Agios Pharmaceuticals (AGIO) 10-K 24 Feb 15 2014 FY Annual report Financial data

Document_and_Entity_Informatio

Document and Entity Information (USD $)	12 Months Ended
	Dec. 31, 2014	Feb. 20, 2015	Jun. 30, 2014
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	FALSE
Document Period End Date	31-Dec-14
Document Fiscal Year Focus	2014
Document Fiscal Period Focus	FY
Trading Symbol	AGIO
Entity Registrant Name	AGIOS PHARMACEUTICALS INC
Entity Central Index Key	1439222
Current Fiscal Year End Date	-19
Entity Well-known Seasoned Issuer	Yes
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		37,218,456
Entity Public Float			$876,210,178

Consolidated_Balance_Sheets

Consolidated Balance Sheets (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Current assets:
Cash and cash equivalents	$14,031	$71,560
Marketable securities	328,034	95,209
Collaboration receivable - related party	6,492	476
Other receivables	2,334
Prepaid expenses and other current assets	4,814	2,502
Refundable income taxes	3,841
Total current assets	359,546	169,747
Marketable securities	125,382	27,125
Property and equipment, net	6,386	3,758
Restricted cash		571
Other assets	590	4
Total assets	491,904	201,205
Current liabilities:
Accounts payable	11,067	3,678
Accrued expenses	14,020	6,586
Income taxes payable		1,462
Deferred revenue - related party	35,686	25,072
Deferred rent	310	123
Other current liabilities	6	9
Total current liabilities	61,089	36,930
Deferred revenue, net of current portion - related party	2,725	32,567
Deferred rent, net of current portion	3,724	220
Other non-current liabilities		6
Commitments and contingencies (Note 6)
Stockholders' equity:
Preferred stock, $0.001 par value; 25,000,000 shares authorized, no shares issued and outstanding.
Common stock, $0.001 par value; and 125,000,000 shares authorized, 37,100,513 and 31,202,542 shares issued and outstanding at December 31, 2014 and 2013, respectively	37	31
Additional paid-in capital	591,334	244,881
Accumulated other comprehensive (loss) income	-57	14
Accumulated deficit	-166,948	-113,444
Total stockholders' equity	424,366	131,482
Total liabilities and stockholders' equity	$491,904	$201,205

Consolidated_Balance_Sheets_Pa

Consolidated Balance Sheets (Parenthetical) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Statement of Financial Position [Abstract]
Preferred stock, par value	$0.00	$0.00
Preferred stock, shares authorized	25,000,000	25,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$0.00	$0.00
Common stock, shares authorized	125,000,000	125,000,000
Common stock, shares issued	37,100,513	31,202,542
Common stock, shares outstanding	37,100,513	31,202,542

Consolidated_Statements_of_Ope

Consolidated Statements of Operations (USD $)	12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Statement [Abstract]
Collaboration revenue - related party	$65,358	$25,548	$25,072
Grant revenue			34
Total revenue	65,358	25,548	25,106
Operating expenses:
Research and development	100,371	54,502	41,037
General and administrative	19,120	9,929	7,064
Total operating expenses	119,491	64,431	48,101
Loss from operations	-54,133	-38,883	-22,995
Interest income	203	55	69
(Loss) before benefit provision for income taxes	-53,930	-38,828	-22,926
(Benefit) provision for income taxes	-426	579	-2,824
Net loss	-53,504	-39,407	-20,102
Cumulative preferred stock dividends		-4,162	-7,190
Net loss applicable to common stockholders	($53,504)	($43,569)	($27,292)
Net loss per share applicable to common stockholders - basic and diluted	($1.59)	($2.83)	($8.02)
Weighted-average number of common shares used in net loss per share applicable to common stockholders - basic and diluted	33,667,024	15,415,373	3,401,719

Consolidated_Statements_of_Com

Consolidated Statements of Comprehensive Loss (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Statement of Comprehensive Income [Abstract]
Net loss	($53,504)	($39,407)	($20,102)
Other comprehensive (loss) income:
Unrealized (loss) gain on available-for-sale securities	-71	16	-25
Comprehensive loss	($53,575)	($39,391)	($20,127)

Consolidated_Statements_of_Con

Consolidated Statements of Convertible Preferred Stock and Stockholders' (Deficit) Equity (USD $)	Total	Series A Convertible Preferred Stock [Member]	Series B Convertible Preferred Stock [Member]	Series C Convertible Preferred Stock [Member]	Common Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]
In Thousands, except Share data
Beginning balance, Value at Dec. 31, 2011	($52,782)	$32,940	$5,681	$77,301	$3	$1,127	$23	($53,935)
Beginning balance, Shares at Dec. 31, 2011		33,188,889	5,190,551	15,882,389	3,197,420
Unrealized gain/loss on marketable securities	-25						-25
Net loss	-20,102							-20,102
Stock-based compensation expense	742					742
Vesting of restricted stock, Value	56					56
Vesting of restricted stock, Shares					183,713
Issuance of common stock upon exercise of stock options, Value	87					87
Issuance of common stock upon exercise of stock options, Shares					234,968
Ending balance, Value at Dec. 31, 2012	-72,024	32,940	5,681	77,301	3	2,012	-2	-74,037
Ending balance, Shares at Dec. 31, 2012		33,188,889	5,190,551	15,882,389	3,616,101
Unrealized gain/loss on marketable securities	16						16
Net loss	-39,407							-39,407
Stock-based compensation expense	3,030					3,030
Vesting of restricted stock, Value	64					64
Vesting of restricted stock, Shares					136,758
Issuance of common stock upon exercise of stock options, Value	150					150
Issuance of common stock upon exercise of stock options, Shares					237,565
Conversion of convertible preferred stock upon initial public offering, Value	115,923	-32,940	-5,681	-77,301	20	115,903
Conversion of convertible preferred stock upon initial public offering, Shares		-33,188,889	-5,190,551	-15,882,389	19,731,564
Issuance of Common Stock for private placement, Value	110,980				7	110,973
Issuance of Common Stock for private placement, Shares					6,772,221
Issuance of Common Stock for private placement, Value	12,750				1	12,749
Issuance of Common Stock for private placement, Shares					708,333
Ending balance, Value at Dec. 31, 2013	131,482				31	244,881	14	-113,444
Ending balance, Shares at Dec. 31, 2013					31,202,542
Unrealized gain/loss on marketable securities	-71						-71
Net loss	-53,504							-53,504
Stock-based compensation expense	11,506					11,506
Vesting of restricted stock, Value	9					9
Vesting of restricted stock, Shares					14,773
Issuance of common stock upon exercise of stock options, Value	2,323				1	2,322
Issuance of common stock upon exercise of stock options, Shares	1,298,775				1,298,775
Issuance of Common Stock for private placement, Value	332,621				5	332,616
Issuance of Common Stock for private placement, Shares					4,584,423
Ending balance, Value at Dec. 31, 2014	$424,366				$37	$591,334	($57)	($166,948)
Ending balance, Shares at Dec. 31, 2014					37,100,513

Consolidated_Statements_of_Con1

Consolidated Statements of Convertible Preferred Stock and Stockholders' (Deficit) Equity (Parenthetical) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Issuance costs of common stock	$900	$2,400
Common Stock [Member]
Issuance costs of common stock	900	2,400
Additional Paid-In Capital [Member]
Issuance costs of common stock	$900	$2,400

Consolidated_Statements_of_Cas

Consolidated Statements of Cash Flows (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operating activities
Net loss	($53,504)	($39,407)	($20,102)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	1,367	1,440	1,179
Net loss on disposal of fixed assets			10
Stock-based compensation expense	11,506	3,030	742
Deferred taxes		3,842	6,707
Net amortization of premiums and discounts on investments	538	284	287
Changes in operating assets and liabilities:
Collaboration receivable - related party	-6,016	-476
Other receivables	-2,334
Prepaid expenses and other assets	-2,878	-1,562	-94
Accounts payable	7,577	30	-322
Accrued expenses and other liabilities	5,231	4,878	156
Deferred rent	3,691	-85	-46
Refundable income taxes and income taxes payable	-5,303	-3,302	-12,993
Deferred revenue - related party	-19,228	-25,072	-25,072
Net cash used in operating activities	-59,353	-56,400	-49,548
Investing activities
Purchases of marketable securities	-837,219	-146,049	-88,524
Proceeds from maturities and sales of marketable securities	505,528	60,126	113,041
Purchases of property and equipment	-2,216	-1,294	-1,475
Release of restricted cash	571
Net cash (used in) provided by investing activities	-333,336	-87,217	23,042
Financing activities
Proceeds from public offering of common stock, net of commissions	333,577	113,367
Proceeds from private placement		12,750
Payment of public offering costs	-720	-2,387
Net proceeds from stock option exercises and issuance of common and restricted common stock	2,303	150	142
Net cash provided by financing activities	335,160	123,880	142
Net decrease in cash and cash equivalents	-57,529	-19,737	-26,364
Cash and cash equivalents at beginning of the period	71,560	91,297	117,661
Cash and cash equivalents at end of the period	14,031	71,560	91,297
Supplemental cash flow information
Cash paid for income taxes	5,980		3,549
Supplemental disclosure of non-cash investing and financing transactions:
Conversion of convertible preferred stock upon initial public offering		115,923
Vesting of restricted stock	9	64
Additions to property, plant and equipment included in accounts payable and accrued expenses	2,118	339	57
Proceeds from stock option exercises in other current assets	20
Public offering costs in accounts payable and accrued expenses	$236

Nature_of_Business

Nature of Business	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
Nature of Business	1. Nature of Business
	Agios Pharmaceuticals, Inc. (“Agios” or “the Company”) is a biopharmaceutical company committed to the fundamental transformation of patients’ lives through scientific leadership in the field of cancer metabolism and rare genetic disorders of metabolism. The Company has built a unique set of core capabilities in the field of cellular metabolism, with the goal of making transformative, first or best in class medicines. The Company’s therapeutic areas of focus are cancer and rare genetic disorders of metabolism, which are a broad group of more than 600 rare genetic diseases caused by mutations, or defects, of single metabolic genes. In both of these areas, the Company is seeking to unlock the biology of cellular metabolism to create transformative therapies. The Company is located in Cambridge, Massachusetts.
	Liquidity
	The Company has an accumulated deficit as of December 31, 2014 of $166.9 million and will require substantial capital for research and product development. The future success of the Company is dependent on its ability to develop its product candidates and ultimately upon its ability to attain profitable operations. The Company is subject to a number of risks similar to other life science companies, including, but not limited to, risks related to the successful discovery and development of its drug candidates, raising additional capital, development of new technological innovations by its competitors, protection of proprietary technology, and market acceptance of the Company’s products.
	On July 29, 2013, the Company closed an initial public offering (“IPO”) of its common stock, which resulted in the sale of 6,772,221 shares of its common stock at a public offering price of $18.00 per share, including 883,333 shares of common stock issued upon the exercise in full by the underwriters of their option to purchase additional shares at the public offering price to cover over-allotments. The Company received net proceeds from the IPO of $111.0 million, after deducting underwriting discounts, commissions and expenses payable by the Company. Additionally, an affiliate of Celgene Corporation (“Celgene”), the Company’s cancer metabolism strategic alliance partner, purchased 708,333 shares of common stock in a separate private placement concurrent with the completion of the IPO at a purchase price of $18.00 per share for aggregate proceeds of $12.8 million.
	In April 2014, the Company completed a public offering of 2,000,000 shares of its common stock at a public offering price of $44.00 per share. The Company received net proceeds from this offering of $82.3 million, after deducting underwriting discounts, commissions and expenses payable by the Company. Celgene purchased 294,800 shares of the Company’s common stock in the offering. In addition, the Company granted the underwriters the right to purchase up to an additional 300,000 shares of its common stock which was exercised in May 2014 resulting in additional net proceeds to the Company of $12.4 million, after underwriting discounts and commissions paid by the Company.
	In December 2014, the Company completed a public offering of 1,986,455 shares of its common stock at a public offering price of $110.75 per share. The Company received net proceeds from this offering of $206.9 million, after deducting underwriting discounts, commissions and expenses payable by the Company. In addition, the Company granted the underwriters the right to purchase up to an additional 297,968 shares of its common stock which was exercised in December 2014 resulting in additional net proceeds to the Company of $31.0 million, after underwriting discounts and commissions paid by the Company.
	In addition to the Company’s existing cash, cash equivalents and marketable securities, the Company is eligible to earn a significant amount of milestone payments, an additional extension payment of $20.0 million and is entitled to additional cost reimbursements under its collaboration agreement with Celgene. At December 31, 2014, the Company believes its cash, cash equivalents and marketable securities, totaling $467.4 million, are sufficient to fund operations for a period of at least 12 months from the balance sheet date.

Summary_of_Significant_Account

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies
	Principles of Consolidation
	The Company’s consolidated financial statements include the Company’s accounts and the accounts of the Company’s wholly owned subsidiary, Agios Securities Corporation. All intercompany transactions have been eliminated in consolidation. The consolidated financial statements have been prepared in conformity with U.S. generally accepted accounting principles (“GAAP”).
	Use of Estimates
	The preparation of financial statements in conformity with U.S. GAAP requires the Company’s management to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenue and expenses during the reporting period. The Company bases its estimates on historical experience and on various other assumptions that are believed to be reasonable under the circumstances. Actual results could differ from those estimates.
	Prior to the IPO, the Company utilized significant estimates and assumptions in determining the fair value of its common stock. The board of directors determined the estimated fair value of the Company’s common stock based on a number of objective and subjective factors, including external market conditions affecting the biotechnology industry sector and the prices at which the Company sold shares of convertible preferred stock, the superior rights and preferences of securities senior to the Company’s common stock at the time and the likelihood of achieving a liquidity event, such as an initial public offering or sale of the Company. The Company utilized various valuation methodologies in accordance with the framework of the American Institute of Certified Public Accountants, or AICPA, Audit and Accounting Practice Aid Series: Valuation of Privately Held Company Equity Securities Issued as Compensation, or the AICPA Practice Aid, to estimate the fair value of its common stock and in performing retrospective valuation analyses for certain grant dates prior to the IPO. The methodologies included the option pricing method utilizing the back-solve method (a form of the market approach defined in the AICPA Practice Aid) and the probability-weighted expected return method based upon the probability of occurrence of certain future liquidity events such as an initial public offering or sale of the Company. Each valuation methodology included estimates and assumptions that require the Company’s judgment. Significant changes to the key assumptions used in the valuations could result in different fair values of common stock at each valuation date.
	Revenue Recognition
	The Company recognizes revenue in accordance with the Financial Accounting Standards Board’s (“FASB”) Accounting Standards Codification (“ASC”) 605, Revenue Recognition. Accordingly, revenue is recognized for each unit of accounting when all of the following criteria are met:
		•		persuasive evidence of an arrangement exists;
		•		delivery has occurred or services have been rendered;
		•		the seller’s price to the buyer is fixed or determinable; and
		•		collectability is reasonably assured.
	The Company’s revenue has primarily been generated from a Discovery and Development Collaboration and License Agreement with Celgene (“the Celgene Agreement”) and from research grant agreements.
	Collaboration and License Revenue
	In January 2011, the Company adopted the FASB Accounting Standards Update (“ASU”) No. 2009-13, Multiple-Element Revenue Arrangements (“ASU No. 2009-13”), on a prospective basis for all revenue arrangements entered into or materially modified after the adoption date. The Celgene Agreement was entered into prior to January 1, 2011 and the Company initially applied its prior accounting policy with respect to the arrangement. Under this policy, when evaluating multiple element arrangements, the Company considered whether the components of the arrangement should be accounted for individually as separate units of accounting if (1) the elements have stand-alone value, and (2) the Company is able to estimate the fair value of all undelivered elements under the arrangement.
	In July 2014, the Company amended its collaboration agreement with Celgene. As a result of the amendment, the Company was required to reevaluate the agreement under ASU No. 2009-13. The amendment was determined to be a material modification pursuant to ASU No. 2009-13, and the Company began recognizing revenue for the arrangement under this guidance on a prospective basis, as discussed further in Note 3.
	Pursuant to ASU 2009-13, revenue arrangements where multiple products or services are sold together are evaluated to determine if each deliverable represents a separate unit of accounting based on the following criteria:
		•		Delivered item or items have value to the customer on a standalone basis, and
		•		If the arrangement includes a general right of return relative to the delivered item or items, delivery or performance of the undelivered item or items is considered probable and substantially in the control of the vendor.
	The arrangement consideration is then allocated to each separately identified unit of accounting based on the relative selling price, using the Company’s best estimate of selling price of each deliverable. The provisions of ASC 605-25, Multiple-Element Arrangements are then applied to each unit of accounting to determine the appropriate revenue recognition. In the event that a deliverable of a multiple element arrangement does not represent a separate unit of accounting, the Company recognizes revenue from the combined units of accounting over the term of the related contract or as undelivered items are delivered, as appropriate.
	In determining the current and noncurrent classification of deferred revenue, the Company considers the total consideration expected to be earned in the next twelve months for services to be performed under certain units of accounting and the estimated proportional performance and timing of delivery of certain deliverables to determine the deferred revenue balance that will remain twelve months from the balance sheet date.
	Revenue is recognized under the proportional performance method for certain units of accounting. The amount recognized is determined based on the consideration allocated to each unit of accounting based on the ratio of the level of effort incurred to date compared to the total estimated level of effort required to complete the Company’s performance obligations under the unit of accounting. Determining the total estimated level of effort required to complete all performance obligations requires management judgment and estimation, including assumptions regarding future operating performance, the timelines of the clinical trials approvals and the estimated patient populations.
	In January 2011, the Company adopted the FASB’s ASU No. 2010-17, Revenue Recognition – Milestone Method, on a prospective basis. ASU 2010-17 provides guidance in applying the milestone method of revenue recognition to research or development arrangements. Under this guidance, management may recognize revenue contingent upon the achievement of a milestone in its entirety in the period in which the milestone is achieved, only if the milestone meets all the criteria within the guidance to be considered substantive. In accordance with ASU 2010-17, at the inception of each arrangement that includes milestone payments, the Company evaluates each contingent payment on an individual basis to determine whether they are considered substantive milestones, specifically reviewing factors such as the degree of certainty in achieving the milestone, the research and development risk and other risks that must be overcome to achieve the milestone, as well as the level of effort and investment required and whether the milestone consideration is reasonable relative to all deliverables and payment terms in the arrangement. This evaluation includes an assessment of whether (a) the consideration is commensurate with either (1) the entity’s performance to achieve the milestone, or (2) the enhancement of the value of the delivered item(s) as a result of a specific outcome resulting from the entity’s performance to achieve the milestone, (b) the consideration relates solely to past performance and (c) the consideration is reasonable relative to all of the deliverables and payment terms within the arrangement.
	Revenues from milestones, if they are nonrefundable and deemed substantive, are recognized upon achievement of the milestones. The Company recognizes revenue associated with the non-substantive milestones upon achievement of the milestone if there are no undelivered elements and the Company has no remaining performance obligations.
	Reimbursement of research and development costs by Celgene is recognized as revenue, provided the Company has determined that it is acting primarily as a principal in the transaction according to the provisions outlined in FASB ASC 605-45, Revenue Recognition – Principal Agent Considerations, the amounts are determinable and collection of the related receivable is reasonably assured.
	Grant Revenue
	Revenue related to research grant agreements is recognized as the underlying services are performed and delivered. Revenues from grants totaled approximately $0.1 million for the year ended December 31, 2012. The Company did not recognize any grant revenue for the years ended December 31, 2014 and 2013.
	Research and Development Costs
	Research and development costs are expensed as incurred. Research and development costs include salaries and personnel-related costs, consulting fees, fees paid for contract research services, fees paid to clinical research organizations and other third parties associated with clinical trials, the costs of laboratory equipment and facilities, and other external costs.
	Nonrefundable advance payments for goods or services to be received in the future for use in research and development activities are deferred and capitalized. The capitalized amounts are expensed as the related goods are delivered or the services are performed.
	Stock-Based Compensation
	The Company accounts for its stock-based compensation awards in accordance with ASC Topic 718, Compensation – Stock Compensation (“ASC 718”). ASC 718 requires all share-based payments to employees, including grants of employee stock options, to be recognized in the consolidated statements of operations based on their grant date fair values. For stock options granted to employees and to members of the board of directors for their services on the board of directors, the Company estimates the grant date fair value of each option award using the Black-Scholes option-pricing model. The use of the Black-Scholes option-pricing model requires management to make assumptions with respect to the expected term of the option, the expected volatility of the common stock consistent with the expected life of the option, risk-free interest rates and expected dividend yields of the common stock. For awards subject to service-based vesting conditions, the Company recognizes stock-based compensation expense, net of estimated forfeitures, equal to the grant date fair value of stock options on a straight-line basis over the requisite service period. For awards subject to both performance and service-based vesting conditions, the Company recognizes stock-based compensation expense using an accelerated recognition method if achievement of the performance criteria is considered probable.
	Share-based payments issued to non-employees are recorded at their fair values, and are revalued at each reporting date and as the equity instruments vest and are recognized as expense over the related service period in accordance with the provisions of ASC Topic 505, Equity. For equity instruments granted to non-employees, the Company recognizes stock-based compensation expense using an accelerated recognition method.
	During the years ended December 31, 2014, 2013, and 2012, the Company recorded stock-based compensation expense for employee and non-employee stock options, the employee stock purchase plan and restricted stock, which was allocated as follows in the consolidated statements of operations (in thousands):
			Years Ended December 31,
			2014			2013		2012
	Research and development expense		$	6,688		$	2,001	$	605
	General and administrative expense			4,818			1,029		137
			$	11,506		$	3,030	$	742
	No related tax benefits were recognized for the years ended December 31, 2014, 2013 and 2012.
	Income Taxes
	Income taxes are recorded in accordance with ASC 740, Accounting for Income Taxes (“ASC 740”), which provides for deferred taxes using an asset and liability approach. The Company recognizes deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the financial statements or tax returns. The Company determines its deferred tax assets and liabilities based on differences between financial reporting and tax bases of assets and liabilities, which are measured using the enacted tax rates and laws that will be in effect when the differences are expected to reverse. Valuation allowances are provided, if based upon the weight of available evidence, it is more likely than not that some or all of the deferred tax assets will not be realized.
	The Company accounts for uncertain tax positions in accordance with the provisions of ASC 740. When uncertain tax positions exist, the Company recognizes the tax benefit of tax positions to the extent that the benefit will more likely than not be realized. The determination as to whether the tax benefit will more likely than not be realized is based upon the technical merits of the tax position as well as consideration of the available facts and circumstances. As of December 31, 2014 and 2013, the Company did not have any uncertain tax positions.
	Comprehensive Income (Loss)
	Comprehensive income (loss) is defined as the change in equity of a business enterprise during a period from transactions, and other events and circumstances from non-owner sources, and currently consists of net loss and changes in unrealized gains and losses on available-for-sale securities. Accumulated other comprehensive loss consists entirely of unrealized gains and losses from available for sale securities as of December 31, 2014 and 2013.
	Cash and Cash Equivalents
	The Company considers highly liquid investments with a maturity of ninety days or less when purchased to be cash equivalents. Cash equivalents, which consist primarily of money market funds, are stated at fair value.
	Marketable Securities
	Marketable securities at December 31, 2014 and 2013 consisted primarily of investments in United States Treasuries and certificates of deposit. Management determines the appropriate classification of the securities at the time they are acquired and evaluates the appropriateness of such classifications at each balance sheet date. The Company classifies its marketable securities as available-for-sale pursuant to ASC 320, Investments – Debt and Equity Securities. Marketable securities are recorded at fair value, with unrealized gains and losses included as a component of accumulated other comprehensive income (loss) in stockholders’ equity and a component of total comprehensive loss in the consolidated statements of comprehensive loss, until realized. The fair value of these securities is based on quoted prices for identical or similar assets. Realized gains and losses are included in investment income on a specific-identification basis. There were no realized gains or losses on marketable securities for the years ended December 31, 2014, 2013 and 2012.
	The Company reviews marketable securities for other-than-temporary impairment whenever the fair value of a marketable security is less than the amortized cost and evidence indicates that a marketable security’s carrying amount is not recoverable within a reasonable period of time. Other-than-temporary impairments of investments are recognized in the consolidated statements of operations if the Company has experienced a credit loss, has the intent to sell the marketable security, or if it is more likely than not that the Company will be required to sell the marketable security before recovery of the amortized cost basis. Evidence considered in this assessment includes reasons for the impairment, compliance with the Company’s investment policy, the severity and the duration of the impairment and changes in value subsequent to the end of the period.
	Marketable securities at December 31, 2014 consist of the following (in thousands):
			Amortized			Unrealized		Unrealized			Fair
			Cost			Gains		Losses			Value
	Current:
	Certificates of deposit		$	13,160		$	—	$	(5	)	$	13,155
	U.S. Treasuries			314,866			45		(32	)		314,879
	Non-current:
	U.S. Treasuries			125,447			5		(70	)		125,382
			$	453,473		$	50	$	(107)		$	453,416
	Marketable securities at December 31, 2013 consist of the following (in thousands):
			Amortized			Unrealized		Unrealized			Fair
			Cost			Gains		Losses			Value
	Current:
	Certificates of deposit		$	6,920		$	—	$	(5	)	$	6,915
U.S. Treasuries			88,287			8		(1	)		88,294
Non-current:
U.S. Treasuries			27,113			12		—			27,125
		$	122,320		$	20	$	-6		$	122,334
At December 31, 2014 and 2013, the Company held both current and non-current investments. Investments classified as current have maturities of less than one year. Investments classified as non-current are those that (i) have maturities of one to two years and (ii) management does not intend to liquidate within the next twelve months, although these funds are available for use and therefore classified as available-for-sale.
At December 31, 2014 and 2013, the Company held 92 and 33 debt securities, respectively, that were in an unrealized loss position for less than one year. The aggregate fair value of debt securities in an unrealized loss position at December 31, 2014 and 2013 was $236.9 million and $31.7 million, respectively. There were no individual securities that were in a significant unrealized loss position or that had been in an unrealized loss position for greater than one year as of December 31, 2014 and 2013. The Company evaluated its securities for other-than-temporary impairment and considered the decline in market value for the securities to be primarily attributable to current economic and market conditions. It is not more likely than not that the Company will be required to sell the securities, and the Company does not intend to do so prior to the recovery of the amortized cost basis. Based on this analysis, these marketable securities were not considered to be other-than-temporarily impaired as of December 31, 2014 and 2013.
Concentrations of Credit Risk
Financial instruments which potentially subject the Company to credit risk consist primarily of cash, cash equivalents and marketable securities. The Company holds these investments in highly rated financial institutions, and, by policy, limits the amounts of credit exposure to any one financial institution. These amounts at times may exceed federally insured limits. The Company has not experienced any credit losses in such accounts and does not believe it is exposed to any significant credit risk on these funds. The Company has no off-balance sheet concentrations of credit risk, such as foreign currency exchange contracts, option contracts or other hedging arrangements.
The Company is also subject to credit risk from its collaboration receivable. The Company evaluates the creditworthiness of its collaborator and has determined it is credit worthy. To date the Company has not experienced any losses with respect to its collaboration receivable.
Fair Value Measurements
The Company records cash equivalents and marketable securities at fair value. ASC Topic 820, Fair Value Measurements and Disclosures, establishes a fair value hierarchy for those instruments measured at fair value that distinguishes between assumptions based on market data (observable inputs) and the Company’s own assumptions (unobservable inputs). The hierarchy consists of three levels:
Level 1 – Unadjusted quoted prices in active markets for identical assets or liabilities.
Level 2 – Quoted prices for similar assets and liabilities in active markets, quoted prices in markets that are not active, or inputs which are observable, either directly or indirectly, for substantially the full term of the asset or liability.
Level 3 – Unobservable inputs that reflect the Company’s own assumptions about the assumptions market participants would use in pricing the asset or liability in which there is little, if any, market activity for the asset or liability at the measurement date.
The following table summarizes the cash equivalents and marketable securities measured at fair value on a recurring basis as of December 31, 2014 (in thousands):
		Level 1			Level 2		Level 3			Total
Cash equivalents		$	11,410		$	960	$	—		$	12,370
Marketable securities:
Certificates of deposit			—			13,155		—			13,155
U.S. Treasuries			440,261			—		—			440,261
		$	451,671		$	14,115	$	—		$	465,786
The following table summarizes the cash equivalents and marketable securities measured at fair value on a recurring basis as of December 31, 2013 (in thousands):
		Level 1			Level 2		Level 3			Total
Cash equivalents		$	68,792		$	—	$	—		$	68,792
Marketable securities:
Certificates of deposit			—			6,915		—			6,915
U.S. Treasuries			115,419			—		—			115,419
		$	184,211		$	6,915	$	—		$	191,126
Cash equivalents and marketable securities have been initially valued at the transaction price and subsequently valued, at the end of each reporting period, utilizing third party pricing services or other market observable data. The pricing services utilize industry standard valuation models, including both income and market based approaches and observable market inputs to determine value. The Company validates the prices provided by its third party pricing services by reviewing their pricing methods and obtaining market values from other pricing sources. After completing its validation procedures, the Company did not adjust or override any fair value measurements provided by the pricing services as of December 31, 2014 or 2013.
The carrying amounts reflected in the consolidated balance sheets for cash, restricted cash, collaboration receivable – related party, prepaid expenses and other current assets, other assets, accounts payable, and accrued expenses approximate their fair values at December 31, 2014 and 2013, due to their short-term nature.
There have been no changes to the valuation methods during the years ended December 31, 2014 and 2013. The Company evaluates transfers between levels at the end of each reporting period. There were no transfers of assets or liabilities between Level 1 and Level 2 during the years ended December 31, 2014 and 2013. The Company had no financial assets or liabilities that were classified as Level 3 at any point during the years ended December 31, 2014 and 2013.
Collaboration and Other Receivables
Collaboration receivables as of December 31, 2014 represent amounts due from Celgene for cost reimbursements of certain on-going phase 1 studies under the Celgene Agreement. As of December 31, 2013, collaboration receivables represented an unbilled collaboration receivable for revenues recognized related to Celgene’s election to extend the term of the initial discovery period from four to five years. Other receivables represent amounts due from the Company’s landlord for reimbursement of tenant improvements under the Company’s lease agreement.
The Company estimates an allowance for doubtful accounts based on credit worthiness, historical payment patterns, aging of accounts receivable balances, and general economic conditions. As of December 31, 2014 and 2013, the Company had no allowance for doubtful accounts.
Property and Equipment
Property and equipment consist of laboratory equipment, computer equipment and software, leasehold improvements, furniture and fixtures, and office equipment. Property and equipment is stated at cost, and depreciated using the straight-line method over the estimated useful lives of the respective assets:
		Laboratory equipment		5 years
		Computer equipment and software		3 years
		Leasehold improvements		Shorter of asset’s useful
				life or remaining term of lease
		Furniture and fixtures		5 years
		Office equipment		5 years
Costs of major additions and betterments are capitalized; maintenance and repairs, which do not improve or extend the life of the respective assets, are charged to expense as incurred. Upon retirement or sale, the cost of the disposed asset and the related accumulated depreciation are removed from the accounts and the resulting gain or loss is recognized.
Impairment of Long-Lived Assets
The Company periodically evaluates its long-lived assets for potential impairment in accordance with ASC Topic 360, Property, Plant and Equipment. Potential impairment is assessed when there is evidence that events or changes in circumstances indicate that the carrying amount of an asset may not be recovered. Recoverability of these assets is assessed based on undiscounted expected future cash flows from the assets, considering a number of factors, including past operating results, budgets and economic projections, market trends and product development cycles. If impairments are identified, assets are written down to their estimated fair value. The Company has not recognized any impairment charges through December 31, 2014.
Segment and Geographic Information
Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision maker, or decision making group, in making decisions on how to allocate resources and assess performance. The Company’s chief operating decision maker is the chief executive officer. The Company and the chief decision maker view the Company’s operations and manage its business as one operating segment. The Company operates in only one geographic segment.
Net Loss per Share Applicable to Common Stockholders
Basic net loss per share applicable to common stockholders is calculated by dividing net loss applicable to common stockholders by the weighted average shares outstanding during the period, without consideration for common stock equivalents. Net loss applicable to common stockholders is calculated by adjusting the net loss of the Company for cumulative preferred stock dividends. Diluted net loss per share applicable to common stockholders is calculated by adjusting weighted average shares outstanding for the dilutive effect of common stock equivalents outstanding for the period, determined using the treasury-stock method. For purposes of the dilutive net loss per share applicable to common stockholders calculation, preferred stock, stock options, and unvested restricted stock are considered to be common stock equivalents but are excluded from the calculation of diluted net loss per share applicable to common stockholders as their effect would be anti-dilutive; therefore, basic and diluted net loss per share applicable to common stockholders were the same for all periods presented. The following common stock equivalents were excluded from the calculation of diluted net loss per share applicable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect:
		Years ended December 31,
		2014			2013		2012
Convertible preferred stock			-			-		19,731,564
Stock options			3,805,420			3,846,168		3,145,544
Unvested restricted stock			8,522			23,295		160,053
Employee stock purchase plan shares			7,159			-		-
			3,821,101			3,869,463		23,037,161
Recent Accounting Pronouncements
In August 2014, the FASB issued ASU No. 2014-15, Presentation of Financial Statements—Going Concern (Subtopic 205-40). The ASU requires all entities to evaluate for the existence of conditions or events that raise substantial doubt about the entity’s ability to continue as a going concern within one year after the issuance date of the financial statements. The accounting standard is effective for interim and annual periods ending after December 15, 2016, and will not have a material impact on the consolidated financial statements, but may impact the Company’s footnote disclosures.
In June 2014, the FASB issued ASU No. 2014-12, Compensation—Stock Compensation (Topic 718). The ASU clarifies how entities should treat performance targets that can be achieved after the requisite service period of a share-based payment award. The accounting standard is effective for interim and annual periods beginning after December 15, 2015 and is not expected to have a material impact on the consolidated financial statements.
In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606). The ASU provides for a single comprehensive model for use in accounting for revenue arising from contracts with customers and supersedes most current revenue recognition guidance. The accounting standard is effective for interim and annual periods beginning after December 15, 2016 with no early adoption permitted. The Company is required to adopt the amendments in the ASU using one of two acceptable methods. The Company is currently in the process of determining which adoption method it will apply and evaluating the impact of the guidance on its consolidated financial statements.
Other accounting standards that have been issued by the FASB or other standards-setting bodies that do not require adoption until a future date are not expected to have a material impact on the Company’s financial statements upon adoption.
Subsequent Events
The Company considered events or transactions occurring after the balance sheet date but prior to the issuance of the consolidated financial statements are available to be issued for potential recognition or disclosure in its consolidated financial statements. All significant subsequent events have been properly disclosed in the consolidated financial statements.

Collaboration_Agreement

Collaboration Agreement	12 Months Ended
	Dec. 31, 2014
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Collaboration Agreement	3. Collaboration Agreement
	In April 2010, the Company entered into a collaboration agreement focused on cancer metabolism with Celgene, a related party through ownership of the Company’s common stock. The agreement was amended in October 2011 and in July 2014, as described below. The goal of the collaboration is to discover, develop and commercialize disease-altering therapies in oncology based on the Company’s cancer metabolism research platform. The Company is leading discovery, preclinical and early clinical development for all cancer metabolism programs under the collaboration. The discovery phase of the amended collaboration was to expire in April 2014, subject to Celgene’s option to extend the discovery phase for up to an additional two years with additional funding to the Company. In December 2013, Celgene elected to extend the term of the initial discovery phase from four years to five years, to April 2015, in exchange for the payment of a $20.0 million extension fee which was received in May 2014. In December 2014, Celgene elected to exercise its final option to extend the term of the initial discovery phase one additional year, to April 2016, in exchange for the payment of a $20.0 million extension fee which is expected to be received in the second quarter of 2015.
	Pursuant to the collaboration agreement and subsequent amendments, the Company was responsible for nominating development candidates, of which two required confirmation by the Joint Research Committee (“JRC”) during the discovery phase. During the year ended December 31, 2012 the Company nominated its first development candidate and during the year ended December 31, 2013 the Company nominated its second development candidate, both of which have been confirmed by the JRC pursuant to the agreement. For each development candidate, Celgene elected to progress such development candidate into preclinical development requiring the Company to conduct studies to meet the requirements for filing an Investigational New Drug application (“IND”), or IND-enabling studies. Subsequently, the Company was required to file an IND for each development candidate and, upon the FDA’s acceptance of the INDs, Celgene requested that the Company conduct an initial phase 1 study.
	Celgene may elect to convert each discovery program for which the Company has nominated a development candidate into a co-commercialized licensed program, the attributes of which are described below. The Company has the right, exercisable during a specified period following FDA acceptance of the applicable IND, to convert one of every three co-commercialized licensed programs into a split licensed program, for which the Company will retain the United States rights, other attributes of which are further described below. In June 2014, Celgene exercised its option to an exclusive global license for the development and commercialization of the Company’s isocitrate dehydrogenase 2 (“IDH2”) program, AG-221. The Company elected to retain U.S. rights to its isocitrate dehydrogenase 1 (“IDH1”) program, AG-120, in January 2014. In January 2015, Celgene agreed to exercise its rights to this program subject to receipt of any required regulatory approvals upon which the program will become a split licensed program. In addition, Celgene may license certain discovery programs that the Company does not nominate or the JRC does not confirm as a development candidate and for which Celgene will lead and fund global development and commercialization.
	The Company will retain the rights to development candidates and certain other compounds that Celgene does not elect to progress into preclinical development or convert into a co-commercialized licensed program. In addition, if the JRC or Celgene elects not to continue collaboration activities with respect to a particular target, either the Company or Celgene would have the right to independently undertake a discovery program on such target and would have rights to specified compounds from such program, subject to certain “buy-in” rights granted to the other party.
	The agreement provides for three types of licensed programs as discussed above:
	Co-Commercialized Licensed Programs: Celgene will lead and, following either IND acceptance by the FDA or, if Celgene requests the Company to conduct the initial phase 1 study upon completion of such phase 1 study, will fund global development and commercialization. The Company has the right to participate in a portion of sales activities in the United States for products from co-commercialized programs in accordance with the applicable commercialization plan. The Company will be eligible to receive milestone payments and royalties arising from the licensed program.
	Split Licensed Programs: Celgene will lead development and commercialization outside the United States and the Company will lead development and commercialization in the United States. The Company and Celgene will equally fund the global development costs of each split licensed program that are not specific to any particular region or country, Celgene will be responsible for development and commercialization costs specific to countries outside the United States, and the Company will be responsible for development and commercialization costs specific to the United States. The Company will retain profits generated in the United States and will also be eligible to receive milestone payments and royalties arising from net sales outside the United States. The Company will be obligated to pay Celgene royalties arising from net sales in the United States.
	Buy-In Programs: If a party elects to independently undertake a discovery program, with respect to a particular target under the agreement, the party that is conducting the independent program that becomes a buy-in program will lead the development and commercialization of such program. The party that elects to buy in to such program will be responsible for funding a portion of development costs incurred after acceptance of an IND for a buy-in program compound, and the lead party will be responsible for all other development costs and all commercialization costs for products from such buy-in program. The commercializing party will be obligated to pay the buy-in party specified royalties on worldwide net sales.
	The term of the agreement will continue, unless earlier terminated by either party, until the expiration of the last-to-expire of all royalty terms with respect to all royalty-bearing products. Celgene may terminate the agreement for convenience in its entirety or with respect to one or more programs upon ninety days written notice to the Company. Either the Company or Celgene may terminate the agreement in its entirety or with respect to one or more programs, if the other party is in material breach and fails to cure such breach within the specified cure period; however, if such breach relates solely to a specific program, the non-breaching party may only terminate the agreement with respect to such program. Either the Company or Celgene may terminate the agreement in the event of specified insolvency events involving the other party.
	Under the terms of the agreement, the Company received an upfront payment of approximately $121.2 million. In addition, Celgene purchased 5,190,551 shares of Series B convertible preferred stock (“Series B Preferred Stock”) at a price of $1.70 per share, resulting in net proceeds to the Company of approximately $8.8 million. The Company determined the price paid by Celgene for the Series B Preferred Stock represented a premium over the fair value of the Company’s Series B Preferred Stock as determined by the implied value of the Series B Preferred Stock pursuant to a contemporaneous valuation analysis that allocated the equity value of the Company to the various classes of its then-outstanding securities. The Company accounted for the $3.1 million premium as additional consideration under the agreement and the Series B Preferred Stock was recorded at its fair value of $5.7 million. In connection with the 1-for-2.75 reverse stock split of the Company’s common stock, the shares of Series B Preferred Stock converted into 1,887,473 shares of common stock upon the closing of the Initial Public Offering in July 2013.
	In October 2011, the agreement was amended to extend the term of the initial discovery period from three to four years, to April 2014. The amendment was not deemed to be a material modification to the arrangement, pursuant to ASU No. 2009-13, since there were no changes in the deliverables or the total arrangement consideration, as the provisions of the original agreement provided Celgene with the option to extend the research period for the same consideration. Celgene made a payment to the Company of $20.0 million pursuant to the amendment. The payment was combined with the unamortized upfront payment and premium and was recognized as revenue on a straight-line basis over the estimated performance period prior to the July 2014 amendment described below.
	In December 2013, Celgene elected to extend the term of the initial discovery period from four to five years, to April 2015. As a result of the extension, the Company received a $20.0 million extension payment from Celgene in May 2014. The payment was combined with the unamortized upfront payment, premium, and prior extension payment and was being recognized as revenue on a straight-line basis over the estimated performance period prior to the July 2014 amendment described below.
	In July 2014, the Company amended the collaboration agreement to allow for more flexibility in the design and conduct of phase 1 clinical trials and additional nonclinical and/or clinical activities that the Company agrees to perform at Celgene’s request. The amendment further modifies the mechanism and timing for payments to be made with respect to such development activities. The amendment was determined to be a material modification pursuant to ASU No. 2009-13, due to a change in the total potential consideration that was more than insignificant and changes to certain of the deliverables in the arrangement. The amendment impacts the co-commercialized and split licensed programs as follows:
		•	Co-commercialized licensed programs: The amendment modifies the timing and nature of the consideration for the development efforts related to an initial phase 1 study from a milestone due at the completion of the study to payments due upon the earlier of the determination of the maximum tolerated dose or Celgene’s election to license the program.
		•	Split licensed programs: The amendment allows for the Company to receive reimbursement for costs and expenses it incurs for any disease-specific expansion cohort within a phase 1 clinical trial design, provided that the disease-specific expansion cohort supports the initiation of a subsequent pivotal clinical trial. The milestone reimbursement is the lesser of fifty percent of the costs incurred by the Company for disease specific cohorts and $10 million and is payable upon the first patient dosed within the corresponding pivotal trial.
	Prior to the amendment, the Company concluded that none of the identified deliverables had stand-alone value and, therefore, accounted for the deliverables as a single unit of accounting. The Company further concluded it was unable to estimate the fair value of the undelivered items within the agreement. Consideration received was recognized on a straight-line basis through the period over which the Company expected to fulfill its performance obligations (the performance period), which was initially determined to be 6 years.
	Upon concluding the arrangement had been materially modified in July 2014, the Company identified the remaining deliverables under the arrangement and determined its best estimate of selling price for the undelivered elements as of the modification date. The Company then allocated the total arrangement consideration, which included the remaining deferred revenue balance at the modification date and other consideration that was deemed to be determinable at the modification date, to each unit of accounting based on its best estimate of selling price. The difference between the total consideration and the best estimate of selling price of the undelivered items was recorded as revenue at the modification date. The undelivered items, which are each considered by the Company to have stand-alone value and therefore are separate units of accounting, the related best estimate of selling price, and the method of recognizing the allocated consideration, for each unit of accounting are as follows:
		•	License for the split licensed program – AG-120: The Company developed the best estimate of selling price of the license by probability weighting multiple cash flow scenarios using the income approach. Management estimates within the models include the expected, probability-weighted net profits from estimated future sales, an estimated royalty rate using comparable industry royalty agreements, an estimate of the direct costs incurred to generate future cash flows, a discount rate, an estimated contributory asset charge rate to reflect the cost associated with the use of other assets to generate the cash flow, an estimated income tax rate and other business forecast factors. There are significant judgments and estimates inherent in the determination of the best estimate of selling price of this unit of accounting. These judgments and estimates include assumptions regarding future operating performance, the timelines of the clinical trials and regulatory approvals and the estimated patient populations. Should different reasonable assumptions be utilized, the best estimate of selling price and the associated revenues recognized would be different. Based on the analysis using management’s best estimate, the Company allocated $21.2 million to the license and will recognize revenue upon Celgene’s election to exercise its option to the split licensed program – AG-120. The Company will immediately recognize the non-contingent allocated consideration on the exercise date. On January 12, 2015, Celgene agreed to exercise its option to obtain an exclusive license outside the United States for AG-120, subject to receipt of any required regulatory approvals including any applicable clearance under the Hart-Scott-Rodino Act.
		•	Development services for five separate on-going phase 1 studies (each of which is a separate unit of accounting): The Company developed the best estimate of selling price of the on-going phase 1 study development services of $50.8 million for all five studies using management’s best estimate of the cost of obtaining these services from a third-party provider, as well as internal full time equivalent costs to support the development services. The estimated costs were determined to represent management’s best estimate of the price these services could be sold for separately. The amount allocated to these units of accounting is being recognized as revenue on a proportional performance basis as services are provided. The Company expects the performance period for these units of accounting to be delivered through the second quarter of 2016.
		•	On-going research and development: The Company developed the best estimate of selling price of the research and development services of $13.6 million using management’s best estimate of the cost of obtaining these services from a third-party provider. The amount allocated to this unit of accounting is being recognized as revenue ratably over the performance period. The performance period has been determined to be through April 2015.
		•	Committee participation: The Company developed the best estimate of selling price of the committee participation services of $0.2 million using management’s best estimate of the anticipated participation hours multiplied by a market rate for comparable participants. The amount allocated to this unit of accounting is being recognized as revenue ratably over the performance period. The performance period has been determined to be through April 2015.
	The total estimated arrangement consideration, as well as the expected timing of revenue recognition, is adjusted based on changes in estimated arrangement consideration as a result of changes in estimate for certain on-going phase 1 studies. The allocable consideration will increase as the Company performs certain services for which it is eligible to receive reimbursement. These amounts will be recognized on a cumulative catch-up basis for any in-process units of accounting or immediately for any fully delivered units of accounting. For the period January 1, 2014 through the amendment date, the Company recognized a total of $42.7 million in revenues under the previous accounting guidance and upon the material modification. The Company recognized total revenue of $65.4 million, $25.5 million, and $25.1 million in connection with the Celgene collaboration during the years ended December 31, 2014, 2013, and 2012, respectively.
	In December 2014, Celgene elected to extend the term of the initial discovery period from five to six years, to April 2016. As a result of the extension, the Company is entitled to receive a $20.0 million extension payment from Celgene. The Company evaluated this substantive option upon the material modification and concluded that upon exercise it is obligated provide its committee participation and research and development services for a period of one year from April 2015 through April 2016. Revenue will be recognized ratably over the performance period of April 2015 to April 2016 as the services are performed. The Company did not recognize any revenue related to this substantive option during the year ended December 31, 2014.
	Under the arrangement, the Company is eligible to receive up to $120.0 million in potential milestone payments payable for each program selected by Celgene. The potential milestone payments for each such program are comprised of: (i) a $25.0 million milestone payment upon achievement of a specified clinical development milestone event, (ii) up to $70.0 million in milestone payments upon achievement of specified regulatory milestone events, and (iii) a $25.0 million milestone payment upon achievement of a specified commercial milestone event. The Company is also eligible to receive additional milestone payments specific to co-commercialized licensed programs and split licensed programs. In addition, the Company is eligible to receive a substantive milestone payment of $22.5 million upon achievement of an early clinical development milestone event for certain co-commercialized licensed programs. In connection with the first split licensed program under the collaboration, the Company’s IDH1 program, AG-120, the Company is eligible to receive an additional one-time payment of $25.0 million upon the dosing of the last patient in a Company-sponsored phase 2 clinical trial.
	In addition to the milestone payments described above, for each co-commercialized licensed program, the Company will be reimbursed for all eligible development costs of the related phase 1 multiple ascending dose (MAD) study. The initial costs will be reimbursed as a milestone payment equal to the greater of $5.0 million or eligible development costs incurred by the Company upon the earlier of the determination of the maximum tolerated dose (MTD) or Celgene’s election to license the program. Subsequent to the initial milestone payment, development costs will be reimbursed on a quarterly basis. Through December 31, 2014, the Company had earned $26.6 million in cost reimbursements which includes the initial milestone payment. As of December 31, 2014, the Company has recorded a collaboration receivable of $6.5 million related to reimbursable development costs for AG-221.
	In addition to the milestone payments described above, for each split licensed program, the Company is eligible for reimbursement of the costs of disease-specific expansion cohort(s) that support the initiation of a subsequent pivotal clinical trial. Costs will be reimbursed as a milestone payment equal to the lesser of $10.0 million or fifty percent of the eligible costs for the disease-specific expansion cohort(s) upon the first patient dosed under the pivotal clinical trial. The maximum amount for the milestone payment will be $10.0 million for each split program regardless of the number of disease-specific expansion cohorts and pivotal trials undertaken for each split program.
	The Company has concluded that certain of the clinical development and regulatory milestones that may be received under the Celgene agreement, if the Company is involved in future product development and commercialization, are substantive. Factors considered in the evaluation of the milestones included the degree of risk associated with performance of the milestone, the level of effort and investment required, whether the milestone consideration was reasonable relative to the deliverables and whether the milestone was earned at least in part based on the Company’s performance. Revenues from substantive milestones, if they are nonrefundable, are recognized as revenue upon successful accomplishment of the milestones. Clinical and regulatory milestones are deemed non-substantive if they are based solely on the collaborator’s performance. Non-substantive milestones will be recognized when achieved to the extent the Company has no remaining performance obligations under the arrangement. Milestone payments earned upon achievement of commercial milestone events will be recognized when earned.
	The Company may also receive royalties at tiered, low- to mid-teen percentage rates on net sales and has the option to participate in the development and commercialization of certain products in the United States. The royalty payments are recognized as revenue in the period in which they are earned. No other milestone or royalty payments under the agreement have been earned.

Property_and_Equipment

Property and Equipment	12 Months Ended
	Dec. 31, 2014
Property, Plant and Equipment [Abstract]
Property and Equipment	4. Property and Equipment
	Property and equipment consists of the following (in thousands):
		December 31,
		2014			2013
	Laboratory equipment	$	6,885		$	5,754
	Computer equipment and software		1,458			1,056
	Leasehold improvements		97			97
	Furniture and fixtures		345			342
	Office equipment		218			218
	Construction in progress		2,941			482
	Total property and equipment		11,944			7,949
	Less accumulated depreciation		(5,558	)		(4,191	)
	Total property and equipment, net	$	6,386		$	3,758
	Depreciation expense for the years ended December 31, 2014, 2013 and 2012 was $1.4 million, $1.4 million and $1.2 million, respectively.

Accrued_Expenses_and_Other_Cur

Accrued Expenses and Other Current Liabilities	12 Months Ended
	Dec. 31, 2014
Payables and Accruals [Abstract]
Accrued Expenses and Other Current Liabilities	5. Accrued Expenses and Other Current Liabilities
	Accrued expenses and other current liabilities consist of the following (in thousands):
		December 31,
		2014		2013
	Accrued compensation	$	5,689	$	3,642
	Accrued contracted research and development costs		7,340		2,484
	Accrued professional fees		549		320
	Accrued other		442		140
	Total	$	14,020	$	6,586

Commitments_and_Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2014
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	6. Commitments and Contingencies
	Operating Lease
	On September 15, 2014, the Company entered into an operating lease agreement (the “Lease”) for approximately 74,500 square feet of office and laboratory space located at 88 Sidney Street, Cambridge, Massachusetts. Concurrently, the Company also entered into an agreement to terminate its existing lease under which the Company currently leases approximately 38,500 square feet of office and laboratory space located at 38 Sidney Street, Cambridge, Massachusetts. On November 21, 2014, the Company entered into a first amendment to the Lease (the “Lease Amendment”) to expand the rentable square footage of the leased space at 88 Sidney Street to approximately 113,200 square feet.
	The date on which the Company will become responsible for paying rent under the Lease, as amended by the Lease Amendment (the “Commencement Date”) will be the earlier of May 15, 2015 or the date upon which the Company first begins conducting business at the new location. The Company’s existing lease at 38 Sidney Street will terminate thirty days after the Commencement Date. The initial lease term will be for a period of seven years from the Commencement Date. At the end of the lease term, the Company has the option to extend the Lease for two consecutive terms of five years at the fair market rent at the time of the extension. The lease agreement includes rent escalation clauses and a tenant improvement allowance of $16.5 million. The Company gained physical access to the leased space in September 2014 and began to record rent expense on a straight-line basis over the effective term of the Lease. The Company gained physical access to the expanded space in November 2014. The Company provided a standby letter of credit of $2.2 million as security for its obligations under the Lease in December 2014. The Company was not required to maintain any cash collateral for the standby letter of credit due to its good standing with the lender.
	At December 31, 2014, the Company recorded approximately $2.3 million in tenant improvement allowance to be received from the Company’s landlord and is recorded as construction in progress within property and equipment, net, and in deferred rent and deferred rent, net of current portion, in the consolidated balance sheets. The deferred rent will be recorded as a reduction in rent expense ratably over the lease term.
	Future annual minimum lease payments due under non-cancellable operating leases at December 31 of each year are as follows (in thousands):
	2015	$	4,990
	2016		6,685
	2017		6,842
	2018		7,004
	2019		7,169
	Thereafter		17,691
		$	50,381
	Rent expense was $3.7 million, $2.2 million and $2.2 million for the years ended December 31, 2014, 2013 and 2012, respectively. The operating leases require the Company to share in prorated operating expenses and property taxes based upon actual amounts incurred; those amounts are not fixed for future periods and, therefore, are not included in the future commitments listed above.
	Lead Program License Agreement
	In August 2012, the Company entered into a license agreement pursuant to which the licensor granted the Company a worldwide exclusive license to certain intellectual property rights for the development of diagnostic products to detect the metabolism of certain cancers. The Company is obligated to pay the licensor up to $100,000 in milestone payments, contingent upon the issuance of certain patents. For each product developed under the agreement the Company has the right to elect to develop and commercialize the product or to grant the licensor an exclusive license to develop and commercialize the product. Under the agreement, the applicable party will pay to the other party a royalty based on worldwide net sales of products. As of December 31, 2014, the Company accrued $50,000 related to milestones achieved; however, there have been no sales of products licensed.
	The term of the agreement will continue, unless earlier terminated by either party, until the expiration of the last-to-expire issued patent. Either party may terminate the agreement in the event of the failure of the other party to make required payments under the agreement or an uncured material breach by the other party. In addition, the licensor may terminate the agreement if the Company becomes insolvent or challenges certain licensed patent rights.
	Other Program License Agreements
	The Company has entered into various cancelable license agreements for certain technology. None of the Company’s lead product candidates utilize technology covered by these licenses. In consideration for the licensed rights the Company made up-front payments totaling $470,000 and issued a total of 162,545 shares of common stock to certain licensors. During the year ended December 31, 2014, the Company paid annual maintenance payments totaling $42,000 to certain of the licensors, which are recorded as research and development expense. The Company has the option to renew these licenses on an annual basis in exchange for payments approximating $45,000 for 2015. The Company could be required to make patent-related, clinical development, regulatory and sales-based milestones of up to $0.1 million, $1.6 million, $5.4 million and $4.2 million, respectively, to the licensors. The license agreements also require the Company to remit royalties in amounts ranging from 0.5% to 2.5% based on net sales of products utilizing the licensed technology. The Company is also required to make payments in amounts ranging from 7.0% to 25.0% for non-royalty income received from any sublicense of the rights granted to the Company under the agreements. Total license expense incurred under the license agreements amounted to approximately $83,000, $72,000 and $30,000 during the years ended December 31, 2014, 2013 and 2012, respectively. The Company incurred $50,000 and $25,000 in patent-based milestones expenses in 2014 and 2013, respectively. The Company did not pay any milestones in 2012 and the Company has paid no royalties to date.
	Milestone Payment Agreements
	The Company entered into an agreement with a service provider to receive discounted upfront labor costs for a defined program in consideration of a milestone payment in five years from the Effective Date, as defined in the agreement. The milestone is dependent on the Company declaring a development candidate within the five-year contract term and is dependent on the origins of the development candidate. If the development candidate is derived from a new chemical class created by the service provider, the milestone will be two times the discounted upfront labor costs. If the development candidate is not derived from a new chemical class created by the service provider, the milestone will be equal to the discount on services provided to date. No milestone payment is due if no development candidate is declared within the five year period. In addition, should no development candidate be declared within three years and the Company remains active with the program, the service provider may, at its discretion, elect to request reimbursement of the discount on services provided to date and forgo the milestone payment. The election must be provided in writing within thirty days of the end of the three-year period. The accumulated discounted labor costs through December 31, 2014 are approximately $1.0 million. The Company has not accrued for the accumulated discount under the reimbursement option as the Company is currently unable to determine the probability of a development candidate being declared within a three-year period.
	The Company entered into an agreement to utilize certain technologies and services in exchange for agreed upon rates over a period of time. The Company made an up-front payment of approximately $0.1 million upon execution of the agreement. The agreement includes milestone payments related to regulatory and preclinical events. The Company paid $0.3 million in regulatory milestones in the year ended December 31, 2014 and paid $0.2 million in preclinical milestones and $0.2 million in regulatory milestones in the year ended December 31, 2013. The Company did not pay any milestones in the year ended December 31, 2012. Total expenses incurred under this agreement amounted to approximately $0.3 and $0.4 million in the years ended December 31, 2014 and 2013, respectively. There were no expenses incurred under this agreement in the year ended December 31, 2012. As of December 31, 2013, the Company no longer had any milestone payment obligation remaining under this agreement.
	Other Agreement
	On November 10, 2009, the Company entered into a research funding agreement whereby the Company received upfront grant consideration to be utilized in the development and testing of brain cancer therapies in exchange for three separate $178,538 milestone payments. The milestones can be earned over a ten-year period from the agreement date and are dependent on the Company successfully commercializing a brain cancer therapy product. The Company will make separate milestone payments when it accumulates net profits of $5.0 million, $50.0 million and $250.0 million, respectively, from sales of the product. As of December 31, 2014, the Company has not commercialized a brain cancer therapy product.
	Legal Contingencies
	From time to time, the Company may be involved in disputes and legal proceedings in the ordinary course of its business. These proceedings may include allegations of infringement of intellectual property, employment or other matters. The Company does not have any ongoing legal proceedings that, based on management estimates, could have a material effect on the Company’s consolidated financial statements.

Convertible_Preferred_Stock

Convertible Preferred Stock	12 Months Ended
	Dec. 31, 2014
Equity [Abstract]
Convertible Preferred Stock	7. Convertible Preferred Stock
	In 2008 and 2009, the Company sold a total of 33,188,889 shares of Series A convertible preferred stock to investors at $1.00 per share, resulting in aggregate proceeds of $33.1 million, including the conversion of the principal and interest on $2.0 million of convertible notes.
	In April 2010, the Company executed a strategic collaboration agreement with Celgene Corporation (Note 3). In connection with the Celgene Agreement, the Company sold 5,190,551 shares of Series B convertible preferred stock (Series B Preferred Stock) to Celgene at $1.70 per share, resulting in aggregate proceeds of $8.8 million. The Company determined the fair value per share of the Series B Preferred Stock on the date of issuance to be $1.11 and has considered the premium paid over the fair value of the Series B Preferred Stock to be additional consideration under the Celgene Agreement. Refer to Note 3 for further discussion of the treatment of the implied premium on the Series B Preferred Stock.
	In November 2011, the Company completed a Series C convertible preferred stock financing, pursuant to which the Company sold 15,882,389 shares of Series C convertible preferred stock to investors at $4.91 per share, resulting in aggregate proceeds of $78.0 million. The shares of Series C convertible preferred stock included 7,395,829 shares of Series C-1 convertible preferred stock (the C-1 Preferred Stock) and 8,486,560 shares of Series C-2 convertible preferred stock (the C-2 Preferred Stock) (collectively, the Series C Preferred Stock).
	The Company assessed the Series A, B and C Preferred Stock (collectively, the “Preferred Stock”) for any embedded derivatives that would require bifurcation from the Preferred Stock and receive separate accounting treatment. No embedded derivatives were identified that would require bifurcation.
	In connection with the closing of the IPO, all of the Company’s outstanding convertible Preferred Stock automatically converted to common stock, resulting in an additional 19,731,564 shares of common stock of the Company becoming outstanding. Further, under the terms of the certificate of incorporation, the Board of Directors is authorized to direct the Company to issue shares of Preferred Stock in one or more series without stockholder approval. The Board of Directors has the discretion to determine the rights, preferences, privileges and restrictions, including voting rights, dividend rights, conversion rights, redemption privileges and liquidation preferences, of each series of Preferred Stock. As of December 31, 2014, there are no shares of Preferred Stock outstanding.
	Prior to the IPO, the holders of Series A, Series B, and Series C Preferred Stock were entitled to receive cumulative dividends at the rate of $0.06, $0.10, and $0.294666 per share per annum, respectively, in preference to any dividends on common stock, when, as, and if declared by the Board of Directors. These dividends were cumulative and accrued whether or not declared. As of December 31, 2012, dividends accrued but unpaid were $7.8 million for Series A Preferred Stock, $1.4 million for Series B Preferred Stock, and $5.3 million for Series C Preferred Stock. Immediately prior to the IPO, dividends accrued but unpaid were $8.9 million for Series A Preferred Stock, $1.7 million for Series B Preferred Stock, and $8.0 million for Series C Preferred Stock.

Common_Stock

Common Stock	12 Months Ended
	Dec. 31, 2014
Equity [Abstract]
Common Stock	8. Common Stock
	In connection with the IPO, the Company’s Board of Directors and stockholders approved a 1-for-2.75 reverse stock split of the Company’s common stock. The reverse stock split became effective on July 11, 2013. All share and per share amounts in the consolidated financial statements have been retroactively adjusted for all periods presented to give effect to this reverse stock split, including reclassifying an amount equal to the reduction in par value of common stock to additional paid-in capital. The Company’s common stock has the following characteristics:
	Voting
	The holders of shares of common stock are entitled to one vote for each share of common stock held at all meetings of stockholders and written actions in lieu of meetings.
	Dividends
	The holders of shares of common stock are entitled to receive dividends, if and when declared by the board of directors and are subject to any preferential dividend or other right of any then outstanding preferred stock. No dividends have been declared or paid since the Company’s inception.
	Liquidation
	The holders of shares of common stock are entitled to share ratably in the Company’s assets available for distribution to stockholders, in the event of any voluntary or involuntary liquidation, subject to any preferential or other rights of any then outstanding preferred stock.

ShareBased_Payments

Share-Based Payments	12 Months Ended
	Dec. 31, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Share-Based Payments	9. Share-Based Payments
	Stock Incentive Plans
	Prior to the IPO, the Company maintained the 2007 Stock Incentive Plan (the “2007 Plan”) for employees, directors, consultants, and advisors to the Company. The 2007 Plan provided for the grant of incentive and non-qualified stock options and restricted stock grants as determined by the Board of Directors. Under the 2007 Plan, the Company reserved 5,079,462 shares of common stock and, at December 31, 2014 and December 31, 2013, the Company had no shares available for future issuance.
	In June 2013, the Company’s Board of Directors adopted, and in July 2013, the Company’s stockholders approved, the 2013 Stock Incentive Plan (the “2013 Plan”). The 2013 Plan became effective upon the closing of the IPO and provides for the grant of incentive stock options, non-qualified stock options, stock appreciation rights, restricted stock awards, restricted stock units and other stock-based awards. The Company will grant no further stock options or other awards under the 2007 Plan. Any options or awards outstanding under the 2007 Plan at the time of adoption of the 2013 Plan remain outstanding and effective. As of December 31, 2014, the total number of shares reserved under all equity plans is 4,445,395, and the Company had 639,975 shares available for future issuance under such plans. The 2013 Plan provides for an annual increase, to be added on the first day of each fiscal year, beginning with the fiscal year ending December 31, 2014 and continuing until the expiration of the 2013 Plan, equal to the lesser of (i) 2,000,000 shares of common stock, (ii) 4% of the outstanding shares of common stock on such date or (iii) an amount determined by the Company’s Board of Directors. On January 1, 2015, the annual increase for the 2013 Plan resulted in an additional 1,484,020 shares authorized for issuance.
	During the year ended December 31, 2014 the Company did not grant any stock options to consultants and advisors of the Company. During the year ended December 31, 2013, consultants and advisors were granted 27,272 stock options. These awards are included within the following table which summarizes the stock option activity of all stock incentive plans for the year ended December 31, 2014.
		Number of			Weighted-			Weighted-			Aggregate
		Stock			Average			Average			Intrinsic
		Options			Exercise			Remaining			Value (in
					Price			Contractual			thousands)
								Term
								(in years)
	Outstanding at December 31, 2013		3,846,168		$	4.14			7.43		$	76,189
	Granted		1,339,483			39.49
	Exercised		(1,298,775	)		1.79
	Forfeited/Expired		(81,456	)		13.22
	Outstanding at December 31, 2014		3,805,420		$	17.19			7.58		$	360,935
	Exercisable at December 31, 2014		1,577,101		$	2.58			5.82		$	172,636
	Vested and expected to vest at December 31, 2014		3,516,545		$	16.98			7.5		$	334,292
	The weighted-average grant date fair value of options granted was $27.26, $9.96 and $2.09 during the years ended December 31, 2014, 2013 and 2012, respectively. The total intrinsic value of options exercised was $60.4 million, $4.2 million and $0.8 million during the years ended December 31, 2014, 2013 and 2012, respectively.
	At December 31, 2014, the total unrecognized compensation expense related to unvested stock option awards, including estimated forfeitures, was $32.5 million, which the Company expects to recognize over a weighted-average period of approximately 3.1 years. The Company also has unrecognized stock-based compensation expense of $1.1 million related to stock options with performance-based vesting criteria that are not considered probable of achievement as of December 31, 2014.
	Restricted Stock Units
	The Company may grant awards of restricted stock units (“RSUs”) to non-employee directors, members of the management team and employees on a discretionary basis pursuant to the 2013 Plan. Each RSU entitles the holder to receive, at the end of each vesting period, a specified number of shares of the Company’s common stock. The total number of unvested RSUs at December 31, 2014 was 10,000. The issued and outstanding RSUs vest on the first anniversary of the grant date.
	The fair value of the RSUs granted in the year ended December 31, 2014 was approximately $0.5 million. No RSUs were granted in prior years. The Company recorded stock-based compensation expense related to RSUs of $0.1 million for the year ended December 31, 2014. No compensation expense related to RSUs was recorded in prior years. As of December 31, 2014, there was approximately $0.4 million of total unrecognized compensation expense related to RSUs, which is expected to be recognized over a period of nine months.
	Unvested RSU activity for the year ended December 31, 2014 is summarized as follows:
	Unvested shares at December 31, 2013		—
	Granted		10,000
	Vested		—
	Unvested shares end of period		10,000
	Restricted Stock and Early Exercise of Stock Options
	Certain employees were granted restricted stock and certain directors were permitted to early exercise their stock options, upon approval by the Company’s Board, at which time the awards became subject to restricted stock agreements. These shares of restricted stock granted upon early exercise of the options are subject to the same vesting provisions as the original stock option awards. Accordingly, the Company has recorded the exercise proceeds from early exercises as a restricted stock liability in the consolidated balance sheets. The restricted stock liability is reclassified into stockholders’ equity as the restricted stock and options vest.
	Unvested restricted stock activity for the year ended December 31, 2014 is summarized as follows:
	Unvested shares at December 31, 2013		23,295
	Granted		—
	Vested		(14,773	)
	Forfeited		—
	Unvested shares at December 31, 2014		8,522
	There were no shares of restricted stock granted during the years ended December 31, 2014 and 2013. The weighted-average purchase price of restricted stock granted during the year ended December 31, 2012 was $0.91. The fair value of awards vested during the years ended December 31, 2014, 2013 and 2012 was $0.7 million, $1.7 million and $0.5 million, respectively.
	Performance-Based Stock Option Grants
	During the year ended December 31, 2014, no options to purchase shares of common stock that contain performance-based or a combination of performance-based and service-based vesting criteria were granted by the Company. During the year ended December 31, 2013, the Company granted options to purchase 355,454 shares of common stock, which contain performance-based and service-based vesting criteria, to employees. Performance-based vesting criteria for these options primarily relate to milestone events specific to the Company’s corporate goals, including but not limited to certain preclinical and clinical development milestones related to the Company’s product candidates. Stock-based compensation expense associated with these performance-based stock options is recognized if the performance condition is considered probable of achievement using management’s best estimates. As of December 31, 2014, certain of the performance-based milestones had been achieved and the achievement of certain other milestones have been deemed probable and therefore the related expense has either been fully recognized or is being recognized over the remaining service period. The achievement of the remaining milestones was deemed to be not probable as of December 31, 2014 and therefore no expense has been recognized related to these awards. During the years ended December 31, 2014 and 2013, the Company recognized stock-based compensation expense of $0.9 million, related to stock options with performance-based vesting criteria. No stock-based compensation expense related to stock options with performance-based criteria was recorded during the year ended December 31, 2012.
	Stock-Based Compensation Expense
	The fair value of each stock option granted to employees is estimated on the date of grant using the Black-Scholes option-pricing model. The following table summarizes the weighted average assumptions used in calculating the grant date fair value of the awards:
		Years Ending December 31,
		2014			2013			2012
	Risk-free interest rate		1.83	%		1.31	%		1.09	%
	Expected dividend yield		—			—			—
	Expected term (in years)		6.03			6.42			6.08
	Expected volatility		78.61	%		92.49	%		97.75	%
	Risk-Free Rate
	The risk-free rate is based on the yield curve of U.S. Treasury securities with periods commensurate with the expected term of the options being valued.
	Dividends
	The Company has never paid, and does not anticipate paying, any cash dividends in the foreseeable future, and therefore uses an expected dividend yield of zero in the option-pricing model.
	Volatility
	Since the Company was privately held through July 2013, it does not have the relevant company-specific historical data to support its expected volatility. As such, the Company has used a weighted-average of expected volatility based on the volatilities of a representative group of publicly-traded biopharmaceutical companies. For purposes of identifying representative companies, the Company considered characteristics such as number of product candidates in earlier stages of product development, area of therapeutic focus, length of trading history, companies’ stage of life cycle, size, and relevant financial metrics. The expected volatility has been determined using a weighted-average of the historical volatilities of the representative group of companies for a period equal to the expected term of the option grant. The Company intends to continue to consistently apply this process using similar entities until a sufficient amount of historical information regarding the volatility of the Company’s own share price becomes available or until circumstances change, such that the identified entities are no longer representative companies. In the latter case, more suitable, similar entities whose share prices are publicly available would be utilized in the calculation.
	Expected Term
	The Company uses the “simplified method” as prescribed by the Securities and Exchange Commission Staff Accounting Bulletin No. 107, Share Based Payments, to estimate the expected term of stock option grants. Under this approach, the weighted-average expected life is presumed to be the average of the contractual term (ten years) and the weighted-average vesting term of the Company’s stock options, taking into consideration multiple vesting tranches. The Company utilizes this method due to lack of historical exercise data and the plain-vanilla nature of the Company’s share-based awards.
	Forfeitures
	Forfeitures are required to be estimated at the time of grant and revised, if necessary, in subsequent periods if actual forfeitures differ from those estimates. The Company based its estimate of forfeitures on data from a representative group of publicly-traded biopharmaceutical companies, as the Company does not currently have sufficient history, and records the stock-based compensation expense only on the awards that are expected to vest. To date forfeitures have been less than 5.0% of total grants.
	2013 Employee Stock Purchase Plan
	In June 2013, the Company’s Board of Directors adopted, and in July 2013 the Company’s stockholders approved, the 2013 Employee Stock Purchase Plan (the “2013 ESPP”). The 2013 ESPP will be administered by the Company’s Board of Directors or by a committee appointed by the Company’s Board of Directors. Under the 2013 ESPP, each offering period is six months, at the end of which employees may purchase shares of common stock through payroll deductions made over the term of the offering period. The per-share purchase price at the end of each offering period is equal to 85% of the closing price of one share of the Company’s common stock at the beginning or end of the offering period, whichever is lower, subject to Internal Revenue Service limits. The first offering period was initiated on September 1, 2014. No shares were issued during the year ended December 31, 2014 under the 2013 ESPP. The 2013 ESPP initially provides participating employees with the opportunity to purchase up to an aggregate of 327,272 shares of the Company’s common stock.
	The Company recorded $0.1 million of stock-based compensation expense for the year ended December 31, 2014 related to the 2013 ESPP. No stock-based compensation expense related to the 2013 ESPP was recorded during the year ended December 31, 2013.

Income_Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Income Taxes	10. Income Taxes
	The provision (benefit) for income taxes is as follows for the years ended December 31, 2014, 2013 and 2012 (in thousands):
		Years Ended December 31,
		2014			2013			2012
	Current:
	Federal		$  (444	)		$  (3,263	)	$	(9,531	)
	State		18			—			—
	Total current		(426	)		(3,263	)		(9,531	)
	Deferred:
	Federal		—			3,842			6,707
	State		—			—			—
	Total deferred		—			3,842			6,707
	Total	$	(426	)	$	579		$	(2,824	)
	A reconciliation of the expected income tax benefit (expense) computed using the federal statutory income tax rate to the Company’s effective income tax rate is as follows for the years ended December 31, 2014, 2013 and 2012:
		December 31,
		2014			2013			2012
	Income tax benefit computed at federal statutory tax rate		35.00%			35.00%			35.00%
	State taxes, net of federal benefit		4.90			4.02			7.07
	Change in valuation allowance		(35.80)			(38.02)			(28.08)
	General business credits and other credits		(1.90)			0.52			0.12
	Permanent differences		(2.10)			(1.75)			(0.63)
	Interest and penalties		—			(1.19)			(1.95)
	Other		0.80			(0.07)			0.79
	Total		0.90%			(1.49)%			12.32%
	During the years ended December 31, 2013 and 2012, the Company incurred $0.6 million for interest and penalties related to the non-payment of U.S. federal income taxes, respectively. No interest and penalties were incurred during the year ended December 31, 2014.
	Deferred income taxes reflect the net tax effects of temporary differences between the carrying amount of assets and liabilities for financial reporting purposes and the amounts used for income tax purposes. Significant components of the Company’s deferred tax assets and liabilities for the years ended December 31, 2014 and 2013 are as follows (in thousands):
		2014			2013
	Deferred tax assets:
	Net operating loss carryforwards	$	53,545		$	21,901
	Deferred revenue		8,713			23,171
	Tax credit carryforwards		3,159			637
	Purchased intangible assets		137			147
	Stock-based compensation		3,461			768
	Deferred rent		1,622			138
	Other		325			231
	Total deferred tax assets		70,962			46,993
	Valuation allowance		(69,433)			(46,549)
	Total deferred tax assets	$	1,529		$	444
	Deferred tax liabilities:
	Depreciation and amortization	$	(1,529)		$	(444)
	Total deferred tax liabilities	$	(1,529)		$	(444)
	Net deferred tax asset	$	-		$	-
	As of December 31, 2014, the Company had net operating loss carryforwards available to reduce federal and state income taxes of approximately $166.9 million and $208.1 million, respectively. If not utilized, these carryforwards expire at various dates through 2034. At December 31, 2014 the Company also has available research and development tax credits for federal and state income tax purposes of approximately $2.5 million and $1.1 million, respectively.
	Utilization of the net operating loss carryforwards and credits may be subject to annual limitations as prescribed by federal and state statutory provisions. The annual limitation may result in the expiration of net operating loss carryforwards prior to its utilization.
	The Company has federal and state net operating loss carry forward (NOLs) related to stock compensation in the amount of $39.1 million and $39.2 million, respectively, that is not included in deferred tax assets. When the excess stock-based compensation related to NOL carryover tax assets are realized, the benefit will be credited directly to stockholders’ equity.
	Utilization of the NOLs and tax credit carryforwards may be subject to a substantial annual limitation due to ownership change limitations that have occurred previously or that could occur in the future, as provided by Section 382 of the Internal Revenue Code of 1986 (“Section 382”), as well as similar state provisions. Ownership changes may limit the amount of NOLs and tax credit carryforwards that can be utilized annually to offset future taxable income and tax, respectively. In general, an ownership change, as defined by Section 382, results from transactions that increase the ownership of 5% shareholders in the stock of a corporation by more than 50 percent in the aggregate over a three-year period. During 2011, the Company completed a study through December 31, 2011, to determine whether any ownership change has occurred since the Company’s formation and has determined that transactions have resulted in two ownership changes, as defined by Section 32. The impact of the ownership changes have been reflected in the Company’s deferred tax assets in the table above. There could be additional ownership changes in the future that could further limit the amount of NOLs and tax credit carryforwards that the Company can utilize.
	As required by ASC 740, management of the Company has evaluated the positive and negative evidence bearing upon the realizability of its deferred tax assets. During the year ended December 31, 2011, management determined that it was more likely than not that it would realize a portion of its deferred tax assets because of the Company’s ability to carryback future losses for U.S. federal income tax purposes. As a result, the Company reversed approximately $10.7 million of the valuation allowance on its deferred tax assets in the year ended December 31, 2011, representing the amount of deferred tax assets that will be realized in 2012 and 2013, the years available for carryback. The Company utilized certain of the deferred tax assets, including net operating losses, generated in the year ended December 31, 2013 to reduce its federal income taxes payable in the years ended December 31, 2013 and 2012. For the remainder of the Company’s deferred tax assets, management determined that it is more likely than not that the Company may not realize the benefit and has recorded a valuation allowance of approximately $69.4 million and $46.5 million at December 31, 2014 and 2013, respectively. The valuation allowance increased by $22.9 million in the year ended December 31, 2014.
	The Company applies the accounting guidance in ASC 740 related to accounting for uncertainty in income taxes. The Company’s reserves related to taxes are based on a determination of whether, and how much of, a tax benefit taken by the Company in its tax filings or positions is more likely than not to be realized following resolution of any potential contingencies present related to the tax benefit. As of December 31, 2014 and 2013 the Company had no unrecognized tax benefits. The Company recognizes interest and penalties related to uncertain tax positions in income tax expense.
	The statute of limitations for assessment by the Internal Revenue Service (IRS) and state tax authorities is open for tax years ending December 31, 2014, 2013, 2012, 2011 and 2010 although carryforward attributes that were generated for tax years prior to 2010 may still be adjusted upon examination by the IRS or state tax authorities if they either have been, or will be, used in a future period. There are currently no federal or state audits in progress.

Defined_Contribution_Benefit_P

Defined Contribution Benefit Plan	12 Months Ended
	Dec. 31, 2014
Compensation and Retirement Disclosure [Abstract]
Defined Contribution Benefit Plan	11. Defined Contribution Benefit Plan
	The Company sponsors a 401(k) retirement plan, in which substantially all of its full-time employees are eligible to participate. Participants may contribute a percentage of their annual compensation to this plan, subject to statutory limitations. The Company did not provide any contributions to this plan during the years ended December 31, 2014, 2013 or 2012.
	Beginning January 1, 2015, the Company will make matching contributions equal to 50% of the employee’s contributions, subject to a maximum of 6% of eligible compensation.

Selected_Quarterly_Financial_D

Selected Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Selected Quarterly Financial Data (Unaudited)	12. Selected Quarterly Financial Data (Unaudited)
	2014	First		Second		Third		Fourth
		Quarter		Quarter		Quarter		Quarter
		(In thousands, except per share data)
	Total revenue	$	8,411	$	8,411	$	33,900	$	14,636
	Income (loss) from operations		(12,284)		(18,330)		3,208		(26,727)
	Net income (loss)		-12,248		-18,296		3,704		-26,664
	Net income (loss) applicable to common stockholders		-12,248		-18,296		3,704		-26,664
	Net income (loss) per share applicable to common stockholders – basic		-0.39		-0.54		0.11		-0.76
	Net income (loss) per share applicable to common stockholders – diluted		-0.39		-0.54		0.1		-0.76
	2013	First		Second		Third		Fourth
		Quarter		Quarter		Quarter		Quarter
		(In thousands, except per share data)
	Total revenue	$	6,268	$	6,268	$	6,268	$	6,744
	Loss from operations		-7,046		-8,526		-11,069		(12,242)
	Net loss		-7,228		-8,620		-11,177		-12,382
	Net loss applicable to common stockholders		-9,025		-10,418		-11,744		-12,382
	Net loss per share applicable to common stockholders – basic and diluted		-2.47		-2.8		-0.52		-0.4

Summary_of_Significant_Account1

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
Principles of Consolidation	Principles of Consolidation
	The Company’s consolidated financial statements include the Company’s accounts and the accounts of the Company’s wholly owned subsidiary, Agios Securities Corporation. All intercompany transactions have been eliminated in consolidation. The consolidated financial statements have been prepared in conformity with U.S. generally accepted accounting principles (“GAAP”).
Use of Estimates	Use of Estimates
	The preparation of financial statements in conformity with U.S. GAAP requires the Company’s management to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenue and expenses during the reporting period. The Company bases its estimates on historical experience and on various other assumptions that are believed to be reasonable under the circumstances. Actual results could differ from those estimates.
	Prior to the IPO, the Company utilized significant estimates and assumptions in determining the fair value of its common stock. The board of directors determined the estimated fair value of the Company’s common stock based on a number of objective and subjective factors, including external market conditions affecting the biotechnology industry sector and the prices at which the Company sold shares of convertible preferred stock, the superior rights and preferences of securities senior to the Company’s common stock at the time and the likelihood of achieving a liquidity event, such as an initial public offering or sale of the Company. The Company utilized various valuation methodologies in accordance with the framework of the American Institute of Certified Public Accountants, or AICPA, Audit and Accounting Practice Aid Series: Valuation of Privately Held Company Equity Securities Issued as Compensation, or the AICPA Practice Aid, to estimate the fair value of its common stock and in performing retrospective valuation analyses for certain grant dates prior to the IPO. The methodologies included the option pricing method utilizing the back-solve method (a form of the market approach defined in the AICPA Practice Aid) and the probability-weighted expected return method based upon the probability of occurrence of certain future liquidity events such as an initial public offering or sale of the Company. Each valuation methodology included estimates and assumptions that require the Company’s judgment. Significant changes to the key assumptions used in the valuations could result in different fair values of common stock at each valuation date.
Revenue Recognition	Revenue Recognition
	The Company recognizes revenue in accordance with the Financial Accounting Standards Board’s (“FASB”) Accounting Standards Codification (“ASC”) 605, Revenue Recognition. Accordingly, revenue is recognized for each unit of accounting when all of the following criteria are met:
		•		persuasive evidence of an arrangement exists;
		•		delivery has occurred or services have been rendered;
		•		the seller’s price to the buyer is fixed or determinable; and
		•		collectability is reasonably assured.
	The Company’s revenue has primarily been generated from a Discovery and Development Collaboration and License Agreement with Celgene (“the Celgene Agreement”) and from research grant agreements.
	Collaboration and License Revenue
	In January 2011, the Company adopted the FASB Accounting Standards Update (“ASU”) No. 2009-13, Multiple-Element Revenue Arrangements (“ASU No. 2009-13”), on a prospective basis for all revenue arrangements entered into or materially modified after the adoption date. The Celgene Agreement was entered into prior to January 1, 2011 and the Company initially applied its prior accounting policy with respect to the arrangement. Under this policy, when evaluating multiple element arrangements, the Company considered whether the components of the arrangement should be accounted for individually as separate units of accounting if (1) the elements have stand-alone value, and (2) the Company is able to estimate the fair value of all undelivered elements under the arrangement.
	In July 2014, the Company amended its collaboration agreement with Celgene. As a result of the amendment, the Company was required to reevaluate the agreement under ASU No. 2009-13. The amendment was determined to be a material modification pursuant to ASU No. 2009-13, and the Company began recognizing revenue for the arrangement under this guidance on a prospective basis, as discussed further in Note 3.
	Pursuant to ASU 2009-13, revenue arrangements where multiple products or services are sold together are evaluated to determine if each deliverable represents a separate unit of accounting based on the following criteria:
		•		Delivered item or items have value to the customer on a standalone basis, and
		•		If the arrangement includes a general right of return relative to the delivered item or items, delivery or performance of the undelivered item or items is considered probable and substantially in the control of the vendor.
	The arrangement consideration is then allocated to each separately identified unit of accounting based on the relative selling price, using the Company’s best estimate of selling price of each deliverable. The provisions of ASC 605-25, Multiple-Element Arrangements are then applied to each unit of accounting to determine the appropriate revenue recognition. In the event that a deliverable of a multiple element arrangement does not represent a separate unit of accounting, the Company recognizes revenue from the combined units of accounting over the term of the related contract or as undelivered items are delivered, as appropriate.
	In determining the current and noncurrent classification of deferred revenue, the Company considers the total consideration expected to be earned in the next twelve months for services to be performed under certain units of accounting and the estimated proportional performance and timing of delivery of certain deliverables to determine the deferred revenue balance that will remain twelve months from the balance sheet date.
	Revenue is recognized under the proportional performance method for certain units of accounting. The amount recognized is determined based on the consideration allocated to each unit of accounting based on the ratio of the level of effort incurred to date compared to the total estimated level of effort required to complete the Company’s performance obligations under the unit of accounting. Determining the total estimated level of effort required to complete all performance obligations requires management judgment and estimation, including assumptions regarding future operating performance, the timelines of the clinical trials approvals and the estimated patient populations.
	In January 2011, the Company adopted the FASB’s ASU No. 2010-17, Revenue Recognition – Milestone Method, on a prospective basis. ASU 2010-17 provides guidance in applying the milestone method of revenue recognition to research or development arrangements. Under this guidance, management may recognize revenue contingent upon the achievement of a milestone in its entirety in the period in which the milestone is achieved, only if the milestone meets all the criteria within the guidance to be considered substantive. In accordance with ASU 2010-17, at the inception of each arrangement that includes milestone payments, the Company evaluates each contingent payment on an individual basis to determine whether they are considered substantive milestones, specifically reviewing factors such as the degree of certainty in achieving the milestone, the research and development risk and other risks that must be overcome to achieve the milestone, as well as the level of effort and investment required and whether the milestone consideration is reasonable relative to all deliverables and payment terms in the arrangement. This evaluation includes an assessment of whether (a) the consideration is commensurate with either (1) the entity’s performance to achieve the milestone, or (2) the enhancement of the value of the delivered item(s) as a result of a specific outcome resulting from the entity’s performance to achieve the milestone, (b) the consideration relates solely to past performance and (c) the consideration is reasonable relative to all of the deliverables and payment terms within the arrangement.
	Revenues from milestones, if they are nonrefundable and deemed substantive, are recognized upon achievement of the milestones. The Company recognizes revenue associated with the non-substantive milestones upon achievement of the milestone if there are no undelivered elements and the Company has no remaining performance obligations.
	Reimbursement of research and development costs by Celgene is recognized as revenue, provided the Company has determined that it is acting primarily as a principal in the transaction according to the provisions outlined in FASB ASC 605-45, Revenue Recognition – Principal Agent Considerations, the amounts are determinable and collection of the related receivable is reasonably assured.
	Grant Revenue
	Revenue related to research grant agreements is recognized as the underlying services are performed and delivered. Revenues from grants totaled approximately $0.1 million for the year ended December 31, 2012. The Company did not recognize any grant revenue for the years ended December 31, 2014 and 2013.
Research and Development Costs	Research and Development Costs
	Research and development costs are expensed as incurred. Research and development costs include salaries and personnel-related costs, consulting fees, fees paid for contract research services, fees paid to clinical research organizations and other third parties associated with clinical trials, the costs of laboratory equipment and facilities, and other external costs.
	Nonrefundable advance payments for goods or services to be received in the future for use in research and development activities are deferred and capitalized. The capitalized amounts are expensed as the related goods are delivered or the services are performed.
Stock-Based Compensation	Stock-Based Compensation
	The Company accounts for its stock-based compensation awards in accordance with ASC Topic 718, Compensation – Stock Compensation (“ASC 718”). ASC 718 requires all share-based payments to employees, including grants of employee stock options, to be recognized in the consolidated statements of operations based on their grant date fair values. For stock options granted to employees and to members of the board of directors for their services on the board of directors, the Company estimates the grant date fair value of each option award using the Black-Scholes option-pricing model. The use of the Black-Scholes option-pricing model requires management to make assumptions with respect to the expected term of the option, the expected volatility of the common stock consistent with the expected life of the option, risk-free interest rates and expected dividend yields of the common stock. For awards subject to service-based vesting conditions, the Company recognizes stock-based compensation expense, net of estimated forfeitures, equal to the grant date fair value of stock options on a straight-line basis over the requisite service period. For awards subject to both performance and service-based vesting conditions, the Company recognizes stock-based compensation expense using an accelerated recognition method if achievement of the performance criteria is considered probable.
	Share-based payments issued to non-employees are recorded at their fair values, and are revalued at each reporting date and as the equity instruments vest and are recognized as expense over the related service period in accordance with the provisions of ASC Topic 505, Equity. For equity instruments granted to non-employees, the Company recognizes stock-based compensation expense using an accelerated recognition method.
	During the years ended December 31, 2014, 2013, and 2012, the Company recorded stock-based compensation expense for employee and non-employee stock options, the employee stock purchase plan and restricted stock, which was allocated as follows in the consolidated statements of operations (in thousands):
			Years Ended December 31,
			2014			2013		2012
	Research and development expense		$	6,688		$	2,001	$	605
	General and administrative expense			4,818			1,029		137
			$	11,506		$	3,030	$	742
	No related tax benefits were recognized for the years ended December 31, 2014, 2013 and 2012.
Income Taxes	Income Taxes
	Income taxes are recorded in accordance with ASC 740, Accounting for Income Taxes (“ASC 740”), which provides for deferred taxes using an asset and liability approach. The Company recognizes deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the financial statements or tax returns. The Company determines its deferred tax assets and liabilities based on differences between financial reporting and tax bases of assets and liabilities, which are measured using the enacted tax rates and laws that will be in effect when the differences are expected to reverse. Valuation allowances are provided, if based upon the weight of available evidence, it is more likely than not that some or all of the deferred tax assets will not be realized.
	The Company accounts for uncertain tax positions in accordance with the provisions of ASC 740. When uncertain tax positions exist, the Company recognizes the tax benefit of tax positions to the extent that the benefit will more likely than not be realized. The determination as to whether the tax benefit will more likely than not be realized is based upon the technical merits of the tax position as well as consideration of the available facts and circumstances. As of December 31, 2014 and 2013, the Company did not have any uncertain tax positions.
Comprehensive Income (Loss)	Comprehensive Income (Loss)
	Comprehensive income (loss) is defined as the change in equity of a business enterprise during a period from transactions, and other events and circumstances from non-owner sources, and currently consists of net loss and changes in unrealized gains and losses on available-for-sale securities. Accumulated other comprehensive loss consists entirely of unrealized gains and losses from available for sale securities as of December 31, 2014 and 2013.
Cash and Cash Equivalents	Cash and Cash Equivalents
	The Company considers highly liquid investments with a maturity of ninety days or less when purchased to be cash equivalents. Cash equivalents, which consist primarily of money market funds, are stated at fair value.
Marketable Securities	Marketable Securities
	Marketable securities at December 31, 2014 and 2013 consisted primarily of investments in United States Treasuries and certificates of deposit. Management determines the appropriate classification of the securities at the time they are acquired and evaluates the appropriateness of such classifications at each balance sheet date. The Company classifies its marketable securities as available-for-sale pursuant to ASC 320, Investments – Debt and Equity Securities. Marketable securities are recorded at fair value, with unrealized gains and losses included as a component of accumulated other comprehensive income (loss) in stockholders’ equity and a component of total comprehensive loss in the consolidated statements of comprehensive loss, until realized. The fair value of these securities is based on quoted prices for identical or similar assets. Realized gains and losses are included in investment income on a specific-identification basis. There were no realized gains or losses on marketable securities for the years ended December 31, 2014, 2013 and 2012.
	The Company reviews marketable securities for other-than-temporary impairment whenever the fair value of a marketable security is less than the amortized cost and evidence indicates that a marketable security’s carrying amount is not recoverable within a reasonable period of time. Other-than-temporary impairments of investments are recognized in the consolidated statements of operations if the Company has experienced a credit loss, has the intent to sell the marketable security, or if it is more likely than not that the Company will be required to sell the marketable security before recovery of the amortized cost basis. Evidence considered in this assessment includes reasons for the impairment, compliance with the Company’s investment policy, the severity and the duration of the impairment and changes in value subsequent to the end of the period.
	Marketable securities at December 31, 2014 consist of the following (in thousands):
			Amortized			Unrealized		Unrealized			Fair
			Cost			Gains		Losses			Value
	Current:
	Certificates of deposit		$	13,160		$	—	$	(5	)	$	13,155
	U.S. Treasuries			314,866			45		(32	)		314,879
	Non-current:
	U.S. Treasuries			125,447			5		(70	)		125,382
			$	453,473		$	50	$	(107)		$	453,416
	Marketable securities at December 31, 2013 consist of the following (in thousands):
			Amortized			Unrealized		Unrealized			Fair
			Cost			Gains		Losses			Value
	Current:
	Certificates of deposit		$	6,920		$	—	$	(5	)	$	6,915
	U.S. Treasuries			88,287			8		(1	)		88,294
	Non-current:
	U.S. Treasuries			27,113			12		—			27,125
			$	122,320		$	20	$	-6		$	122,334
	At December 31, 2014 and 2013, the Company held both current and non-current investments. Investments classified as current have maturities of less than one year. Investments classified as non-current are those that (i) have maturities of one to two years and (ii) management does not intend to liquidate within the next twelve months, although these funds are available for use and therefore classified as available-for-sale.
	At December 31, 2014 and 2013, the Company held 92 and 33 debt securities, respectively, that were in an unrealized loss position for less than one year. The aggregate fair value of debt securities in an unrealized loss position at December 31, 2014 and 2013 was $236.9 million and $31.7 million, respectively. There were no individual securities that were in a significant unrealized loss position or that had been in an unrealized loss position for greater than one year as of December 31, 2014 and 2013. The Company evaluated its securities for other-than-temporary impairment and considered the decline in market value for the securities to be primarily attributable to current economic and market conditions. It is not more likely than not that the Company will be required to sell the securities, and the Company does not intend to do so prior to the recovery of the amortized cost basis. Based on this analysis, these marketable securities were not considered to be other-than-temporarily impaired as of December 31, 2014 and 2013.
Concentrations of Credit Risk	Concentrations of Credit Risk
	Financial instruments which potentially subject the Company to credit risk consist primarily of cash, cash equivalents and marketable securities. The Company holds these investments in highly rated financial institutions, and, by policy, limits the amounts of credit exposure to any one financial institution. These amounts at times may exceed federally insured limits. The Company has not experienced any credit losses in such accounts and does not believe it is exposed to any significant credit risk on these funds. The Company has no off-balance sheet concentrations of credit risk, such as foreign currency exchange contracts, option contracts or other hedging arrangements.
	The Company is also subject to credit risk from its collaboration receivable. The Company evaluates the creditworthiness of its collaborator and has determined it is credit worthy. To date the Company has not experienced any losses with respect to its collaboration receivable.
Fair Value Measurements	Fair Value Measurements
	The Company records cash equivalents and marketable securities at fair value. ASC Topic 820, Fair Value Measurements and Disclosures, establishes a fair value hierarchy for those instruments measured at fair value that distinguishes between assumptions based on market data (observable inputs) and the Company’s own assumptions (unobservable inputs). The hierarchy consists of three levels:
	Level 1 – Unadjusted quoted prices in active markets for identical assets or liabilities.
	Level 2 – Quoted prices for similar assets and liabilities in active markets, quoted prices in markets that are not active, or inputs which are observable, either directly or indirectly, for substantially the full term of the asset or liability.
	Level 3 – Unobservable inputs that reflect the Company’s own assumptions about the assumptions market participants would use in pricing the asset or liability in which there is little, if any, market activity for the asset or liability at the measurement date.
	The following table summarizes the cash equivalents and marketable securities measured at fair value on a recurring basis as of December 31, 2014 (in thousands):
			Level 1			Level 2		Level 3			Total
	Cash equivalents		$	11,410		$	960	$	—		$	12,370
	Marketable securities:
	Certificates of deposit			—			13,155		—			13,155
	U.S. Treasuries			440,261			—		—			440,261
			$	451,671		$	14,115	$	—		$	465,786
	The following table summarizes the cash equivalents and marketable securities measured at fair value on a recurring basis as of December 31, 2013 (in thousands):
			Level 1			Level 2		Level 3			Total
	Cash equivalents		$	68,792		$	—	$	—		$	68,792
	Marketable securities:
	Certificates of deposit			—			6,915		—			6,915
	U.S. Treasuries			115,419			—		—			115,419
			$	184,211		$	6,915	$	—		$	191,126
	Cash equivalents and marketable securities have been initially valued at the transaction price and subsequently valued, at the end of each reporting period, utilizing third party pricing services or other market observable data. The pricing services utilize industry standard valuation models, including both income and market based approaches and observable market inputs to determine value. The Company validates the prices provided by its third party pricing services by reviewing their pricing methods and obtaining market values from other pricing sources. After completing its validation procedures, the Company did not adjust or override any fair value measurements provided by the pricing services as of December 31, 2014 or 2013.
	The carrying amounts reflected in the consolidated balance sheets for cash, restricted cash, collaboration receivable – related party, prepaid expenses and other current assets, other assets, accounts payable, and accrued expenses approximate their fair values at December 31, 2014 and 2013, due to their short-term nature.
	There have been no changes to the valuation methods during the years ended December 31, 2014 and 2013. The Company evaluates transfers between levels at the end of each reporting period. There were no transfers of assets or liabilities between Level 1 and Level 2 during the years ended December 31, 2014 and 2013. The Company had no financial assets or liabilities that were classified as Level 3 at any point during the years ended December 31, 2014 and 2013.
Collaboration and Other Receivables	Collaboration and Other Receivables
	Collaboration receivables as of December 31, 2014 represent amounts due from Celgene for cost reimbursements of certain on-going phase 1 studies under the Celgene Agreement. As of December 31, 2013, collaboration receivables represented an unbilled collaboration receivable for revenues recognized related to Celgene’s election to extend the term of the initial discovery period from four to five years. Other receivables represent amounts due from the Company’s landlord for reimbursement of tenant improvements under the Company’s lease agreement.
	The Company estimates an allowance for doubtful accounts based on credit worthiness, historical payment patterns, aging of accounts receivable balances, and general economic conditions. As of December 31, 2014 and 2013, the Company had no allowance for doubtful accounts.
Property and Equipment	Property and Equipment
	Property and equipment consist of laboratory equipment, computer equipment and software, leasehold improvements, furniture and fixtures, and office equipment. Property and equipment is stated at cost, and depreciated using the straight-line method over the estimated useful lives of the respective assets:
			Laboratory equipment		5 years
			Computer equipment and software		3 years
			Leasehold improvements		Shorter of asset’s useful
					life or remaining term of lease
			Furniture and fixtures		5 years
			Office equipment		5 years
	Costs of major additions and betterments are capitalized; maintenance and repairs, which do not improve or extend the life of the respective assets, are charged to expense as incurred. Upon retirement or sale, the cost of the disposed asset and the related accumulated depreciation are removed from the accounts and the resulting gain or loss is recognized.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets
	The Company periodically evaluates its long-lived assets for potential impairment in accordance with ASC Topic 360, Property, Plant and Equipment. Potential impairment is assessed when there is evidence that events or changes in circumstances indicate that the carrying amount of an asset may not be recovered. Recoverability of these assets is assessed based on undiscounted expected future cash flows from the assets, considering a number of factors, including past operating results, budgets and economic projections, market trends and product development cycles. If impairments are identified, assets are written down to their estimated fair value. The Company has not recognized any impairment charges through December 31, 2014.
Segment and Geographic Information	Segment and Geographic Information
	Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision maker, or decision making group, in making decisions on how to allocate resources and assess performance. The Company’s chief operating decision maker is the chief executive officer. The Company and the chief decision maker view the Company’s operations and manage its business as one operating segment. The Company operates in only one geographic segment.
Subsequent Events	Subsequent Events
	The Company considered events or transactions occurring after the balance sheet date but prior to the issuance of the consolidated financial statements are available to be issued for potential recognition or disclosure in its consolidated financial statements. All significant subsequent events have been properly disclosed in the consolidated financial statements.
Net Loss Per Share Applicable to Common Stockholders	Net Loss per Share Applicable to Common Stockholders
	Basic net loss per share applicable to common stockholders is calculated by dividing net loss applicable to common stockholders by the weighted average shares outstanding during the period, without consideration for common stock equivalents. Net loss applicable to common stockholders is calculated by adjusting the net loss of the Company for cumulative preferred stock dividends. Diluted net loss per share applicable to common stockholders is calculated by adjusting weighted average shares outstanding for the dilutive effect of common stock equivalents outstanding for the period, determined using the treasury-stock method. For purposes of the dilutive net loss per share applicable to common stockholders calculation, preferred stock, stock options, and unvested restricted stock are considered to be common stock equivalents but are excluded from the calculation of diluted net loss per share applicable to common stockholders as their effect would be anti-dilutive; therefore, basic and diluted net loss per share applicable to common stockholders were the same for all periods presented. The following common stock equivalents were excluded from the calculation of diluted net loss per share applicable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect:
			Years ended December 31,
			2014			2013		2012
	Convertible preferred stock			-			-		19,731,564
	Stock options			3,805,420			3,846,168		3,145,544
	Unvested restricted stock			8,522			23,295		160,053
	Employee stock purchase plan shares			7,159			-		-
				3,821,101			3,869,463		23,037,161
Recent Accounting Pronouncements	Recent Accounting Pronouncements
	In August 2014, the FASB issued ASU No. 2014-15, Presentation of Financial Statements—Going Concern (Subtopic 205-40). The ASU requires all entities to evaluate for the existence of conditions or events that raise substantial doubt about the entity’s ability to continue as a going concern within one year after the issuance date of the financial statements. The accounting standard is effective for interim and annual periods ending after December 15, 2016, and will not have a material impact on the consolidated financial statements, but may impact the Company’s footnote disclosures.
	In June 2014, the FASB issued ASU No. 2014-12, Compensation—Stock Compensation (Topic 718). The ASU clarifies how entities should treat performance targets that can be achieved after the requisite service period of a share-based payment award. The accounting standard is effective for interim and annual periods beginning after December 15, 2015 and is not expected to have a material impact on the consolidated financial statements.
	In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606). The ASU provides for a single comprehensive model for use in accounting for revenue arising from contracts with customers and supersedes most current revenue recognition guidance. The accounting standard is effective for interim and annual periods beginning after December 15, 2016 with no early adoption permitted. The Company is required to adopt the amendments in the ASU using one of two acceptable methods. The Company is currently in the process of determining which adoption method it will apply and evaluating the impact of the guidance on its consolidated financial statements.
	Other accounting standards that have been issued by the FASB or other standards-setting bodies that do not require adoption until a future date are not expected to have a material impact on the Company’s financial statements upon adoption.

Summary_of_Significant_Account2

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
Stock-Based Compensation Expense for Employee and Non-Employee Stock Options, Employee Stock Purchase Plan and Restricted Stock	During the years ended December 31, 2014, 2013, and 2012, the Company recorded stock-based compensation expense for employee and non-employee stock options, the employee stock purchase plan and restricted stock, which was allocated as follows in the consolidated statements of operations (in thousands):
		Years Ended December 31,
		2014			2013		2012
	Research and development expense	$	6,688		$	2,001	$	605
	General and administrative expense		4,818			1,029		137
		$	11,506		$	3,030	$	742
Marketable Securities	Marketable securities at December 31, 2014 consist of the following (in thousands):
		Amortized			Unrealized		Unrealized			Fair
		Cost			Gains		Losses			Value
	Current:
	Certificates of deposit	$	13,160		$	—	$	(5	)	$	13,155
	U.S. Treasuries		314,866			45		(32	)		314,879
	Non-current:
	U.S. Treasuries		125,447			5		(70	)		125,382
		$	453,473		$	50	$	(107)		$	453,416
	Marketable securities at December 31, 2013 consist of the following (in thousands):
		Amortized			Unrealized		Unrealized			Fair
		Cost			Gains		Losses			Value
	Current:
	Certificates of deposit	$	6,920		$	—	$	(5	)	$	6,915
	U.S. Treasuries		88,287			8		(1	)		88,294
	Non-current:
	U.S. Treasuries		27,113			12		—			27,125
		$	122,320		$	20	$	-6		$	122,334
Cash Equivalents and Marketable Securities Measured at Fair Value on a Recurring Basis	The following table summarizes the cash equivalents and marketable securities measured at fair value on a recurring basis as of December 31, 2014 (in thousands):
		Level 1			Level 2		Level 3			Total
	Cash equivalents	$	11,410		$	960	$	—		$	12,370
	Marketable securities:
	Certificates of deposit		—			13,155		—			13,155
	U.S. Treasuries		440,261			—		—			440,261
		$	451,671		$	14,115	$	—		$	465,786
	The following table summarizes the cash equivalents and marketable securities measured at fair value on a recurring basis as of December 31, 2013 (in thousands):
		Level 1			Level 2		Level 3			Total
	Cash equivalents	$	68,792		$	—	$	—		$	68,792
	Marketable securities:
	Certificates of deposit		—			6,915		—			6,915
	U.S. Treasuries		115,419			—		—			115,419
		$	184,211		$	6,915	$	—		$	191,126
Estimated Useful Lives of Property and Equipment	Property and equipment is stated at cost, and depreciated using the straight-line method over the estimated useful lives of the respective assets:
		Laboratory equipment		5 years
		Computer equipment and software		3 years
		Leasehold improvements		Shorter of asset’s useful
				life or remaining term of lease
		Furniture and fixtures		5 years
		Office equipment		5 years
Common Stock Excluded from Calculation of Diluted Net Loss Per Share	The following common stock equivalents were excluded from the calculation of diluted net loss per share applicable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect:
		Years ended December 31,
		2014			2013		2012
	Convertible preferred stock		-			-		19,731,564
	Stock options		3,805,420			3,846,168		3,145,544
	Unvested restricted stock		8,522			23,295		160,053
	Employee stock purchase plan shares		7,159			-		-
			3,821,101			3,869,463		23,037,161

Property_and_Equipment_Tables

Property and Equipment (Tables)	12 Months Ended
	Dec. 31, 2014
Property, Plant and Equipment [Abstract]
Summary of Property and Equipment	Property and equipment consists of the following (in thousands):
		December 31,
		2014			2013
	Laboratory equipment	$	6,885		$	5,754
	Computer equipment and software		1,458			1,056
	Leasehold improvements		97			97
	Furniture and fixtures		345			342
	Office equipment		218			218
	Construction in progress		2,941			482
	Total property and equipment		11,944			7,949
	Less accumulated depreciation		(5,558	)		(4,191	)
	Total property and equipment, net	$	6,386		$	3,758

Accrued_Expenses_and_Other_Cur1

Accrued Expenses and Other Current Liabilities (Tables)	12 Months Ended
	Dec. 31, 2014
Payables and Accruals [Abstract]
Summary of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities consist of the following (in thousands):
		December 31,
		2014		2013
	Accrued compensation	$	5,689	$	3,642
	Accrued contracted research and development costs		7,340		2,484
	Accrued professional fees		549		320
	Accrued other		442		140
	Total	$	14,020	$	6,586

Commitments_and_Contingencies_

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2014
Commitments and Contingencies Disclosure [Abstract]
Summary of Future Annual Minimum Lease Payments Due Under Operating Leases	Future annual minimum lease payments due under non-cancellable operating leases at December 31 of each year are as follows (in thousands):
	2015	$	4,990
	2016		6,685
	2017		6,842
	2018		7,004
	2019		7,169
	Thereafter		17,691
		$	50,381

ShareBased_Payments_Tables

Share-Based Payments (Tables)	12 Months Ended
	Dec. 31, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Summary of Stock Option Activity of All Stock Incentive Plans	These awards are included within the following table which summarizes the stock option activity of all stock incentive plans for the year ended December 31, 2014.
		Number of			Weighted-			Weighted-			Aggregate
		Stock			Average			Average			Intrinsic
		Options			Exercise			Remaining			Value (in
					Price			Contractual			thousands)
								Term
								(in years)
	Outstanding at December 31, 2013		3,846,168		$	4.14			7.43		$	76,189
	Granted		1,339,483			39.49
	Exercised		(1,298,775	)		1.79
	Forfeited/Expired		(81,456	)		13.22
	Outstanding at December 31, 2014		3,805,420		$	17.19			7.58		$	360,935
	Exercisable at December 31, 2014		1,577,101		$	2.58			5.82		$	172,636
	Vested and expected to vest at December 31, 2014		3,516,545		$	16.98			7.5		$	334,292
Unvested Stock Unit Activity	Unvested RSU activity for the year ended December 31, 2014 is summarized as follows:
	Unvested shares at December 31, 2013		—
	Granted		10,000
	Vested		—
	Unvested shares end of period		10,000
Unvested Stock Unit Activity	Unvested restricted stock activity for the year ended December 31, 2014 is summarized as follows:
	Unvested shares at December 31, 2013		23,295
	Granted		—
	Vested		(14,773	)
	Forfeited		—
	Unvested shares at December 31, 2014		8,522
Schedule of Grant Date Fair Value Option Award Weighted Average Assumptions Used	The following table summarizes the weighted average assumptions used in calculating the grant date fair value of the awards:
		Years Ending December 31,
		2014			2013			2012
	Risk-free interest rate		1.83	%		1.31	%		1.09	%
	Expected dividend yield		—			—			—
	Expected term (in years)		6.03			6.42			6.08
	Expected volatility		78.61	%		92.49	%		97.75	%

Income_Taxes_Tables

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Provision (Benefit) for Income Taxes	The provision (benefit) for income taxes is as follows for the years ended December 31, 2014, 2013 and 2012 (in thousands):
		Years Ended December 31,
		2014			2013			2012
	Current:
	Federal		$  (444	)		$  (3,263	)	$	(9,531	)
	State		18			—			—
	Total current		(426	)		(3,263	)		(9,531	)
	Deferred:
	Federal		—			3,842			6,707
	State		—			—			—
	Total deferred		—			3,842			6,707
	Total	$	(426	)	$	579		$	(2,824	)
Reconciliation of Expected Income Tax Benefit (Expense) Computed Using Federal Statutory Income Tax Rate
	A reconciliation of the expected income tax benefit (expense) computed using the federal statutory income tax rate to the Company’s effective income tax rate is as follows for the years ended December 31, 2014, 2013 and 2012:
		December 31,
		2014			2013			2012
	Income tax benefit computed at federal statutory tax rate		35.00%			35.00%			35.00%
	State taxes, net of federal benefit		4.90			4.02			7.07
	Change in valuation allowance		(35.80)			(38.02)			(28.08)
	General business credits and other credits		(1.90)			0.52			0.12
	Permanent differences		(2.10)			(1.75)			(0.63)
	Interest and penalties		—			(1.19)			(1.95)
	Other		0.80			(0.07)			0.79
	Total		0.90%			(1.49)%			12.32%
Company's Deferred Tax Assets and Liabilities	Significant components of the Company’s deferred tax assets and liabilities for the years ended December 31, 2014 and 2013 are as follows (in thousands):
		2014			2013
	Deferred tax assets:
	Net operating loss carryforwards	$	53,545		$	21,901
	Deferred revenue		8,713			23,171
	Tax credit carryforwards		3,159			637
	Purchased intangible assets		137			147
	Stock-based compensation		3,461			768
	Deferred rent		1,622			138
	Other		325			231
	Total deferred tax assets		70,962			46,993
	Valuation allowance		(69,433)			(46,549)
	Total deferred tax assets	$	1,529		$	444
	Deferred tax liabilities:
	Depreciation and amortization	$	(1,529)		$	(444)
	Total deferred tax liabilities	$	(1,529)		$	(444)
	Net deferred tax asset	$	-		$	-

Selected_Quarterly_Financial_D1

Selected Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Summary of Quarterly Financial Data
	2014	First		Second		Third		Fourth
		Quarter		Quarter		Quarter		Quarter
		(In thousands, except per share data)
	Total revenue	$	8,411	$	8,411	$	33,900	$	14,636
	Income (loss) from operations		(12,284)		(18,330)		3,208		(26,727)
	Net income (loss)		-12,248		-18,296		3,704		-26,664
	Net income (loss) applicable to common stockholders		-12,248		-18,296		3,704		-26,664
	Net income (loss) per share applicable to common stockholders – basic		-0.39		-0.54		0.11		-0.76
	Net income (loss) per share applicable to common stockholders – diluted		-0.39		-0.54		0.1		-0.76
	2013	First		Second		Third		Fourth
		Quarter		Quarter		Quarter		Quarter
		(In thousands, except per share data)
	Total revenue	$	6,268	$	6,268	$	6,268	$	6,744
	Loss from operations		-7,046		-8,526		-11,069		(12,242)
	Net loss		-7,228		-8,620		-11,177		-12,382
	Net loss applicable to common stockholders		-9,025		-10,418		-11,744		-12,382
	Net loss per share applicable to common stockholders – basic and diluted		-2.47		-2.8		-0.52		-0.4

Nature_of_Business_Additional_

Nature of Business - Additional Information (Detail) (USD $)	12 Months Ended		1 Months Ended		0 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Apr. 30, 2014	31-May-14	Jul. 29, 2013
Basis Of Presentation [Line Items]
Accumulated deficit	($166,948,000)	($113,444,000)
Proceeds from private placement		12,750,000
Proceeds from public offering of common stock	333,577,000	113,367,000
Cash, cash equivalents and marketable securities	467,400,000
Additional extension payment	20,000,000	20,000,000
Minimum [Member]
Basis Of Presentation [Line Items]
Number of rare genetic disease under entity therapeutic group	600
Public Offering [Member]
Basis Of Presentation [Line Items]
Common stock shares purchased			2,000,000
Public offering price			$44
Proceeds from public offering of common stock			82,300,000
Public Offering [Member] | Underwriter [Member]
Basis Of Presentation [Line Items]
Common stock shares purchased				300,000
Proceeds from public offering of common stock				12,400,000
Public Offering [Member] | Celgene [Member]
Basis Of Presentation [Line Items]
Common stock shares purchased			294,800
IPO [Member]
Basis Of Presentation [Line Items]
Common stock shares purchased	1,986,455				6,772,221
Public offering price	$110.75				$18
Proceeds from public offering of common stock	206,900,000
IPO [Member] | Underwriter [Member]
Basis Of Presentation [Line Items]
Common stock shares purchased					883,333
Net proceeds from the IPO					111,000,000
Private Placement [Member]
Basis Of Presentation [Line Items]
Proceeds from public offering of common stock	31,000,000
Private Placement [Member] | Celgene [Member]
Basis Of Presentation [Line Items]
Common stock shares purchased					708,333
Public offering price					$18
Proceeds from private placement					$12,800,000
Underwriter [Member]
Basis Of Presentation [Line Items]
Common stock shares purchased	297,968

Summary_of_Significant_Account3

Summary of Significant Accounting Policies - Stock-Based Compensation Expense for Employee and Non-Employee Stock Options, Employee Stock Purchase Plan and Restricted Stock (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated share based compensation expense	$11,506,000	$3,030,000	$742,000
Research and development expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated share based compensation expense	6,688,000	2,001,000	605,000
General and administrative expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated share based compensation expense	$4,818,000	$1,029,000	$137,000

Summary_of_Significant_Account4

Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)	1 Months Ended	12 Months Ended
	Apr. 30, 2010	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
		Segment	Securities
		Securities
Significant Of Accounting Policies [Line Items]
Tax benefits related to stock based compensation		$0	$0	$0
Significant uncertain tax positions		0	0
Realized gains or losses on marketable securities		0	0	0
Minimum period to liquidate		12 months	12 months
Debt securities in an unrealized loss position		92	33
Aggregate fair value of debt securities in an unrealized loss position		236,900,000	31,700,000
Concentration of credit risk		The Company has not experienced any credit losses in such accounts and does not believe it is exposed to any significant credit risk on these funds
Off- balance sheet concentrations of credit risk		The Company has no off-balance sheet concentrations of credit risk, such as foreign currency exchange contracts, option contracts or other hedging arrangements
Financial assets or liabilities that were classified as Level 3		0	0
Transfers of assets from Level 1 to Level 2		0	0
Transfers of assets from Level 2 to Level 1		0	0
Transfers of liabilities from Level 1 to Level 2		0	0
Transfers of liabilities from Level 2 to Level 1		0	0
Initial discovery period under collaboration	2 years
Number of geographic segment in which Company operates		1
Level 3 [Member]
Significant Of Accounting Policies [Line Items]
Financial assets classified as Level 3		0	0
Financial liabilities classified as Level 3		$0	$0
Maximum [Member]
Significant Of Accounting Policies [Line Items]
Investment maturity period		2 years	2 years
Initial discovery period under collaboration			5 years
Minimum [Member]
Significant Of Accounting Policies [Line Items]
Investment maturity period		1 year	1 year
Initial discovery period under collaboration			4 years

Summary_of_Significant_Account5

Summary of Significant Accounting Policies - Marketable Securities (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$453,473	$122,320
Unrealized Gains	50	20
Unrealized Losses	-107	-6
Fair Value	453,416	122,334
Current: [Member] | Certificates of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	13,160	6,920
Unrealized Losses	-5	-5
Fair Value	13,155	6,915
Current: [Member] | U.S. Treasuries [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	314,866	88,287
Unrealized Gains	45	8
Unrealized Losses	-32	-1
Fair Value	314,879	88,294
Non-Current: [Member] | U.S. Treasuries [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	125,447	27,113
Unrealized Gains	5	12
Unrealized Losses	-70
Fair Value	$125,382	$27,125

Summary_of_Significant_Account6

Summary of Significant Accounting Policies - Cash Equivalents and Marketable Securities Measured at Fair Value on a Recurring Basis (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Marketable securities	$453,416	$122,334
Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents	12,370	68,792
Cash equivalents and marketable securities, total	465,786	191,126
Fair Value, Measurements, Recurring [Member] | Certificates of Deposit [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Marketable securities	13,155	6,915
Fair Value, Measurements, Recurring [Member] | U.S. Treasuries [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Marketable securities	440,261	115,419
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents	11,410	68,792
Cash equivalents and marketable securities, total	451,671	184,211
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | U.S. Treasuries [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Marketable securities	440,261	115,419
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents	960
Cash equivalents and marketable securities, total	14,115	6,915
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Certificates of Deposit [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Marketable securities	$13,155	$6,915

Summary_of_Significant_Account7

Summary of Significant Accounting Policies - Estimated Useful Lives of Property and Equipment (Detail)	12 Months Ended
	Dec. 31, 2014
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment useful life	5 years
Computer Equipment and Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment useful life	3 years
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment useful life description	Shorter of asset's useful life or remaining term of lease
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property and equipment useful life	5 years
Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment useful life	5 years

Summary_of_Significant_Account8

Summary of Significant Accounting Policies - Common Stock Excluded from Calculation of Diluted Net Loss Per Share (Detail)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	3,821,101	3,869,463	23,037,161
Convertible Preferred Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share			19,731,564
Stock Options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	3,805,420	3,846,168	3,145,544
Restricted Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	8,522	23,295	160,053
Employee Stock Purchase Plan Shares [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	7,159

Collaboration_Agreement_Additi

Collaboration Agreement - Additional Information (Detail) (USD $)	1 Months Ended				3 Months Ended								12 Months Ended			1 Months Ended	0 Months Ended	7 Months Ended	1 Months Ended
	31-May-14	Dec. 31, 2013	Oct. 31, 2011	Apr. 30, 2010	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jul. 31, 2014	Jul. 31, 2014	Jul. 31, 2014	Jul. 31, 2013
													Candidate
													Licensed_Program
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Agreement amended date				2011-10
Amended collaboration expiration date		2015-04		2014-04									2016-04	2015-04
Discovery phase years				2 years
Term of the initial discovery period before amendment to extend		4 years	3 years										5 years	4 years
Term of extended period after amendment of the initial discovery period		5 years	4 years										6 years	5 years
Eligible payment to be received from extension of the discovery phase		$20,000,000			$20,000,000				$20,000,000				$20,000,000	$20,000,000
Date of extension of discovery phase													2015-06	2014-05
Number of candidates confirmed by the Joint Research Committee													2
Number of licensed programs type under collaboration agreements													3
Period considered for agreement termination notice													90 days
Upfront payment received													121,200,000
Series B convertible preferred stock, purchased		0			0				0				0	0
Reverse stock split													1-for-2.75
Reverse stock split ratio													2.75
Payment made in pursuant to the amendment	20,000,000		20,000,000										20,000,000
Collaboration revenue					14,636,000	33,900,000	8,411,000	8,411,000	6,744,000	6,268,000	6,268,000	6,268,000	65,358,000	25,548,000	25,106,000
Recognized revenue to extension													0
Achievement of an early clinical development milestone event					22,500,000								22,500,000
Dosing of the last patient in a Company-sponsored					25,000,000								25,000,000
Development cost reimbursed													26,600,000
Collaboration receivable					6,500,000								6,500,000
Celgene [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payment upon achievement of a specified clinical development milestone event					10,000,000								10,000,000			10,000,000	10,000,000	10,000,000
Milestone payment reimbursement, percentage													50.00%			50.00%
Milestone or royalty payments received													0
Celgene [Member] | Split License [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue recognition, selling price																	21,200,000
Celgene [Member] | In Process Research and Development [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue recognition, selling price													50,800,000			13,600,000		42,700,000
Celgene [Member] | Participation Service [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue recognition, selling price																200,000
Celgene [Member] | MTD [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Received amount of development cost													5,000,000
Celgene [Member] | Before Amendment [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Number of years company expected to fulfill its performance obligations													6 years
Collaboration revenue													65,400,000	25,500,000	25,100,000
Maximum [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Discovery phase years														5 years
Milestone payment upon achievement of a specified clinical development milestone event					25,000,000								25,000,000
Receipts of potential milestone payments					120,000,000								120,000,000
Milestone payments upon achievement of specified regulatory milestone events					70,000,000								70,000,000
Milestone payment upon achievement of a specified commercial milestone event					25,000,000								25,000,000
Maximum [Member] | Celgene [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payment upon achievement of a specified clinical development milestone event					10,000,000								10,000,000
Series B Convertible Preferred Stock [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Series B convertible preferred stock, purchased				5,190,551
Series B convertible preferred stock price per share					$1.70								$1.70
Net proceeds of Series B convertible preferred stock													8,800,000
Additional consideration under agreement													3,100,000
Series B convertible preferred stock at fair value					$5,700,000								$5,700,000
Reverse stock split													1-for-2.75
Reverse stock split ratio													0.3636
Additional outstanding shares common stock														-5,190,551					1,887,473
Series B Convertible Preferred Stock [Member] | Celgene [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Series B convertible preferred stock, purchased					5,190,551								5,190,551

Property_and_Equipment_Summary

Property and Equipment - Summary of Property and Equipment (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Property, Plant and Equipment [Line Items]
Total property and equipment	$11,944	$7,949
Less accumulated depreciation	-5,558	-4,191
Total property and equipment, net	6,386	3,758
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	6,885	5,754
Computer Equipment and Software [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	1,458	1,056
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	97	97
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	345	342
Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	218	218
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	$2,941	$482

Property_and_Equipment_Additio

Property and Equipment - Additional Information (Detail) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Property, Plant and Equipment [Abstract]
Depreciation expense	$1,367	$1,440	$1,179

Accrued_Expenses_and_Other_Cur2

Accrued Expenses and Other Current Liabilities - Summary of Accrued Expenses and Other Current Liabilities (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Payables and Accruals [Abstract]
Accrued compensation	$5,689	$3,642
Accrued contracted research and development costs	7,340	2,484
Accrued professional fees	549	320
Accrued other	442	140
Total	$14,020	$6,586

Commitments_and_Contingencies_1

Commitments and Contingencies - Additional Information (Detail) (USD $)	0 Months Ended	12 Months Ended
	Nov. 10, 2009	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
		Lawsuits
		sqft
Schedule Of Commitments And Contingencies Future Maturities [Line Items]
Operating lease		74,500
Lease agreement tenant incentive		$16,500,000
Initial lease term		7 years
Lease contract additional extension option		Option of extending for 2 consecutive terms of 5 years
Stand by letter of credit		2,200,000
Tenant improvement allowance		2,300,000
Rent expense		3,700,000	2,200,000	2,200,000
Annual maintenance payments recorded as research and development expense		100,371,000	54,502,000	41,037,000
License agreements amount		83,000	72,000	30,000
Milestone payment period		5 years
Contract term		5 years
Milestone payment		0
Accumulated discounted labor cost		1,000,000
Number of days for election		30 days
Milestones payment		100,000
Total expense incurred under milestone payment agreements		300,000	400,000	0
Period under contract term to declare development candidate for milestone Payments forgo		3 years
Milestone possible earning period	10 years
Legal proceedings		0
Minimum [Member]
Schedule Of Commitments And Contingencies Future Maturities [Line Items]
Range related to royalties amount		0.50%
Range related to non royalties amount		7.00%
Maximum [Member]
Schedule Of Commitments And Contingencies Future Maturities [Line Items]
Range related to royalties amount		2.50%
Range related to non royalties amount		25.00%
Preclinical Milestones [Member]
Schedule Of Commitments And Contingencies Future Maturities [Line Items]
Milestones payment		200,000
Regulatory Milestones [Member]
Schedule Of Commitments And Contingencies Future Maturities [Line Items]
Milestones payment		300,000	200,000
Milestone One [Member]
Schedule Of Commitments And Contingencies Future Maturities [Line Items]
Upfront grant money to be utilized in development and testing brain cancer therapy	178,538
Accumulated net profit	5,000,000
Milestone Two [Member]
Schedule Of Commitments And Contingencies Future Maturities [Line Items]
Upfront grant money to be utilized in development and testing brain cancer therapy	178,538
Accumulated net profit	50,000,000
Milestone Three [Member]
Schedule Of Commitments And Contingencies Future Maturities [Line Items]
Upfront grant money to be utilized in development and testing brain cancer therapy	178,538
Accumulated net profit	250,000,000
After Amendment [Member]
Schedule Of Commitments And Contingencies Future Maturities [Line Items]
Operating lease		113,200
License Agreement Terms [Member]
Schedule Of Commitments And Contingencies Future Maturities [Line Items]
Milestone payments			100,000
Accruals from milestones achieved			50,000
Payments made in consideration for the licensed rights		470,000
Shares of common stock issued to certain licensors		162,545
Annual maintenance payments recorded as research and development expense		42,000
Payment of patent related to licensor		100,000
Payment of clinical development related to licensor		1,600,000
Regulatory payment related to licensor		5,400,000
Sales based payment related to licensor		4,200,000
Patent-based payment related to licensor		50,000	25,000	0
Royalties paid		$0
Option to renew licenses		Option to renew these licenses on an annual basis in exchange for payments approximating $45,000 for 2015
Existing Leases [Member]
Schedule Of Commitments And Contingencies Future Maturities [Line Items]
Operating lease		38,500
Existing lease termination Period		30 days

Commitments_and_Contingencies_2

Commitments and Contingencies - Future Annual Minimum Lease Payments Due Under Operating Leases (Detail) (USD $)	Dec. 31, 2014
In Thousands, unless otherwise specified
Commitments and Contingencies Disclosure [Abstract]
2015	$4,990
2016	6,685
2017	6,842
2018	7,004
2019	7,169
Thereafter	17,691
Total minimum lease payments	$50,381

Convertible_Preferred_Stock_Ad

Convertible Preferred Stock - Additional Information (Detail) (USD $)	24 Months Ended	12 Months Ended		1 Months Ended
In Millions, except Share data, unless otherwise specified	Dec. 31, 2009	Dec. 31, 2014	Dec. 31, 2012	Apr. 30, 2010	Nov. 30, 2011	Dec. 31, 2013
Class of Stock [Line Items]
Number of preferred stock sold		0				0
Conversion of the principal and interest of convertible notes	$2
Common stock shares outstanding		19,731,564
Preferred stock shares outstanding		0				0
Series A Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Number of preferred stock sold	33,188,889
Preferred stock per share	$1
Preferred stock aggregate proceeds	33.1
Cumulative dividend per share		$0.06
Dividends accrued but unpaid		8.9	7.8
Series B Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Number of preferred stock sold				5,190,551
Preferred stock per share				$1.70
Preferred stock aggregate proceeds				8.8
Preferred stock fair value per share considered as premium				$1.11
Cumulative dividend per share		$0.10
Dividends accrued but unpaid		1.7	1.4
Series C Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Number of preferred stock sold					15,882,389
Preferred stock per share					$4.91
Preferred stock aggregate proceeds					78
Cumulative dividend per share		$0.29
Dividends accrued but unpaid		$8	$5.30
Series C- One Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Number of preferred stock sold					7,395,829
Series C-Two Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Number of preferred stock sold					8,486,560

Common_Stock_Additional_Inform

Common Stock - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014
Equity [Abstract]
Shareholders right to vote	1
Dividends declared or paid since the Company's inception	$0
Reverse stock split, description	1-for-2.75
Reverse stock split, ratio	2.75
Reverse stock split, effective date	11-Jul-13

ShareBased_Payments_Additional

Share-Based Payments - Additional Information (Detail) (USD $)	12 Months Ended			0 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jan. 01, 2015	Dec. 30, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average grant date fair value of options	$27.26	$9.96	$2.09
Intrinsic value of options exercised	$60,400,000	$4,200,000	$800,000
Stock-based compensation expense	11,506,000	3,030,000	742,000
Stock options granted	1,339,483
Expected contractual term	10 years
Number of shares issued	37,100,513	31,202,542
Consultants [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted	0	27,272
Advisors [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted	0	27,272
Performance-Based Stock Option Grants [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	0
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total number of unvested Stock option	10,000
Restricted stock	10,000
Fair value of awards vested	700,000	1,700,000	500,000
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense related to stock option	400,000
Fair value Granted Amount	500,000	0	0
Stock-based compensation expense	100,000	0	0
Stock based compensation associated with restricted stock, recognition period	9 months
2007 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance	5,079,462
Shares available for future issuance	0	0
2013 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance	4,445,395
Shares available for future issuance	639,975
Unrecognized compensation expense related to stock option	32,500,000
Weighted-average period to recognize compensation expense	3 years 1 month 6 days
Forfeiture of total grants	5.00%
2013 Stock Incentive Plan [Member] | Subsequent Event [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized for issuance				1,484,020
2013 Stock Incentive Plan [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock	2,000,000
Common stock outstanding	4.00%
2013 Stock Incentive Plan [Member] | Performance-Based Stock Option Grants [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense related to stock option	1,100,000
Stock-based compensation expense	900,000	900,000	0
Stock options granted	0
2013 Stock Incentive Plan [Member] | Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock	0	0
Weighted average purchase price of restricted stock granted			$0.91
2013 Stock Incentive Plan [Member] | Stock Option Grant [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted		355,454
2013 Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares issued	0
Purchase price interest rate stock option	85.00%
Opportunity to purchase of common stock	327,272
Stock Based Compensation expense	$100,000				$0

ShareBased_Payments_Summary_of

Share-Based Payments - Summary of Stock Option Activity of All Stock Incentive Plans (Detail) (USD $)	12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Number of Stock Options, Outstanding, Beginning balance	3,846,168
Number of Stock Options, Granted	1,339,483
Number of Stock Options, Exercised	-1,298,775
Number of Stock Options, Forfeited/Expired	-81,456
Number of Stock Options, Outstanding, Ending balance	3,805,420	3,846,168
Number of Stock Options, Exercisable	1,577,101
Number of Stock Options, Vested and expected to vest	3,516,545
Weighted-Average Exercise Price, Outstanding, Beginning balance	$4.14
Weighted-Average Exercise Price, Granted	$39.49
Weighted-Average Exercise Price, Exercised	$1.79
Weighted-Average Exercise Price, Forfeited/Expired	$13.22
Weighted-Average Exercise Price, Outstanding, Ending balance	$17.19	$4.14
Weighted-Average Exercise Price, Exercisable	$2.58
Weighted-Average Exercise Price, Vested and expected to vest	$16.98
Weighted-Average Remaining Contractual Term (in years), Outstanding	7 years 6 months 29 days	7 years 5 months 5 days
Weighted-Average Remaining Contractual Term (in years), Exercisable	5 years 9 months 26 days
Weighted-Average Remaining Contractual Term (in years), Vested and expected to vest	7 years 6 months
Aggregate Intrinsic Value, Outstanding, Beginning balance	$76,189
Aggregate Intrinsic Value, Outstanding, Ending balance	360,935	76,189
Aggregate Intrinsic Value, Exercisable	172,636
Aggregate Intrinsic Value, Vested and expected to vest	$334,292

ShareBased_Payments_Summary_of1

Share-Based Payments - Summary of Unvested RSUs Activity (Detail) (Restricted Stock Units (RSUs) [Member])	12 Months Ended
	Dec. 31, 2014
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unvested shares at December 31, 2013
Granted	10,000
Vested	0
Unvested shares at December 31, 2014	10,000

ShareBased_Payments_Unvested_R

Share-Based Payments - Unvested Restricted Stock Activity (Detail) (Restricted Stock Units (RSUs) [Member])	12 Months Ended
	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unvested shares at December 31, 2013
Granted	10,000
Vested	0
Unvested shares at December 31, 2014	10,000
Unvested [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unvested shares at December 31, 2013	23,295
Granted	0
Vested	-14,773
Forfeited	0
Unvested shares at December 31, 2014	8,522

ShareBased_Payments_Schedule_o

Share-Based Payments - Schedule of Fair Value Option Award Weighted Average Assumptions Used (Detail)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Risk-free interest rate	1.83%	1.31%	1.09%
Expected dividend yield	0.00%	0.00%	0.00%
Expected term (in years)	6 years 11 days	6 years 5 months 1 day	6 years 29 days
Expected volatility	78.61%	92.49%	97.75%

Income_Taxes_Provision_Benefit

Income Taxes - Provision (Benefit) for Income Taxes (Detail) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Current:
Federal	($444)	($3,263)	($9,531)
State	18
Total current	-426	-3,263	-9,531
Deferred:
Federal		3,842	6,707
State	0	0	0
Total deferred		3,842	6,707
Total	($426)	$579	($2,824)

Income_Taxes_Reconciliation_of

Income Taxes - Reconciliation of Expected Income Tax Benefit (Expense) Computed Using Federal Statutory Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Income tax benefit computed at federal statutory tax rate	35.00%	35.00%	35.00%
State taxes, net of federal benefit	4.90%	4.02%	7.07%
Change in valuation allowance	-35.80%	-38.02%	-28.08%
General business credits and other credits	-1.90%	0.52%	0.12%
Permanent differences	-2.10%	-1.75%	-0.63%
Interest and penalties		-1.19%	-1.95%
Other	0.80%	-0.07%	0.79%
Total	0.90%	-1.49%	12.32%

Income_Taxes_Additional_Inform

Income Taxes - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
	Audit			Ownership
Income Taxes Disclosure [Line Items]
Accrued amount for interest and penalties related to the non-payment of U.S. federal income taxes	$0	$600,000	$600,000
Expiration date of carryforwards	31-Dec-34
Percentage of increase the ownership	5.00%
Aggregate period	3 years
Number of ownership change				2
Amount of valuation allowance on deferred tax assets	69,433,000	46,549,000		10,700,000
Increase in valuation allowance	22,900,000
Unrecognized tax benefits	0	0
Number of federal or state audit in process	0
Minimum [Member]
Income Taxes Disclosure [Line Items]
Shareholders in the stock of a corporation	50.00%
Federal Income Tax [Member]
Income Taxes Disclosure [Line Items]
Net operating loss	166,900,000
Federal Income Tax [Member] | Research Tax Credit Carryforward [Member]
Income Taxes Disclosure [Line Items]
Research and development tax credit	2,500,000
State Income Tax [Member]
Income Taxes Disclosure [Line Items]
Net operating loss	208,100,000
State Income Tax [Member] | Research Tax Credit Carryforward [Member]
Income Taxes Disclosure [Line Items]
Research and development tax credit	1,100,000
State and Local Jurisdiction [Member]
Income Taxes Disclosure [Line Items]
Net operating loss	39,200,000
Federal [Member]
Income Taxes Disclosure [Line Items]
Net operating loss	$39,100,000

Income_Taxes_Companys_Deferred

Income Taxes - Company's Deferred Tax Assets and Liabilities (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2011
In Thousands, unless otherwise specified
Deferred tax assets:
Net operating loss carryforwards	$53,545	$21,901
Deferred revenue	8,713	23,171
Tax credit carryforwards	3,159	637
Purchased intangible assets	137	147
Stock-based compensation	3,461	768
Deferred rent	1,622	138
Other	325	231
Total deferred tax assets	70,962	46,993
Valuation allowance	-69,433	-46,549	-10,700
Total deferred tax assets	-1,529	444
Deferred tax liabilities:
Depreciation and amortization	-1,529	-444
Total deferred tax liabilities	-1,529	-444
Net deferred tax asset	$0	$0

Defined_Contribution_Benefit_P1

Defined Contribution Benefit Plan - Additional Information (Detail) (Subsequent Event [Member])	0 Months Ended
	Jan. 01, 2015
Subsequent Event [Member]
Defined Contribution Plan Disclosure [Line Items]
Employer's contributions	50.00%
Employer's contributions of eligible compensation	6.00%

Selected_Quarterly_Financial_D2

Selected Quarterly Financial Data (Unaudited) - Summary of Quarterly Financial Data (Detail) (USD $)	3 Months Ended								12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Total revenue	$14,636	$33,900	$8,411	$8,411	$6,744	$6,268	$6,268	$6,268	$65,358	$25,548	$25,106
Income (loss) from operations	-26,727	3,208	-18,330	-12,284	-12,242	-11,069	-8,526	-7,046	-54,133	-38,883	-22,995
Net income (loss)	-26,664	3,704	-18,296	-12,248	-12,382	-11,177	-8,620	-7,228	-53,504	-39,407	-20,102
Net income (loss) applicable to common stockholders	($26,664)	$3,704	($18,296)	($12,248)	($12,382)	($11,744)	($10,418)	($9,025)	($53,504)	($43,569)	($27,292)
Net income (loss) per share applicable to common stockholders - basic	($0.76)	$0.11	($0.54)	($0.39)
Net loss per share applicable to common stockholders - basic and diluted					($0.40)	($0.52)	($2.80)	($2.47)	($1.59)	($2.83)	($8.02)
Net income (loss) per share applicable to common stockholders - diluted	($0.76)	$0.10	($0.54)	($0.39)