash Equivalents and Restricted Cash (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Financial Instruments Disclosure [Abstract]
Cash and cash equivalents	$ 59,159	$ 60,698
Restricted cash - short-term	194	194
Restricted cash - long-term	1,200	1,200
Total cash, cash equivalents and restricted cash	$ 60,553	$ 62,092	$ 104,597	$ 80,617

Property and Equipment - Schedu

Property and Equipment - Schedule of Property and Equipment (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 65,252	$ 73,653
Accumulated depreciation and amortization	(8,162)	(5,077)
Property and equipment, net	57,090	68,576
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	48,993	47,609
Build-to-Suit Asset
Property Plant And Equipment [Line Items]
Property and equipment, gross	0	10,686
Construction in Progress
Property Plant And Equipment [Line Items]
Property and equipment, gross	3,533	4,682
Computer Equipment and Software
Property Plant And Equipment [Line Items]
Property and equipment, gross	3,235	3,049
Lab Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	4,624	3,019
Machinery and Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	3,193	2,980
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 1,674	$ 1,628

Property and Equipment - Additi

Property and Equipment - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Property Plant And Equipment Net By Type [Abstract]
Depreciation and amortization expense	$ 1,800	$ 700	$ 3,385	$ 1,064

License and Collaboration Agr_2

License and Collaboration Agreements - Additional Information (Detail) - USD ($)	1 Months Ended		3 Months Ended				6 Months Ended
	Oct. 31, 2015	Jun. 30, 2015	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Sep. 30, 2016	Jun. 30, 2019	Jun. 30, 2018
Related Party Transaction [Line Items]
Research and development expense			$ 52,251,000		$ 33,387,000		$ 100,919,000	$ 61,847,000
Contractual obligations due to MSK and QIMR			0	$ 0			0
MSK Agreements
Related Party Transaction [Line Items]
Research and development expense		$ 4,500,000		$ 12,500,000
Potential milestone payments payable			$ 33,000,000				$ 33,000,000
License Agreement | QIMR Berghofer
Related Party Transaction [Line Items]
Research and development expense	$ 3,000,000					$ 3,300,000

Leases - Additional Information

Leases - Additional Information (Detail) $ in Millions	Feb. 15, 2017USD ($)ft²	May 31, 2019	Jun. 30, 2018USD ($)	Jun. 30, 2019USD ($)	Jun. 30, 2018USD ($)	Dec. 31, 2018USD ($)
Lessee Lease Description [Line Items]
Rent expenses under operating leases			$ 0.5		$ 1
South San Francisco California Lease
Lessee Lease Description [Line Items]
Lease expiration date				Apr. 30, 2021
Letter of credit renewal term				12 months
South San Francisco California Lease | Letter of Credit
Lessee Lease Description [Line Items]
Letter of credit maintained				$ 0.2
Thousand Oaks California lease
Lessee Lease Description [Line Items]
Lease expiration date				Feb. 28, 2026
Lease agreement area of office, lab and cellular therapy manufacturing space | ft²	90,580
Lease commencement date	Feb. 15, 2018
Lease initial term	15 years
Contractual obligations	$ 16.4
Lease extension term, option one	10 years
Lease extension term, option two	9 years
Thousand Oaks California lease | Letter of Credit
Lessee Lease Description [Line Items]
Letter of credit issued, classified as long-term restricted cash	$ 1.2
Aurora Colorado lease
Lessee Lease Description [Line Items]
Lease expiration date		Apr. 30, 2024
Build To Suit Lease Arrangement | Building
Lessee Lease Description [Line Items]
Construction in progress						$ 10.3
Capitalized interest costs during the construction period						$ 0.3

Leases - Schedule of Future Min

Leases - Schedule of Future Minimum Payments Under Operating, Finance and Capital Leases (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Leases [Abstract]
Operating leases, 2019		$ 1,107
Operating leases 2020		1,666
Operating leases, 2021		1,555
Operating leases, 2022		1,337
Operating leases, 2023		1,375
Operating leases, Thereafter		3,122
Operating leases, Total minimum payments		10,162
Finance leases, 2019		934
Finance leases, Remaining 2019	$ 272
Finance leases, 2020	236	962
Finance leases, 2021	30	991
Finance leases, 2022	0	1,020
Finance leases, 2023	0	1,051
Finance leases, Thereafter	0	11,458
Finance leases, Total minimum payments	$ 538	16,416
Capital leases, 2019		540
Capital leases, 2020		234
Capital leases, 2021		29
Capital leases, 2022		0
Capital leases, 2023		0
Capital leases, Thereafter		0
Capital leases, Total minimum payments		803
Less: amount representing interest		65
Present value of capital lease obligations		738
Less: current portion		490
Capital lease obligation, net of current portion		$ 248

Leases - Schedule of Maturities

Leases - Schedule of Maturities of Lease Liabilities Under Operating and Finance Leases (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Leases [Abstract]
Operating leases, Remaining 2019	$ 1,248
Operating leases, 2020	2,867
Operating leases, 2021	2,740
Operating leases, 2022	2,611
Operating leases, 2023	2,685
Operating leases, Thereafter	14,477
Operating leases, Total minimum payments	26,628
Less: amount representing interest	(10,748)
Present value of lease liabilities	15,880
Other current liabilities	961
Operating lease liabilities	14,919	$ 0
Other long-term liabilities	0
Total	15,880
Finance leases, Remaining 2019	272
Finance leases, 2020	236	962
Finance leases, 2021	30	991
Finance leases, 2022	0	1,020
Finance leases, 2023	0	1,051
Finance leases, Thereafter	0	11,458
Finance leases, Total minimum payments	538	$ 16,416
Less: amount representing interest	(35)
Present value of lease liabilities	503
Other current liabilities	362
Finance lease liabilities	0
Other long-term liabilities	141
Total	$ 503

Leases - Components of Lease Co

Leases - Components of Lease Cost (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2019
Operating lease cost:
Operating lease cost	$ 649	$ 1,374
Short-term lease cost	200	395
Total operating lease cost	849	1,769
Finance lease cost:
Amortization expense	84	167
Interest on lease liabilities	14	31
Total finance lease cost	$ 98	$ 198

Leases - Summary of Other Infor

Leases - Summary of Other Information Related to Leases (Detail) $ in Thousands	6 Months Ended
	Jun. 30, 2019USD ($)
Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flows for operating leases	$ 1,098
Operating cash flows for finance leases	31
Financing cash flows for finance leases	235
Operating lease assets obtained in exchange for lease obligations:	$ 838
Weighted Average Remaining Lease Term
Operating Leases	10 years 8 months 12 days
Finance Leases	10 months 24 days
Weighted Average Discount Rate
Operating leases	10.40%
Finance leases	9.50%

Leases - Summary of Activity fo

Leases - Summary of Activity for ARO Liabilities (Detail) $ in Thousands	6 Months Ended
	Jun. 30, 2019USD ($)
Leases [Abstract]
Balance as of December 31, 2018	$ 717
Accretion expense	35
Balance as of June 30, 2019	$ 752

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - USD ($)	Jun. 30, 2019	Dec. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
Accrued termination charges	$ 0	$ 0
Liabilities related to indemnification agreements	$ 0	$ 0

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Detail) - USD ($)	Feb. 25, 2019	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2017	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Authorized capital stock		520,000,000				520,000,000
Common stock, par value		$ 0.0001				$ 0.0001			$ 0.0001
Common stock, shares authorized		500,000,000				500,000,000			500,000,000
Preferred stock, par value		$ 0.0001				$ 0.0001
Preferred stock, shares authorized		20,000,000				20,000,000
Preferred stock, shares outstanding		0				0			0
Proceeds from sale of common stock, net						$ 7,630,000	$ 47,586,000
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 generally vest over four years and expire in seven to ten years.
Ownership percent						10.00%
Shares of common stock, reserved for issuance		14,536,178				14,536,178
Common stock, shares exercised						181,664	622,982
Option intrinsic value, exercised						$ 3,300,000	$ 11,300,000
Net tax benefits related to exercised options						0
Stock purchase, value		$ 1,802,000	$ 2,898,000	$ 8,661,000	$ 6,196,000
Stock-based compensation expense		$ 15,201,000		$ 7,998,000		$ 27,470,000	15,013,000
2014 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares of common stock, reserved for issuance		12,135,316				12,135,316
Aggregate number of awards available for grant to be issued		7,893,825				7,893,825
Outstanding options and RSUs		4,241,491				4,241,491
Inducement Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares of common stock, reserved for issuance		1,223,621				1,223,621
Aggregate number of awards available for grant to be issued		404,172				404,172
Outstanding options and RSUs		819,449				819,449
2014 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of employees purchase price of common stock						85.00%
Shares of common stock, reserved for issuance		1,067,575				1,067,575
Unrecognized stock-based compensation expense		$ 1,400,000				$ 1,400,000
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
Stock-based compensation expense						$ 500,000	$ 200,000
Shares purchased						74,439	77,100
Aggregate number of shares authorized		1,355,973				1,355,973
Restricted Stock Units (RSUs)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted average grant date fair value per share						$ 35.17	$ 36.69
Unrecognized stock-based compensation expense		$ 44,600,000				$ 44,600,000
Unrecognized stock-based compensation weighted average recognition period						2 years 10 months 24 days
Aggregate intrinsic value		30,800,000				$ 30,800,000
Restricted stock units, settled						528,589	380,034
RSU settlements, net of shares withheld, shares						482,714	233,836
Restricted stock units withheld for tax obligations						210,865	87,954
Restricted stock units withheld for tax obligations, value						$ 6,700,000	$ 3,400,000
Restricted Stock Units (RSUs) | From Date Of Grant
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share based compensation, vesting period						4 years
Restricted Stock Units (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, 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 10 months 24 days
Stock options, issued								275,000
Unrecognized stock-based compensation		$ 82,900,000				$ 82,900,000
Maximum | 2014 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock purchase, value						$ 25,000
Annual increase in EIP/ESPP, subject to other limitations						1.00%
Maximum increase in number of shares available for issuance						230,769
Maximum | Restricted Stock Units (RSUs) | From Date Of Grant
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share based compensation award expiration period						10 years
Maximum | Employees And Non Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share based compensation award expiration period						10 years
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 | Restricted Stock Units (RSUs) | From Date Of Grant
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share based compensation award expiration period						7 years
Minimum | Employees And Non Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share based compensation award expiration period						7 years
At The Market Offering | Cowen
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, shares issued		359,234				359,234
Common stock average price		$ 22.18				$ 22.18
Proceeds from sale of common stock, gross		$ 8,000,000				$ 8,000,000
Proceeds from sale of common stock, net		7,600,000				7,600,000
Common stock value remaining to be sold		$ 92,000,000				$ 92,000,000
At The Market Offering | Cowen | Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock aggregate offering price	$ 100,000,000
Percentage of commission to be paid on gross sales proceeds of common stock sold	3.00%

Stockholders' Equity - Summary

Stockholders' Equity - Summary of RSU Activity (Detail) - Unvested RSUs - $ / shares	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares, Unvested as of December 31, 2018	1,405,460
Shares, Granted	998,567
Shares, Forfeited	(346,327)
Shares, Vested	(526,489)
Shares, Unvested as of June 30, 2019	1,531,211
Shares, Vested and unreleased	0
Shares, Outstanding as of June 30, 2019	1,531,211
Weighted Average Grant Date Fair Value, Unvested as of December 31, 2018	$ 26.94
Weighted Average Grant Date Fair Value, Granted	35.17	$ 36.69
Weighted Average Grant Date Fair Value, Forfeited	29.37
Weighted Average Grant Date Fair Value, Vested	23.69
Weighted Average Grant Date Fair Value, Unvested as of June 30, 2019	$ 32.87

Stockholders' Equity - Summar_2

Stockholders' Equity - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Exercised, Shares	(181,664)	(622,982)
2014 EIP and Inducement Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Outstanding, Shares, beginning balance	6,276,999
Granted, Shares	1,750,825
Exercised, Shares	(181,664)
Forfeited or expired, Shares	(554,431)
Outstanding, Shares, ending balance	7,291,729
Vested and expected to vest, Shares	7,291,729
Exercisable, Shares	2,972,192
Outstanding, Weighted Average Exercise Price, beginning balance	$ 28.15
Granted, Weighted Average Exercise Price	32.47
Exercised, Weighted Average Exercise Price	18.13
Forfeited or expired, Weighted Average Exercise price	33.29
Outstanding, Weighted Average Exercise Price, ending balance	29.04
Stock options vested and expected to vest, Weighted Average Exercise Price	29.04
Exercisable, Weighted Average Exercise Price	$ 24.40
Outstanding, Weighted Average Remaining Contractual Term	5 years 6 months
Stock options vested and expected to vest, Weighted Average Remaining Contractual Term	5 years 6 months
Exercisable, Weighted Average Remaining Contractual Term	3 years 6 months
Aggregate intrinsic value	$ 8,693
Vested and expected to vest, Aggregate Intrinsic Value	8,693
Exercisable, Aggregate Intrinsic Value	$ 5,697

Stockholders' Equity - Summar_3

Stockholders' Equity - Summary of Estimated Weighted-Average Assumptions (Detail) - Employees - USD ($)	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (years)	5 years 10 months 24 days	4 years 7 months 6 days
Expected volatility	77.00%	73.50%
Risk-free interest rate	2.30%	2.60%
Expected dividend yield	0.00%	0.00%
Weighted-average estimated grant date fair value per share	$ 21.82	$ 22.79
Options granted	1,750,825	1,578,750
Total estimated grant date fair value	$ 38,203,000	$ 35,980,000

Stockholders' Equity - Schedule

Stockholders' Equity - Schedule of Common Stock Reserved for Future Issuance (Detail)	Jun. 30, 2019shares
Class Of Stock [Line Items]
Total reserved shares of common stock	14,536,178
2014 Equity Incentive Plan
Class Of Stock [Line Items]
Total reserved shares of common stock	12,135,316
2018 Inducement Plan
Class Of Stock [Line Items]
Total reserved shares of common stock	1,223,621
2014 Employee Stock Purchase Plan
Class Of Stock [Line Items]
Total reserved shares of common stock	1,067,575
Options Granted Outside the Equity Plans
Class Of Stock [Line Items]
Total reserved shares of common stock	109,666

Stockholders' Equity - Schedu_2

Stockholders' Equity - Schedule of Stock-based Compensation Related to All Employee And Non-employee Stock Awards (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 15,201	$ 7,998	$ 27,470	$ 15,013
Research and development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	6,672	3,384	12,737	6,316
General and administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 8,529	$ 4,614	$ 14,733	$ 8,697

Subsequent Event - Additional I

Subsequent Event - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Jul. 18, 2019	Jul. 31, 2019	Jun. 30, 2019	Jun. 30, 2018
Subsequent Event [Line Items]
Proceeds from sale of common stock, net			$ 7,630	$ 47,586
Subsequent Event | Underwritten Public Offering
Subsequent Event [Line Items]
Common stock, shares issued		6,871,727
Shares issued, price per share		$ 15.28
Number of options to purchase common shares of stock granted	1,472,512
Option price to purchase shares	$ 15.28
Proceeds from sale of common stock, gross		$ 150,000
Proceeds from sale of common stock, net		$ 140,600
Subsequent Event | Underwritten Public Offering | Warrant
Subsequent Event [Line Items]
Number of securities called by warrants		2,945,026
Warrant, price per share		$ 15.2799
Number of securities called by each warrant		1
Warrants, exercise price		$ 0.0001
Warrants, term		7 years