Atara Biotherapeutics (ATRA) 10-Q 11 May 15 2015 Q1 Quarterly report Financial data

Document_and_Entity_Informatio

Document and Entity Information	3 Months Ended
	Mar. 31, 2015	Apr. 30, 2015
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	FALSE
Document Period End Date	31-Mar-15
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	Q1
Trading Symbol	ATRA
Entity Registrant Name	Atara Biotherapeutics, Inc.
Entity Central Index Key	1604464
Current Fiscal Year End Date	-19
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		24,364,115

Condensed_Consolidated_and_Com

Condensed Consolidated and Combined Balance Sheets (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified
Current assets:
Cash and cash equivalents	$71,329	$21,897
Short-term available-for-sale investments	95,367	82,219
Prepaid expenses and other current assets	2,995	1,910
Total current assets	169,691	106,026
Property and equipment, net	47	48
Other assets	79	48
Total assets	169,817	106,122
Current liabilities:
Accounts payable	794	440
Accrued compensation	522	1,225
Income tax payable	1	1
Other accrued liabilities	2,197	1,058
Total current liabilities	3,514	2,724
Other long-term liabilities	209	216
Total liabilities	3,723	2,940
Commitments and contingencies (Note 5)
Stockholders’ equity:
Preferred stock—$0.0001 par value, 20,000,000 authorized; none issued and outstanding as of March 31, 2015 and December 31, 2014
Common stock—$0.0001 par value, 23,911,930 and 19,692,937 shares issued and outstanding as of March 31, 2015 and December 31, 2014, respectively	2	2
Additional paid-in capital	216,159	144,169
Accumulated other comprehensive loss	-18	-100
Accumulated deficit	-50,049	-40,889
Total stockholders’ equity	166,094	103,182
Total liabilities and stockholders’ equity	$169,817	$106,122

Condensed_Consolidated_and_Com1

Condensed Consolidated and Combined Balance Sheets (Parenthetical) (USD $)	Mar. 31, 2015	Dec. 31, 2014
Statement Of Financial Position [Abstract]
Preferred stock, par value	$0.00	$0.00
Preferred stock, shares authorized	20,000,000	20,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$0.00	$0.00
Common stock, shares issued	23,911,930	19,692,937
Common stock, shares outstanding	23,911,930	19,692,937

Condensed_Consolidated_and_Com2

Condensed Consolidated and Combined Statements of Operations and Comprehensive Loss (USD $)	3 Months Ended
In Thousands, except Share data, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Expenses:
Research and development	$5,767	$2,981
General and administrative	3,544	4,096
Total operating expenses	9,311	7,077
Loss from operations	-9,311	-7,077
Interest and other income	153	6
Loss before provision for income taxes	-9,158	-7,071
Provision (benefit) for income taxes	2	-22
Net loss	-9,160	-7,049
Other comprehensive gain (loss), net of tax:
Unrealized gains (losses) on investments	82	-11
Other comprehensive gain (loss)	82	-11
Comprehensive loss	($9,078)	($7,060)
Net loss per common share:
Basic and diluted net loss per common share	($0.42)	($5.58)
Weighted-average common shares outstanding used to calculate basic and diluted net loss per common share	21,918,467	1,263,316

Condensed_Consolidated_and_Com3

Condensed Consolidated and Combined Statements of Cash Flows (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Operating activities
Net loss	($9,160,000)	($7,049,000)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation expense	6,000	1,000
Investment premium amortization, net	358,000	16,000
Stock-based compensation expense	2,483,000	3,317,000
Interest accrued on notes receivable from stockholder		-1,000
Changes in operating assets and liabilities:
Other assets	-31,000	1,000
Prepaid expenses and other current assets	-1,081,000	44,000
Accounts payable	354,000	421,000
Income tax payable		-92,000
Other accrued liabilities	1,139,000	557,000
Accrued compensation	-703,000	-132,000
Other long-term liabilities	13,000
Net cash used in operating activities	-6,622,000	-2,917,000
Investing activities
Purchase of short-term investments	-54,796,000	-22,414,000
Maturities of short-term investments	41,368,000
Purchase of property and equipment	-5,000	-1,000
Net cash used in investing activities	-13,433,000	-22,415,000
Financing activities
Proceeds from sale of common stock, net of offering costs	69,487,000
Repayment of notes receivable from stockholder		37,000
Proceeds from sale of convertible preferred stock		13,500,000
Net cash provided by financing activities	69,487,000	13,471,000
Increase (decrease) in cash and cash equivalents	49,432,000	-11,861,000
Cash and cash equivalents-beginning of period	21,897,000	51,615,000
Cash and cash equivalents-end of period	71,329,000	39,754,000
Non-cash financing activities
Issuance of common stock upon vesting of stock awards	20,000	20,000
Change in other long-term liabilities related to non-vested stock awards	-20,000	-20,000
Offering costs in anticipation of initial public filing included in other accrued liabilities and accounts payable		510,000
Supplemental cash flow disclosure—Cash paid for taxes	2,000	70,000
IPO
Financing activities
Offering costs incurred		-47,000
Convertible Preferred Stock
Financing activities
Offering costs incurred		($19,000)

Organization_and_Description_o

Organization and Description of Business	3 Months Ended
	Mar. 31, 2015
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Organization and Description of Business	1	Organization and Description of Business
	Atara Biotherapeutics, Inc. (“Atara”, “we” or “our”) was incorporated in August 2012 in Delaware. We are a biopharmaceutical company focused on developing innovative therapies for patients with debilitating diseases. Atara’s lead programs target myostatin and activin, members of the TGF-beta family of proteins that have demonstrated the potential to have therapeutic benefit in a number of clinical indications. Our product candidate portfolio was acquired through licensing arrangements with Amgen Inc. (“Amgen”) in exchange for convertible preferred stock, milestone payments and commitments for future royalties. See Note 4 for further information.
	Public Offerings
	In October 2014, we completed our initial public offering of 5,750,000 shares of common stock, including 750,000 shares from the exercise by the underwriters of their overallotment option, at an offering price to the public of $11.00 per share. We received net proceeds of approximately $55.8 million, after deducting underwriting discounts and commissions and offering expenses. In connection with the initial public offering, the Company’s outstanding shares of convertible preferred stock were automatically converted into 12,298,515 shares of common stock, resulting in the reclassification of $74.6 million from mezzanine equity to additional paid-in capital.
	In February 2015, we completed a follow-on offering of 4,147,358 shares of common stock at an offering price to the public of $18.00 per share.  We received net proceeds of approximately $69.5 million, after deducting underwriting discounts and commissions and offering expenses.

Summary_of_Significant_Account

Summary of Significant Accounting Policies	3 Months Ended
	Mar. 31, 2015
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2	Summary of Significant Accounting Policies
	Basis of Presentation and Recapitalization
	The accompanying interim condensed consolidated and combined financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”) and the rules and regulations of the Securities and Exchange Commission (the “SEC”). The accounting policies followed in the preparation of the interim condensed consolidated and combined financial statements are consistent in all material respects with those presented in Note 2 to the consolidated and combined financial statements included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2014.
	Atara was originally formed as a management company with the sole purpose of providing management, financial and administrative services for Nina Biotherapeutics, Inc. (“Nina”), Santa Maria Biotherapeutics, Inc. (“Santa Maria”) and Pinta Biotherapeutics, Inc. (“Pinta”).  Prior to March 31, 2014, the accompanying financial statements include the operations of Atara, Nina, Pinta and Santa Maria on a combined basis as the four individual companies were under common ownership and common management since inception. All intercompany transactions have been eliminated.
	On March 31, 2014, our boards of directors approved and we implemented a recapitalization (the “Recapitalization”) in which (a) all the outstanding shares of common stock of Atara were cancelled and forfeited by existing stockholders and (b) the stockholders of Nina, Pinta and Santa Maria exchanged their existing common and convertible preferred stock for newly-issued shares of Atara, with the same rights and privileges as the outstanding capital stock of Nina, Pinta and Santa Maria. The shares were exchanged on a collective nine-for-one basis. The Recapitalization lacked economic substance as the newly-issued shares have the same rights and privileges as the previously outstanding capital stock of Nina, Pinta and Santa Maria and there was no change in ownership percentages of the individual stockholders. As a result of the Recapitalization, Nina, Pinta and Santa Maria became wholly owned subsidiaries of Atara effective March 31, 2014. The Recapitalization is considered a tax-free exchange for US federal income tax purposes.
	Because the four individual companies were under common ownership and the Recapitalization lacked economic substance, we accounted for the Recapitalization as a combination of businesses under common control. The assets and liabilities of Nina, Pinta and Santa Maria were recorded by Atara at their historical carrying amounts on March 31, 2014 and beginning March 31, 2014, the financial statements of the Company are presented on a consolidated basis.
	Liquidity
	We have incurred significant operating losses since inception and have relied on public and private equity financings to fund our operations. At March 31, 2015, we had an accumulated deficit of $50.0 million. As we continue to incur losses, our transition to profitability will depend on the successful development, approval and commercialization of product candidates and on the achievement of sufficient revenues to support our cost structure. We may never achieve profitability, and unless and until we do, we will need to continue to raise additional capital. Management expects that existing cash and cash equivalents as of March 31, 2015 will be sufficient to fund our current operating plan for at least the next twelve months.
	Net Loss per Common Share
	Basic and diluted net loss per common share is presented, giving effect to the Recapitalization, including cancellation of existing Atara common stock and a nine-for-one share exchange. Basic net loss per common share is calculated by dividing the net loss by the weighted-average number of shares of common stock outstanding during the period, without consideration of common stock equivalents. Diluted net loss per common share is computed by dividing the net loss by the weighted-average number of shares of common stock and common share equivalents outstanding for the period. Common share equivalents are only included in the calculation of diluted net loss per common share when their effect is dilutive. Our convertible preferred stock and restricted stock awards are considered to be participating securities as they are entitled to participate in undistributed earnings with shares of common stock. Due to net losses, there is no impact on the net loss per common share calculation in applying the two-class method since the participating securities have no legal requirement to share in any losses.
	Potential dilutive securities, which include convertible preferred stock, unvested restricted common stock awards, unvested restricted stock units and vested and unvested options have been excluded from the computation of diluted net loss per share as the effect would be to reduce the net loss per common share and be 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 shares of potentially dilutive securities give effect to the Recapitalization, and have been excluded from the computations of diluted net loss per common share as the effect of including such securities would be antidilutive:
		Three months
		ended March 31,
		2015		2014
	Convertible preferred stock		—		12,147,786
	Unvested restricted common stock		487,836		774,374
	Unvested restricted stock units		632,838		—
	Vested and unvested options		340,444		—
			1,461,118		12,922,160
	In addition, 72,567 options have been excluded from the above table as the exercise prices of the underlying options were greater than the average fair value of our common stock for the periods presented.
	Recent Accounting Pronouncements
	In August 2014, the Financial Accounting Standards Board (the “FASB”) issued a new accounting standard to provide guidance on the presentation of management’s plans, when conditions or events raise substantial doubt about the entity’s ability to continue as a going concern within one year after the date that the financial statements are issued.  The new standard is effective for fiscal years ending after December 15, 2016.  The adoption of this standard is not expected to have a material impact on our financial statements.
	In May 2014, the FASB issued a new accounting standard, Revenue from Contracts with Customers (Topic 606), which supersedes the revenue recognition requirements in the current standard, Revenue Recognition.  The new standard is based on the principle that revenue is recognized to depict the transfer of goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services.  It also requires additional disclosure about the nature, amount, timing and uncertainty of revenue and cash flows arising from customer contracts, including significant judgments and changes in judgments and assets recognized from costs incurred to obtain or fulfill a contract.  In April 2015, the FASB voted to propose to defer the effective date of this standard by one year to December 2017.  We will evaluate the application of this standard on our financial statements and disclosures when the standard becomes effective.

Fair_Value_of_Financial_Instru

Fair Value of Financial Instruments	3 Months Ended
	Mar. 31, 2015
Fair Value Disclosures [Abstract]
Fair Value of Financial Instruments	3.Fair Value of Financial Instruments
	Our financial assets and liabilities carried at fair value are primarily comprised of investments in money market funds, corporate bonds, U.S. government securities, asset-backed securities and commercial paper. The fair value accounting guidance requires that assets and liabilities be carried at fair value and classified in one of the following three categories:
	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 were no transfers between Level 1, Level 2, and Level 3 for all periods presented.
	The following table represents the fair value hierarchy for our financial assets and financial liabilities measured at fair value on a recurring basis:
						Quoted			Significant
						Prices in			Other
			Total			Active Markets			Observable Inputs
			Fair Value			(Level 1)			(Level 2)
			(in thousands)
	At March 31, 2015:
	Cash equivalents:
	Money market funds		$	71,329		$	71,329		$	—
	Short-term available-for-sale investments:
	Corporate bonds		$	62,067		$	—		$	62,067
	Agency bonds			17,149			—			17,149
	Treasury bonds			466			—			466
	Asset-backed securities			15,685			—			15,685
	Total short-term available-for-sale investments		$	95,367		$	—		$	95,367
	At December 31, 2014:
	Cash equivalents:
	Money market funds		$	18,141		$	18,141		$	—
	Agency bonds			1,750			—			1,750
	Corporate bonds			2,006			—			2,006
	Total cash equivalents		$	21,897		$	18,141		$	3,756
	Short-term available-for-sale investments:
	Corporate bonds		$	57,958		$	—		$	57,958
	Agency bonds			10,764			—			10,764
	Treasury bonds			465			—			465
	Commercial paper			1,200			—			1,200
	Asset-backed securities			11,832			—			11,832
	Total short-term available-for-sale investments		$	82,219		$	—		$	82,219
	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. Corporate bonds, U.S. government securities, asset-backed securities and commercial paper 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 and liabilities.
	Available-for-sale investments are carried at fair value and are included in the tables above under short-term investments. The aggregate market value, cost basis, and gross unrealized gains and losses of available-for-sale investments by major security type are as follows:
		Total			Total			Total
		Amortized			Unrealized			Unrealized			Total
		Cost			Gain			Loss			Fair Value
		(in thousands)
	At March 31, 2015:
	Corporate bonds	$	62,092		$	11		$	(36	)	$	62,067
	Agency bonds		17,143			7			(1	)		17,149
	Treasury bonds		466			—			—			466
	Asset-backed securities		15,684			4			(3	)		15,685
	Total short-term available-for-sale investments	$	95,385		$	22		$	(40	)	$	95,367
	At December 31, 2014:
	Corporate bonds	$	58,046		$	1		$	(89	)	$	57,958
	Agency bonds		10,769			—			(5	)		10,764
	Treasury bonds		466			—			(1	)		465
	Commercial paper		1,200			—			—			1,200
	Asset-backed securities		11,838			2			(8	)		11,832
	Total short-term available-for-sale investments	$	82,319		$	3		$	(103	)	$	82,219
	The amortized cost and fair value of available-for-sale investments, by contractual maturity, were as follows:
			Total
			Amortized			Total
			Cost			Fair Value
			(in thousands)
	At March 31, 2015:
	Maturing within one year		$	54,252		$	54,232
	Maturing in one to five years			41,133			41,135
	Total short-term available-for-sale investments		$	95,385		$	95,367
	At December 31, 2014:
Maturing within one year		$	56,752		$	56,714
Maturing in one to five years			25,567			25,505
Total short-term available-for-sale investments		$	82,319		$	82,219

Significant_Agreements

Significant Agreements	3 Months Ended
	Mar. 31, 2015
Related Party Transactions [Abstract]
Significant Agreements	4	Significant Agreements
	Related Party License Agreements - In September 2012, we entered into three license agreements with Amgen, one of our investors, for the development, manufacturing, use and distribution of products using certain proprietary compounds. Under the terms of these agreements, we paid $250,000 and issued 5,538,462 shares of Series A-1 convertible preferred stock (615,384 shares after giving effect to the Recapitalization) to Amgen. As described further in Note 5, we may also be required to make additional payments to Amgen based upon the achievement of specified development, regulatory, and commercial milestones, as well as mid-single-digit percentage royalties on future sales of products resulting from development of these purchased technologies, if any. These agreements expire at the end of all royalty obligations to Amgen and, upon expiration, the licenses will be fully paid, royalty-free, irrevocable and non-exclusive.
	At March 31, 2015, Amgen owns 6.0% of our outstanding voting capital stock. Amgen does not have any rights to participate in our product candidates’ development and is not represented on our boards of directors.
	Exclusive Option Agreement – In September 2014, we entered into an exclusive option agreement with Memorial Sloan Kettering Cancer Center (“MSK”) under which we have the right to acquire the exclusive worldwide license rights to the three clinical stage T-cell therapies of MSK.  The initial option period is for twelve months, with extensions available to extend the term up to 27 months at the option of Atara.  Under the terms of the option agreement, we are obligated to use reasonable efforts to prepare a request to be submitted to the US Food and Drug Administration (the “FDA”) regarding a meeting to discuss pivotal trials for one of the clinical stage T-cell therapies.  In exchange for the exclusive option, we paid MSK $1.25 million in cash and issued 59,761 shares of our common stock to MSK.  At the time of issuance, we estimated the fair value of the common stock issued to MSK to be $750,000.  This total of $2.0 million was recorded as research and development expense in our condensed consolidated and combined statement of operations and comprehensive loss in the third quarter of 2014.  We will be obligated to pay MSK an additional amount up to $630,000 if we extend the option period.
	If we exercise the option and enter into the license agreement with MSK, we will be obligated under the license agreement to pay to MSK an upfront cash payment of $4.5 million and additional payments of up to $33.0 million based on a license fee and achievement of specified development, regulatory and sales-related milestones, and to make mid-single-digit percentage royalty payments based on sales of the T-cell therapy products.

Commitments_and_Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 2015
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	5	Commitments and Contingencies
	Operating Leases
	Rent expense for the three months ended March 31, 2015 and 2014 was $81,220 and $14,640, respectively.
	Related Party License Agreements
	Under the terms of our license agreements with Amgen, we are obligated to make additional milestone payments to Amgen of up to $86.0 million upon the achievement of certain development and regulatory approval milestones. Of these milestone payments, $14.0 million relate to milestones for clinical trials. The remaining $72.0 million relate to milestones for regulatory approvals in various territories and are anticipated to be made no earlier than 2017. Thereafter, we are obligated to make tiered payments based on achievement of commercial milestones based upon net sales levels. The maximum payments would be $206.0 million based on sales of over $1 billion for each of three products in a calendar year. We are also obligated to pay mid-single-digit percentage tiered royalties on future net sales of products which are developed and approved as defined by the agreements. Our royalty obligations as to a particular licensed product will be payable, on a country-by-country and product-by-product basis, until the later of (a) the date of expiration of the last to expire valid claim within the licensed patents that covers the manufacture, use or sale, offer to sell, or import of such licensed product by us or a sublicense in such country, (b) loss of regulatory exclusivity or (c) 10 years after the first commercial sale of the applicable licensed product in the applicable country. As of March 31, 2015 and December 31, 2014, there were no outstanding obligations due to Amgen.
	In accordance with terms of the agreements, we use commercially reasonable efforts to pay costs related to the preparation, filing, prosecution, defense and maintenance of the patents covered by the license agreements. During the three months ended March 31, 2015 and 2014, we incurred expenses of $508,919 and $218,072, respectively, related to the preparation, filing and maintenance of patents.
	Indemnification Agreements
	In the normal course of business, we enter into contracts and agreements that contain a variety of representations and warranties and provide for indemnification for certain liabilities. The exposure under these agreements is unknown because it involves claims that may be made against us in the future but have not yet been made. To date, we have not paid any claims or been required to defend any action related to our indemnification obligations. However, we may record charges in the future as a result of these indemnification obligations. We also have indemnification obligations to our directors and executive officers for specified events or occurrences, subject to some limits, while they are serving at our request in such capacities. There have been no claims to date and we believe the fair value of these indemnification agreements is minimal. Accordingly, we have not recorded any liabilities for these agreements as of March 31, 2015 and December 31, 2014.

Stockholders_Equity

Stockholders' Equity	3 Months Ended
	Mar. 31, 2015
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stockholders’ Equity	6	Stockholders’ Equity
	Restricted Common Stock
	In August 2012, in connection with our formation, our CEO purchased 9,595,384 shares of restricted common stock at a nominal per share purchase price. The shares were issued subject to certain vesting conditions, restrictions on transfer and a Company right of repurchase of any unvested share at their original purchase price. These shares are placed in escrow until vested, and have rights to vote and participate in dividends and distributions. The combined grant date intrinsic value for this award was $1,704,094 and 7,996,153 of these shares had service and fundraising vesting conditions. Under the service vesting condition, shares vest monthly over 48 months, commencing from the first closing of Series A convertible preferred stock financing on October 22, 2012. 1,599,231 of these shares are subject to performance milestones and fundraising vesting conditions. The fundraising vesting conditions for all shares were satisfied as of December 31, 2013. All shares subject to service vesting conditions are subject to accelerated vesting in the event of certain change of control transactions.
	In March 2013, an Atara employee purchased 2,423,074 shares of restricted common stock for $331,170. The shares were issued under our 2012 Equity Incentive Plan (as discussed below) and are subject to certain vesting conditions, restrictions on transfer and a Company right of repurchase of any unvested shares at their original purchase price. These shares are placed in escrow until vested, and have rights to vote and participate in dividends and distributions. Under these agreements, the shares vest as follows: 2,319,228 shares vest over four years, with one-quarter vesting after one year of service and the remainder vesting in equal installments over the subsequent thirty-six months, and 103,846 shares vest upon achievement of certain performance milestones. Vesting of all shares is subject to acceleration of vesting in the event of certain change of control transactions.
	The restricted common stock was purchased with secured promissory notes totaling $331,170.
	The amounts paid for both restricted stock purchases were initially recorded as other long-term liabilities. As shares vest, we reclassify liabilities to equity and report shares as outstanding in the condensed consolidated and combined financial statements. On March 31, 2014, the shares were exchanged for 1,335,384 shares of Atara common stock.  At March 31, 2015, 887,067 shares had vested and are classified as equity. Restricted stock shares not vested at March 31, 2015 totaled 448,317 shares and are expected to vest over two years.
	As both the Chief Executive Officer and the Atara employee were consultants of Nina, Pinta and Santa Maria through the Recapitalization date, we accounted for these awards as non-employee stock-based awards. Following the Recapitalization, these awards were accounted as employee awards based upon the fair market value of common stock on March 31, 2014. Stock-based compensation expense related to these awards is recorded using an accelerated graded vesting model and was $323,226 and $3.3 million for the three months ended March 31, 2015 and 2014, respectively.  The unrecognized stock-based compensation expense related to this unvested restricted stock was $860,566 at March 31, 2015 and this expense is expected to be recognized over the remaining service periods through 2016.  The aggregate intrinsic value of unvested restricted stock is $18.6 million at March 31, 2015.
	2014 Equity Incentive Plans
	In March 2014, we adopted the 2014 Equity Incentive Plan (the “2014 plan”) as part of our Recapitalization. In connection with the Recapitalization, Atara assumed the plans of Nina, Pinta and Santa Maria and all outstanding restricted stock units (“RSUs”) and restricted stock awards granted under such plans.  At the date of Recapitalization, RSUs and restricted stock awards issued by Nina, Pinta and Santa Maria to Atara employees became employee awards and the awards’ grant dates were established as the Recapitalization date.  In May 2014, our board of directors amended and restated our 2014 plan and the amended plan became effective on October 15, 2014 upon the pricing of our initial public offering.  The maximum number of shares of our common stock that may be issued pursuant to stock awards under the 2014 plan is 4,536,797 shares, including 1,294,041 shares that were previously available for issuance under the 2012 plans.
	The number of shares of our common stock reserved for issuance pursuant to stock awards under our 2014 plan will automatically increase on January 1 of each year for a period of up to ten years, beginning on January 1, 2015 and ending on and including January 1, 2024, by 5% of the number of shares of our capital stock outstanding on December 31 of the preceding calendar year, or a lesser number of shares determined by our board of directors.  The number of shares of our common stock available for issuance under the 2014 plan is 2,046,541 at March 31, 2015.
	Under the terms of the 2014 plan, we may grant options, restricted stock awards and RSUs to employees, directors, consultants and other service providers. Employees typically receive an award upon commencement of employment and members of our board of directors receive an award in connection with their appointment. Generally, if any shares subject to an award expire, or are forfeited, terminated or cancelled without the issuance of shares, the shares are added back into the total shares available for issuance under the 2014 plan.
	RSUs typically expire at the earlier of seven years from the date of grant or the service termination (or, for RSUs granted prior to February 2014, two years following the service termination date). Stock options are granted at prices no less than 100% of the estimated fair value of the shares on the date of grant as determined by the board of directors, provided, however, that the exercise price of an option granted to a 10% shareholder cannot be less than 110% of the estimated fair value of the shares on the date of grant. Options granted to employees and non-employees generally vest over four years and expire in seven years.
	Restricted Stock Units and Awards
	The RSUs granted prior to our initial public offering had a time-based service condition and a liquidity-based performance condition, and vest when both conditions are met. We determined that the liquidity-based performance condition was not probable of occurring and recorded no stock-based compensation expense related to the RSUs prior to our initial public offering.  Upon the closing of our initial public offering in October 2014, we recorded $3.8 million of stock-based compensation expense in our consolidated and combined statement of operations for the quarter ended December 31, 2014.  The remaining unrecognized stock-based compensation expense relating to nonvested RSUs will be recognized as the RSUs vest over the remaining service periods through 2018. As of March 31, 2015, there was $3.9 million of unrecognized stock-based compensation expense related to RSUs that is expected to be recognized over a weighted average period of 1.42 years. The aggregate intrinsic value of the RSUs outstanding at March 31, 2015 was $37.7 million.
	The following is a summary of RSU activity, including the restricted stock award discussed above, under our 2014 plan:
			Restricted Stock Awards						RSUs
			Shares				Weighted Average Grant Date Fair Value		Shares				Weighted Average Grant Date Fair Value
	Unvested at December 31, 2014			112,740			$	0.4		619,303			$	4.64
	Granted			—				—		87,600			$	25.15
	Forfeited			—				—		—				—
	Vested			(16,106	)		$	0.4		(113,713	)		$	5.66
	Unvested at March 31, 2015			96,634			$	0.4		593,190			$	7.47
	Stock Options
	The following is a summary of option activity under our 2014 plan:
			Number of shares				Weighted Average Exercise Price		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
	Outstanding at December 31, 2014			623,936			$	13.69
	Granted (weighted-average grant date fair value of $14.07 per share)			690,699			$	24.96
	Balance at March 31, 2015			1,314,635			$	19.61	6.67		$	28,866,786
	Stock options vested and expected to vest at March 31, 2015			1,314,635			$	19.61	6.67		$	28,866,786
	Exercisable at March 31, 2015			56,175			$	16.1	6.63		$	1,430,908
	Aggregate intrinsic value represents the difference between the closing stock price of our common stock on March 31, 2015 and the exercise price of outstanding, in-the-money options. As of March 31, 2015, there was $13.6 million of unrecognized stock-based compensation expense related to stock options that is expected to be recognized over a weighted average period of 3.43 years. No options were exercised in the first quarter of 2015.
	The fair value of each option issued during 2015 was estimated at the date of grant using the Black-Scholes valuation model with the following weighted-average assumptions:
			Three Months Ended March 31, 2015
			Employees				Non-Employees
	Risk-free interest rate		1.3% - 1.6%				1.60%
	Expected life of options in years		4.5				6.9
	Expected volatility of underlying stock			71.10%				70.10%
	Expected dividend yield			0.00%				0.00%
	Stock-based Compensation Expense
	Total stock-based compensation expense related to all employee and non-employee awards was as follows (in thousands):
		Three months
		ended March 31,
		2015				2014
	Research and development	$	1,288			$	705
	General and administrative		1,195				2,612
		$	2,483			$	3,317

Summary_of_Significant_Account1

Summary of Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2015
Accounting Policies [Abstract]
Basis of Presentation and Recapitalization	Basis of Presentation and Recapitalization
	The accompanying interim condensed consolidated and combined financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”) and the rules and regulations of the Securities and Exchange Commission (the “SEC”). The accounting policies followed in the preparation of the interim condensed consolidated and combined financial statements are consistent in all material respects with those presented in Note 2 to the consolidated and combined financial statements included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2014.
	Atara was originally formed as a management company with the sole purpose of providing management, financial and administrative services for Nina Biotherapeutics, Inc. (“Nina”), Santa Maria Biotherapeutics, Inc. (“Santa Maria”) and Pinta Biotherapeutics, Inc. (“Pinta”).  Prior to March 31, 2014, the accompanying financial statements include the operations of Atara, Nina, Pinta and Santa Maria on a combined basis as the four individual companies were under common ownership and common management since inception. All intercompany transactions have been eliminated.
	On March 31, 2014, our boards of directors approved and we implemented a recapitalization (the “Recapitalization”) in which (a) all the outstanding shares of common stock of Atara were cancelled and forfeited by existing stockholders and (b) the stockholders of Nina, Pinta and Santa Maria exchanged their existing common and convertible preferred stock for newly-issued shares of Atara, with the same rights and privileges as the outstanding capital stock of Nina, Pinta and Santa Maria. The shares were exchanged on a collective nine-for-one basis. The Recapitalization lacked economic substance as the newly-issued shares have the same rights and privileges as the previously outstanding capital stock of Nina, Pinta and Santa Maria and there was no change in ownership percentages of the individual stockholders. As a result of the Recapitalization, Nina, Pinta and Santa Maria became wholly owned subsidiaries of Atara effective March 31, 2014. The Recapitalization is considered a tax-free exchange for US federal income tax purposes.
	Because the four individual companies were under common ownership and the Recapitalization lacked economic substance, we accounted for the Recapitalization as a combination of businesses under common control. The assets and liabilities of Nina, Pinta and Santa Maria were recorded by Atara at their historical carrying amounts on March 31, 2014 and beginning March 31, 2014, the financial statements of the Company are presented on a consolidated basis.
Liquidity	Liquidity
	We have incurred significant operating losses since inception and have relied on public and private equity financings to fund our operations. At March 31, 2015, we had an accumulated deficit of $50.0 million. As we continue to incur losses, our transition to profitability will depend on the successful development, approval and commercialization of product candidates and on the achievement of sufficient revenues to support our cost structure. We may never achieve profitability, and unless and until we do, we will need to continue to raise additional capital. Management expects that existing cash and cash equivalents as of March 31, 2015 will be sufficient to fund our current operating plan for at least the next twelve months.
Net Loss per Common Share	Net Loss per Common Share
	Basic and diluted net loss per common share is presented, giving effect to the Recapitalization, including cancellation of existing Atara common stock and a nine-for-one share exchange. Basic net loss per common share is calculated by dividing the net loss by the weighted-average number of shares of common stock outstanding during the period, without consideration of common stock equivalents. Diluted net loss per common share is computed by dividing the net loss by the weighted-average number of shares of common stock and common share equivalents outstanding for the period. Common share equivalents are only included in the calculation of diluted net loss per common share when their effect is dilutive. Our convertible preferred stock and restricted stock awards are considered to be participating securities as they are entitled to participate in undistributed earnings with shares of common stock. Due to net losses, there is no impact on the net loss per common share calculation in applying the two-class method since the participating securities have no legal requirement to share in any losses.
	Potential dilutive securities, which include convertible preferred stock, unvested restricted common stock awards, unvested restricted stock units and vested and unvested options have been excluded from the computation of diluted net loss per share as the effect would be to reduce the net loss per common share and be 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 shares of potentially dilutive securities give effect to the Recapitalization, and have been excluded from the computations of diluted net loss per common share as the effect of including such securities would be antidilutive:
		Three months
		ended March 31,
		2015		2014
	Convertible preferred stock		—		12,147,786
	Unvested restricted common stock		487,836		774,374
	Unvested restricted stock units		632,838		—
	Vested and unvested options		340,444		—
			1,461,118		12,922,160
	In addition, 72,567 options have been excluded from the above table as the exercise prices of the underlying options were greater than the average fair value of our common stock for the periods presented.
Recent Accounting Pronouncements	Recent Accounting Pronouncements
	In August 2014, the Financial Accounting Standards Board (the “FASB”) issued a new accounting standard to provide guidance on the presentation of management’s plans, when conditions or events raise substantial doubt about the entity’s ability to continue as a going concern within one year after the date that the financial statements are issued.  The new standard is effective for fiscal years ending after December 15, 2016.  The adoption of this standard is not expected to have a material impact on our financial statements.
	In May 2014, the FASB issued a new accounting standard, Revenue from Contracts with Customers (Topic 606), which supersedes the revenue recognition requirements in the current standard, Revenue Recognition.  The new standard is based on the principle that revenue is recognized to depict the transfer of goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services.  It also requires additional disclosure about the nature, amount, timing and uncertainty of revenue and cash flows arising from customer contracts, including significant judgments and changes in judgments and assets recognized from costs incurred to obtain or fulfill a contract.  In April 2015, the FASB voted to propose to defer the effective date of this standard by one year to December 2017.  We will evaluate the application of this standard on our financial statements and disclosures when the standard becomes effective.

Summary_of_Significant_Account2

Summary of Significant Accounting Policies (Tables)	3 Months Ended
	Mar. 31, 2015
Accounting Policies [Abstract]
Antidilutive Securities Excluded From Computation of Diluted Net Loss per Common Share	The following shares of potentially dilutive securities give effect to the Recapitalization, and have been excluded from the computations of diluted net loss per common share as the effect of including such securities would be antidilutive:
		Three months
		ended March 31,
		2015		2014
	Convertible preferred stock		—		12,147,786
	Unvested restricted common stock		487,836		774,374
	Unvested restricted stock units		632,838		—
	Vested and unvested options		340,444		—
			1,461,118		12,922,160

Fair_Value_of_Financial_Instru1

Fair Value of Financial Instruments (Tables)	3 Months Ended
	Mar. 31, 2015
Fair Value Disclosures [Abstract]
Fair Value Hierarchy for Financial Assets and Financial Liabilities Measured at Fair Value on a Recurring Basis	The following table represents the fair value hierarchy for our financial assets and financial liabilities measured at fair value on a recurring basis:
						Quoted			Significant
						Prices in			Other
			Total			Active Markets			Observable Inputs
			Fair Value			(Level 1)			(Level 2)
			(in thousands)
	At March 31, 2015:
	Cash equivalents:
	Money market funds		$	71,329		$	71,329		$	—
	Short-term available-for-sale investments:
	Corporate bonds		$	62,067		$	—		$	62,067
	Agency bonds			17,149			—			17,149
	Treasury bonds			466			—			466
	Asset-backed securities			15,685			—			15,685
	Total short-term available-for-sale investments		$	95,367		$	—		$	95,367
	At December 31, 2014:
	Cash equivalents:
	Money market funds		$	18,141		$	18,141		$	—
	Agency bonds			1,750			—			1,750
	Corporate bonds			2,006			—			2,006
	Total cash equivalents		$	21,897		$	18,141		$	3,756
	Short-term available-for-sale investments:
	Corporate bonds		$	57,958		$	—		$	57,958
	Agency bonds			10,764			—			10,764
	Treasury bonds			465			—			465
	Commercial paper			1,200			—			1,200
	Asset-backed securities			11,832			—			11,832
	Total short-term available-for-sale investments		$	82,219		$	—		$	82,219
Available for Sale Investments Carried at Fair Value Included Short Term Investments	Available-for-sale investments are carried at fair value and are included in the tables above under short-term investments. The aggregate market value, cost basis, and gross unrealized gains and losses of available-for-sale investments by major security type are as follows:
		Total			Total			Total
		Amortized			Unrealized			Unrealized			Total
		Cost			Gain			Loss			Fair Value
		(in thousands)
	At March 31, 2015:
	Corporate bonds	$	62,092		$	11		$	(36	)	$	62,067
	Agency bonds		17,143			7			(1	)		17,149
	Treasury bonds		466			—			—			466
	Asset-backed securities		15,684			4			(3	)		15,685
	Total short-term available-for-sale investments	$	95,385		$	22		$	(40	)	$	95,367
	At December 31, 2014:
	Corporate bonds	$	58,046		$	1		$	(89	)	$	57,958
	Agency bonds		10,769			—			(5	)		10,764
	Treasury bonds		466			—			(1	)		465
	Commercial paper		1,200			—			—			1,200
	Asset-backed securities		11,838			2			(8	)		11,832
	Total short-term available-for-sale investments	$	82,319		$	3		$	(103	)	$	82,219
Amortized Cost and Fair Value of Available for Sale Investments by Contractual Maturity	The amortized cost and fair value of available-for-sale investments, by contractual maturity, were as follows:
			Total
			Amortized			Total
			Cost			Fair Value
			(in thousands)
	At March 31, 2015:
	Maturing within one year		$	54,252		$	54,232
	Maturing in one to five years			41,133			41,135
	Total short-term available-for-sale investments		$	95,385		$	95,367
	At December 31, 2014:
	Maturing within one year		$	56,752		$	56,714
	Maturing in one to five years			25,567			25,505
	Total short-term available-for-sale investments		$	82,319		$	82,219

Stockholders_Equity_Tables

Stockholders' Equity (Tables)	3 Months Ended
	Mar. 31, 2015
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Summary of RSU Activity and Restricted Stock Award	The following is a summary of RSU activity, including the restricted stock award discussed above, under our 2014 plan:
			Restricted Stock Awards						RSUs
			Shares				Weighted Average Grant Date Fair Value		Shares				Weighted Average Grant Date Fair Value
	Unvested at December 31, 2014			112,740			$	0.4		619,303			$	4.64
	Granted			—				—		87,600			$	25.15
	Forfeited			—				—		—				—
	Vested			(16,106	)		$	0.4		(113,713	)		$	5.66
	Unvested at March 31, 2015			96,634			$	0.4		593,190			$	7.47
Summary of Stock Option Activity	The following is a summary of option activity under our 2014 plan:
			Number of shares				Weighted Average Exercise Price		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
	Outstanding at December 31, 2014			623,936			$	13.69
	Granted (weighted-average grant date fair value of $14.07 per share)			690,699			$	24.96
	Balance at March 31, 2015			1,314,635			$	19.61	6.67		$	28,866,786
	Stock options vested and expected to vest at March 31, 2015			1,314,635			$	19.61	6.67		$	28,866,786
	Exercisable at March 31, 2015			56,175			$	16.1	6.63		$	1,430,908
Summary of Options Estimated with Weighted Average	The fair value of each option issued during 2015 was estimated at the date of grant using the Black-Scholes valuation model with the following weighted-average assumptions:
			Three Months Ended March 31, 2015
			Employees				Non-Employees
	Risk-free interest rate		1.3% - 1.6%				1.60%
	Expected life of options in years		4.5				6.9
	Expected volatility of underlying stock			71.10%				70.10%
	Expected dividend yield			0.00%				0.00%
Schedule of Stock-based Compensation, Nonemployee Director Stock Award Plan, Activity	Total stock-based compensation expense related to all employee and non-employee awards was as follows (in thousands):
		Three months
		ended March 31,
		2015				2014
	Research and development	$	1,288			$	705
	General and administrative		1,195				2,612
		$	2,483			$	3,317

Organization_and_Description_o1

Organization and Description of Business - Additional Information (Detail) (USD $)	3 Months Ended	1 Months Ended
	Mar. 31, 2015	Feb. 28, 2015	Oct. 31, 2014	Dec. 31, 2014
Organization And Description Of Business [Line Items]
Entity incorporation state	Delaware
Entity incorporation date	1-Aug-12
Proceeds from sale of common stock	$69,487,000
Additional paid-in capital	216,159,000			144,169,000
Follow-on Offering
Organization And Description Of Business [Line Items]
Common stock, shares issued		4,147,358
Shares issued, price per share		$18
Proceeds from sale of common stock		69,500,000
IPO
Organization And Description Of Business [Line Items]
Common stock, shares issued			5,750,000
Shares issued, price per share			$11
Proceeds from sale of common stock			55,800,000
Underwriters option to purchase common stock			750,000
Preferred stock conversion into common stock			12,298,515
Additional paid-in capital			$74,600,000

Summary_of_Significant_Account3

Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)	3 Months Ended
In Thousands, except Share data, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Significant Accounting Policies [Line Items]
Stock conversion ratio	Nine-for-one basis
Accumulated deficit	$50,049	$40,889
Employee Stock Option
Significant Accounting Policies [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount	72,567

Summary_of_Significant_Account4

Summary of Significant Accounting Policies - Antidilutive Securities Excluded From Computation of Diluted Net Loss per Common Share (Detail)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount	1,461,118	12,922,160
Convertible Preferred Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount		12,147,786
Unvested Restricted Common Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount	487,836	774,374
Unvested Restricted Stock Units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount	632,838
Vested and Unvested Options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount	340,444

Fair_Value_of_Financial_Instru2

Fair Value of Financial Instruments - Fair Value Hierarchy for Financial Assets and Financial Liabilities Measured at Fair Value on a Recurring Basis (Detail) (Fair Value, Measurements, Recurring, USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		$21,897
Short-term available-for-sale investments	95,367	82,219
Quoted Prices in Active Markets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		18,141
Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		3,756
Short-term available-for-sale investments	95,367	82,219
Money Market Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	71,329	18,141
Money Market Funds | Quoted Prices in Active Markets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	71,329	18,141
Agency Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		1,750
Short-term available-for-sale investments	17,149	10,764
Agency Bonds | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		1,750
Short-term available-for-sale investments	17,149	10,764
Corporate Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		2,006
Short-term available-for-sale investments	62,067	57,958
Corporate Bonds | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		2,006
Short-term available-for-sale investments	62,067	57,958
Treasury Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term available-for-sale investments	466	465
Treasury Bonds | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term available-for-sale investments	466	465
Commercial Paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term available-for-sale investments		1,200
Commercial Paper | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term available-for-sale investments		1,200
Asset-Backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term available-for-sale investments	15,685	11,832
Asset-Backed Securities | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term available-for-sale investments	$15,685	$11,832

Fair_Value_of_Financial_Instru3

Fair Value of Financial Instruments - Available for Sale Investments Carried at Fair Value Included Short Term Investments (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	$95,385	$82,319
Total Unrealized Gain	22	3
Total Unrealized Loss	-40	-103
Total Fair Value	95,367	82,219
Corporate Bonds
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	62,092	58,046
Total Unrealized Gain	11	1
Total Unrealized Loss	-36	-89
Total Fair Value	62,067	57,958
Agency Bonds
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	17,143	10,769
Total Unrealized Gain	7
Total Unrealized Loss	-1	-5
Total Fair Value	17,149	10,764
Treasury Bonds
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	466	466
Total Unrealized Loss		-1
Total Fair Value	466	465
Commercial Paper
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost		1,200
Total Fair Value		1,200
Asset-Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	15,684	11,838
Total Unrealized Gain	4	2
Total Unrealized Loss	-3	-8
Total Fair Value	$15,685	$11,832

Fair_Value_of_Financial_Instru4

Fair Value of Financial Instruments - Amortized Cost and Fair Value of Available for Sale Investments by Contractual Maturity (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified
Amortized cost
Maturing within one year, Amortized cost	$54,252	$56,752
Maturing in one to five years, Amortized cost	41,133	25,567
Total short-term available-for-sale investments, Amortized cost	95,385	82,319
Fair value
Maturing within one year, Fair value	54,232	56,714
Maturing in one to five years, Fair value	41,135	25,505
Total short-term available-for-sale investments, Fair value	$95,367	$82,219

Significant_Agreements_Additio

Significant Agreements - Additional Information (Detail) (USD $)	0 Months Ended	3 Months Ended
	Sep. 30, 2012	Mar. 31, 2015	Sep. 30, 2014
Amgen
Related Party Transaction [Line Items]
Payments under license agreement	$250,000
Percentage of common stock		6.00%
Amgen | Series A One Convertible Preferred Stock | Prior to Recapitalization
Related Party Transaction [Line Items]
Convertible preferred stock, shares issued upon conversion	5,538,462
Amgen | Series A One Convertible Preferred Stock | After Recapitalization
Related Party Transaction [Line Items]
Convertible preferred stock, shares issued upon conversion	615,384
Memorial Sloan Kettering Cancer Center
Related Party Transaction [Line Items]
Payments under license agreement		4,500,000
Payments under exclusive option license agreement		1,250,000
Common stock, Shares, Issued		59,761
Fair value of the common stock issued		750,000
Research and development expense			2,000,000
Cost incurred to extend option		630,000
Estimated future payment for license fee		$33,000,000

Commitments_and_Contingencies_

Commitments and Contingencies - Additional Information (Detail) (USD $)	3 Months Ended		0 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Sep. 30, 2012	Dec. 31, 2014
Commitments And Contingencies [Line Items]
Rental expense	$81,220	$14,640
License Agreement With Amgen
Commitments And Contingencies [Line Items]
Milestone payment payable on achievement of development and regulatory approvals	86,000,000
Sales revenue	1,000,000,000
Payments under license agreement	508,919	218,072
License Agreement With Amgen | Maximum
Commitments And Contingencies [Line Items]
Milestone payment payable on achievement of development and regulatory approvals	206,000,000
License Agreement With Amgen | Milestone For Clinical Trials
Commitments And Contingencies [Line Items]
Milestone payment payable on achievement of development and regulatory approvals	14,000,000
License Agreement With Amgen | Favorable Regulatory Action
Commitments And Contingencies [Line Items]
Milestone payment payable on achievement of development and regulatory approvals	72,000,000
Amgen
Commitments And Contingencies [Line Items]
Payments under license agreement			250,000
Amgen | License Agreements
Commitments And Contingencies [Line Items]
Contractual obligations due	$0			$0

Stockholders_Equity_Additional

Stockholders' Equity - Additional Information (Detail) (USD $)	3 Months Ended		1 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2013	Oct. 31, 2014	Aug. 31, 2012	Dec. 31, 2014
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, shares issued	23,911,930					19,692,937
Proceeds from sale of common stock, net of offering costs	$69,487,000
Aggregate intrinsic value	28,866,786
Estimated fair value of the shares on the date of grant	100.00%
Stock option granted description terms	the exercise price of an option granted to a 10% shareholder cannot be less than 110% of the estimated fair value of the shares on the date of grant. Options granted to employees and non-employees generally vest over four years and expire in seven years.
Stock-based compensation expense	2,483,000	3,317,000
Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Estimated fair value of the shares on the date of grant	10.00%
Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Estimated fair value of the shares on the date of grant	110.00%
Secured Promissory Notes
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Debt instrument face amount	331,170
2012 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, reserved for issuance	1,294,041
2014 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, shares issued	4,536,797
Common stock, reserved for issuance	2,046,541
Share based compensation award expiration period	10 years
Capital stock reserved for issuance, percentage of number of shares of capital stock outstanding	5.00%
Unvested Restricted Common Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Combined grant date intrinsic value for award					1,704,094
Share based compensation, vesting period	2 years
Vested restricted stock awards	16,106
Unvested restricted stock awards	96,634					112,740
Performance-based awards, expenses recognized	323,226	3,300,000
Unrecognized stock-based compensation expense	860,566
Aggregate intrinsic value	18,600,000
Unvested Restricted Common Stock | Prior to Recapitalization
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares exchanged		1,335,384
Vested restricted stock awards	887,067
Unvested restricted stock awards	448,317
Unvested Restricted Common Stock | 2012 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Proceeds from sale of common stock, net of offering costs			331,170
Share-based compensation, number of shares expected to vest			2,319,228
Employees And Non Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share based compensation, vesting period	4 years		4 years
Share based compensation award expiration period	7 years
Unvested Restricted Stock Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vested restricted stock awards	113,713
Unvested restricted stock awards	593,190					619,303
Unrecognized stock-based compensation expense	3,900,000
Aggregate intrinsic value	37,700,000
Stock-based compensation expense				3,800,000
Unrecognized stock-based compensation weighted average recognition period	1 year 5 months 1 day
Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock-based compensation weighted average recognition period	3 years 5 months 5 days
Unrecognized stock-based compensation	$13,600,000
Options exercised	0
Performance milestones and fundraising vesting conditions | Unvested Restricted Common Stock | 2012 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation, number of shares expected to vest			103,846
From Date Of Grant | Unvested Restricted Stock Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share based compensation award expiration period	7 years
Following Service Termination Date | Unvested Restricted Stock Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share based compensation award expiration period	2 years
Chief Executive Officer | Unvested Restricted Common Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, shares issued					9,595,384
Share based compensation, vesting period					48 months
Chief Executive Officer | Performance milestones and fundraising vesting conditions | Unvested Restricted Common Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, shares issued					1,599,231
Chief Executive Officer | Service and fundraising vesting conditions | Unvested Restricted Common Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, shares issued					7,996,153
Atara Employee | Unvested Restricted Common Stock | 2012 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, shares issued			2,423,074

Stockholders_Equity_Summary_of

Stockholders' Equity - Summary of RSU Activity and Restricted Stock Award (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015
Restricted Stock Awards
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unvested at December 31, 2014	112,740
Vested	-16,106
Unvested at March 31, 2015	96,634
Unvested at December 31, 2014	$0.40
Vested	$0.40
Unvested at March 31, 2015	$0.40
Unvested Restricted Stock Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unvested at December 31, 2014	619,303
Granted	87,600
Vested	-113,713
Unvested at March 31, 2015	593,190
Unvested at December 31, 2014	$4.64
Granted	$25.15
Vested	$5.66
Unvested at March 31, 2015	$7.47

Stockholders_Equity_Summary_of1

Stockholders' Equity - Summary of Stock Option Activity (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Outstanding, Number of shares, beginning balance	623,936
Granted, Number of shares	690,699
Outstanding, Number of shares, ending balance	1,314,635
Stock options vested and expected to vest, Number of shares	1,314,635
Exercisable, Number of shares	56,175
Outstanding, Weighted Average Exercise Price, beginning balance	$13.69
Granted, Weighted Average Exercise Price	$24.96
Outstanding, Weighted Average Exercise Price, ending balance	$19.61
Stock options vested and expected to vest, Weighted Average Exercise Price	$19.61
Exercisable, Weighted Average Exercise Price	$16.10
Outstanding, Weighted Average Remaining Contractual Term	6 years 8 months 1 day
Stock options vested and expected to vest, Weighted Average Remaining Contractual Term	6 years 8 months 1 day
Exercisable, Weighted Average Remaining Contractual Term	6 years 7 months 17 days
Outstanding, Aggregate Intrinsic Value	$28,866,786
Stock options vested and expected to vest, Aggregate Intrinsic Value	28,866,786
Exercisable, Aggregate Intrinsic Value	$1,430,908

Stockholders_Equity_Summary_of2

Stockholders' Equity - Summary of Stock Option Activity (Parenthetical) (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Weighted-average grant date fair value per share	$14.07

Stockholders_Equity_Summary_of3

Stockholders' Equity - Summary of Estimated Weighted-Average Assumptions (Detail)	3 Months Ended
	Mar. 31, 2015
Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate, minimum	1.30%
Risk-free interest rate, maximum	1.60%
Expected life of options in years	4 years 6 months
Expected volatility of underlying stock	71.10%
Expected dividend yield	0.00%
Non Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate	1.60%
Expected life of options in years	6 years 10 months 24 days
Expected volatility of underlying stock	70.10%
Expected dividend yield	0.00%

Stockholders_Equity_Schedule_o

Stockholders' Equity - Schedule of Stock-based Compensation Related to All Employee and Non-Employee Awards (Detail) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	$2,483	$3,317
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	1,288	705
General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	$1,195	$2,612