Agios Pharmaceuticals (AGIO) 10-Q 11 May 15 2015 Q1 Quarterly report Financial data

Document_and_Entity_Informatio

Document and Entity Information	3 Months Ended
	Mar. 31, 2015	8-May-15
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	FALSE
Document Period End Date	31-Mar-15
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	Q1
Trading Symbol	AGIO
Entity Registrant Name	AGIOS PHARMACEUTICALS INC
Entity Central Index Key	1439222
Current Fiscal Year End Date	-19
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		37,298,171

Condensed_Consolidated_Balance

Condensed Consolidated Balance Sheets (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified
Current assets:
Cash and cash equivalents	$35,256	$14,031
Marketable securities	309,450	328,034
Collaboration receivable - related party	7,017	6,492
Tenant improvement and other receivables	6,906	2,334
Prepaid expenses and other current assets	5,512	4,814
Refundable income taxes		3,841
Total current assets	364,141	359,546
Marketable securities	95,258	125,382
Property and equipment, net	14,297	6,386
Other assets	640	590
Total assets	474,336	491,904
Current liabilities:
Accounts payable	12,640	11,067
Accrued expenses	15,604	14,020
Deferred revenue - related party	6,886	35,686
Deferred rent	1,245	310
Other current liabilities	4	6
Total current liabilities	36,379	61,089
Deferred revenue, net of current portion - related party	133	2,725
Deferred rent, net of current portion	11,007	3,724
Commitments and contingencies
Stockholders' equity:
Preferred stock, $0.001 par value; 25,000,000 shares authorized; no shares issued or outstanding at March 31, 2015 and December 31, 2014
Common stock, $0.001 par value; 125,000,000 shares authorized; 37,289,050 and 37,100,513 shares issued and outstanding at March 31, 2015 and December 31, 2014, respectively	37	37
Additional paid-in capital	598,494	591,334
Accumulated other comprehensive income (loss)	191	-57
Accumulated deficit	-171,905	-166,948
Total stockholders' equity	426,817	424,366
Total liabilities and stockholders' equity	$474,336	$491,904

Condensed_Consolidated_Balance1

Condensed Consolidated Balance Sheets (Parenthetical) (USD $)	Mar. 31, 2015	Dec. 31, 2014
Statement of Financial Position [Abstract]
Preferred stock, par value	$0.00	$0.00
Preferred stock, shares authorized	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	37,289,050	37,100,513
Common stock, shares outstanding	37,289,050	37,100,513

Condensed_Consolidated_Stateme

Condensed Consolidated Statements of Operations (USD $)	3 Months Ended
In Thousands, except Share data, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Income Statement [Abstract]
Collaboration revenue - related party	$34,202	$8,411
Operating expenses:
Research and development (net of $4,366 of cost reimbursement from related party for the three months ended March 31, 2015)	32,443	17,407
General and administrative	6,954	3,288
Total operating expenses	39,397	20,695
Loss from operations	-5,195	-12,284
Interest income	238	36
Net loss	($4,957)	($12,248)
Net loss per share - basic and diluted	($0.13)	($0.39)
Weighted-average number of common shares used in net loss per share - basic and diluted	37,214,747	31,394,563

Condensed_Consolidated_Stateme1

Condensed Consolidated Statements of Operations (Parenthetical) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015
Income Statement [Abstract]
Cost reimbursement from related party	$4,366

Condensed_Consolidated_Stateme2

Condensed Consolidated Statements of Comprehensive Loss (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Statement of Comprehensive Income [Abstract]
Net loss	($4,957)	($12,248)
Other comprehensive income:
Unrealized gain on available-for-sale securities	248	19
Comprehensive loss	($4,709)	($12,229)

Condensed_Consolidated_Stateme3

Condensed Consolidated Statements of Cash Flows (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Operating activities
Net loss	($4,957)	($12,248)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	370	332
Stock-based compensation expense	5,060	1,505
Net amortization of premium and discounts on investments	172	168
Changes in operating assets and liabilities:
Collaboration receivable - related party	-524	-2,143
Other receivables	-4,573
Prepaid expenses and other assets	-708	-930
Accounts payable	-2,665	720
Accrued expenses and other liabilities	1,574	-2,194
Deferred rent	8,218	-24
Refundable income taxes and income taxes payable	3,841	-5,958
Deferred revenue - related party	-31,393	-6,268
Net cash used in operating activities	-25,585	-27,040
Investing activities
Purchases of marketable securities	-35,526	-34,812
Proceeds from maturities and sales of marketable securities	84,311	22,560
Purchases of property and equipment	-3,797	-13
Net cash provided by (used in) investing activities	44,988	-12,265
Financing activities
Payment of public offering costs	-207
Net proceeds from stock option exercises and employee stock purchase plan	2,029	582
Net cash provided by financing activities	1,822	582
Net increase (decrease) in cash and cash equivalents	21,225	-38,723
Cash and cash equivalents at beginning of the period	14,031	71,560
Cash and cash equivalents at end of the period	35,256	32,837
Supplemental cash flow information
Cash paid for income taxes		5,958
Supplemental disclosure of non-cash investing and financing transactions
Additions to property, plant and equipment in accounts payable and accrued expenses	6,602	13
Vesting of restricted stock	2	3
Public offering costs in accounts payable and accrued expenses		65
Proceeds from stock option exercises in other current assets	$60

Overview_and_Basis_of_Presenta

Overview and Basis of Presentation	3 Months Ended
	Mar. 31, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Overview and Basis of Presentation	1. Overview and Basis of Presentation
	Overview
	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.
	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 Corporation (“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 deducting 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 deducting underwriting discounts and commissions paid by the Company.
	Basis of Presentation
	The condensed consolidated interim balance sheet as of March 31, 2015, and the condensed consolidated interim statements of operations, comprehensive loss and cash flows for the three months ended March 31, 2015 and 2014, are unaudited. The unaudited condensed consolidated interim financial statements have been prepared on the same basis as the annual financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary to present fairly the Company’s condensed consolidated financial position as of March 31, 2015 and its results of operations and cash flows for the three months ended March 31, 2015 and 2014. The financial data and the other financial information disclosed in these notes to the condensed consolidated interim financial statements related to the three-month period is also unaudited. The results of operations for the three months ended March 31, 2015 are not necessarily indicative of the results to be expected for the year ending December 31, 2015 or for any other future annual or interim period. The condensed consolidated interim financial statements should be read in conjunction with the audited consolidated financial statements and notes thereto included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2014 that was filed with the Securities and Exchange Commission (the “SEC”) on February 24, 2015.
	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”).

Summary_of_Significant_Account

Summary of Significant Accounting Policies and Recent Accounting Pronouncements	3 Months Ended
	Mar. 31, 2015
Accounting Policies [Abstract]
Summary of Significant Accounting Policies and Recent Accounting Pronouncements	2. Summary of Significant Accounting Policies and Recent Accounting Pronouncements
	Significant Accounting Policies
	There have been no material changes to the significant accounting policies previously disclosed in the Annual Report on Form 10-K for the year ended December 31, 2014.
	Recent Accounting Pronouncements
	In May 2014, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“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. In April 2015, the FASB proposed a one year deferral of the effective date of this accounting update to annual periods beginning after December 15, 2017, along with an option to permit early adoption. 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.

Fair_Value_Measurements

Fair Value Measurements	3 Months Ended
	Mar. 31, 2015
Fair Value Disclosures [Abstract]
Fair Value Measurements	3. Fair Value Measurements
	The Company records cash equivalents and marketable securities at fair value. Accounting Standards Codification (“ASC”) Topic 820, Fair Value Measurements and Disclosures, established 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, 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 March 31, 2015 (in thousands):
		Level 1		Level 2		Level 3		Total
	Cash equivalents	$	32,562	$	720	$	—	$	33,282
	Marketable securities:
	Certificates of deposit		—		11,480		—		11,480
	U.S. Treasuries		393,228		—		—		393,228
		$	425,790	$	12,200	$	—	$	437,990
	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
	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. After completing its validation procedures, the Company did not adjust or override any fair value measurements provided by the pricing services as of March 31, 2015 or December 31, 2014.
	The carrying amounts reflected in the condensed consolidated balance sheets for cash, collaboration receivable – related party, tenant improvement and other receivables, prepaid expenses and other current assets, other assets, accounts payable, and accrued expenses approximate their fair values at March 31, 2015 and December 31, 2014, due to their short-term nature.
	There have been no changes to the valuation methods during the three months ended March 31, 2015 or 2014. 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 three months ended March 31, 2015 or the year ended December 31, 2014. The Company had no financial assets or liabilities that were classified as Level 3 at any point during the three months ended March 31, 2015 or the year ended December 31, 2014.

Marketable_Securities

Marketable Securities	3 Months Ended
	Mar. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Marketable Securities	4. Marketable Securities
	Marketable securities at March 31, 2015 and December 31, 2014 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 condensed consolidated interim statements of comprehensive loss, until realized. 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 three months ended March 31, 2015 and 2014.
	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 condensed consolidated interim 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 March 31, 2015 consist of the following (in thousands):
		Amortized Cost		Unrealized		Unrealized			Fair
				Gains		Losses			Value
	Current:
	Certificates of deposit	$	11,480	$	1	$	(1	)	$	11,480
	U.S. Treasuries		297,883		90		(3	)		297,970
	Non-current:
	U.S. Treasuries		95,154		104		—			95,258
		$	404,517	$	195	$	(4	)	$	404,708
	Marketable securities at December 31, 2014 consist of the following (in thousands):
		Amortized Cost		Unrealized		Unrealized			Fair
				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
	At March 31, 2015 and December 31, 2014, 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 a maturity 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 March 31, 2015 and December 31, 2014, the Company held 23 and 92 debt securities that were in an unrealized loss position for less than one year, respectively. The aggregate fair value of debt securities in an unrealized loss position at March 31, 2015 and December 31, 2014 was $42.7 million and $236.9 million, respectively. There were no individual securities that were in a significant unrealized loss position as of March 31, 2015 and December 31, 2014. 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 March 31, 2015 and December 31, 2014.

Collaboration_Agreements

Collaboration Agreements	3 Months Ended
	Mar. 31, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Collaboration Agreements	5. Collaboration Agreements
	Celgene 2010 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 was received in May 2015.
	Pursuant to the collaboration agreement and subsequent amendments, the Company is 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. Celgene exercised its rights to this program during the three months ended March 31, 2015. 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.
	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. Celgene exercised its option to obtain an exclusive license outside the United States for AG-120 during the three months ended March 31, 2015 and the Company recognized the non-contingent consideration allocated to this unit of accounting, $15.8 million, as revenue upon delivery of the license. As additional contingent consideration is earned and allocated to this unit of accounting it will be recognized immediately.
		•	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. As committed to on the date of the amendment, the Company has completed services for three of the on-going phase 1 studies and expects services for the remaining two on-going phase 1 studies to be performed through the second quarter of 2016. As additional contingent consideration is earned and allocated to the three fully delivered units of accounting it will be recognized immediately.
		•	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 increases as the Company performs certain services for which it is eligible to receive reimbursement. These amounts are to be recognized on a cumulative catch-up basis for any in-process units of accounting or immediately for any fully delivered units of accounting. The estimated arrangement consideration may decrease if the Company receives less reimbursement than initially estimated.
	Beginning in the first quarter of 2015, the Company and Celgene agreed to plans to advance AG-221 into later stage development studies. Pursuant to the terms of the 2010 Celgene agreement, the parties agreed to transition primary development responsibilities for AG-221 to Celgene for later stage development at which point Celgene would become the lead development party for AG-221. During the transition, the Company continued to manage certain arrangements with third-party service providers whose contracts were assigned to Celgene. The Company determined it is no longer the primary obligor of the arrangements and, when considering the other factors included within ASC 605-45, Revenue Recognition – Principal Agent Considerations, determined reimbursements of amounts incurred under third-party contracts should be reported on a net basis in research and development expense during the three months ended March 31, 2015. The Company re-assessed its estimate of the total level of effort required to perform the development services related to AG-221 and recorded a change in estimate in the three months ended March 31, 2015, accordingly. This change in estimate resulted in the recognition of an additional $5.1 million of revenue. Including the $3.8 million presented as a reduction of research and development expenditures, the net change reduced the Company’s net loss by $8.9 million and caused a decrease in net loss per share of $0.24 for the three months ended March 31, 2015.
	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 three months ended March 31, 2015.
	During the three months ended March 31, 2015, the Company performed planning services on behalf of Celgene related to an expanded phase 1 trial of AG-221. The Company determined the work represented a contingent deliverable under the collaboration agreement amended in July 2014. The Company also determined it is not the primary obligor of the underlying third-party contracts and determined reimbursements of amounts incurred under the contracts should be reported on a net basis in research and development expense. During the three months ended March 31, 2015, the Company recognized $0.1 million in revenues and recorded $0.6 million as a reduction in research and development costs related to these services. Costs reimbursed for services performed directly by the Company are presented as collaboration revenues.
	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 March 31, 2015, the Company had earned $33.1 million in cost reimbursements which includes the initial milestone payment. As of March 31, 2015 and December 31, 2014, the Company has recorded a collaboration receivable of $7.0 million and $6.5 million, respectively, 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 2010 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.

Accrued_Expenses

Accrued Expenses	3 Months Ended
	Mar. 31, 2015
Payables and Accruals [Abstract]
Accrued Expenses	6. Accrued Expenses
	Accrued expenses consist of the following (in thousands):
		March 31,		December 31,
		2015		2014
	Accrued compensation	$	2,370	$	5,689
	Accrued contracted research and development costs		11,829		7,340
	Accrued professional fees		764		549
	Accrued other		641		442
	Total	$	15,604	$	14,020

ShareBased_Payments

Share-Based Payments	3 Months Ended
	Mar. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Share-Based Payments	7. Share-Based Payments
	2013 Stock Incentive Plan
	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. As of March 31, 2015, the total number of shares reserved under all equity plans is 5,754,381 and the Company had 1,201,353 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.
	The following table summarizes the activity of all stock incentive plans for the three months ended March 31, 2015:
		Number of			Weighted-			Weighted-		Aggregate
		Stock Options			Average			Average		Intrinsic
					Exercise			Remaining		Value
					Price			Contractual		(in thousands)
								Term
								(in years)
	Outstanding at December 31, 2014		3,805,420		$	17.19			7.58	$	360,935
	Granted		922,642			108.21
	Exercised		(175,034	)		9.42
	Forfeited/expired		—			—
	Outstanding at March 31, 2015		4,553,028		$	35.94			7.88	$	279,052
	Exercisable at March 31, 2015		1,664,398		$	5.42			5.91	$	147,926
	Vested and expected to vest at March 31, 2015		4,203,171		$	35.39			7.8	$	259,635
	The weighted-average grant date fair value of options granted was $68.60 and $21.81 during the three months ended March 31, 2015 and 2014, respectively. The total intrinsic value of options exercised was $18.2 million and $24.1 million during the three months ended March 31, 2015 and 2014, respectively.
	At March 31, 2015, the total unrecognized compensation expense related to unvested stock option awards, including estimated forfeitures, was $84.8 million, which the Company expects to recognize over a weighted-average period of approximately 3.5 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 March 31, 2015.
	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 March 31, 2015 was 10,000. The issued and outstanding RSUs vest on the first anniversary of the grant date.
	No RSUs were granted in the three months ended March 31, 2015 and 2014. The Company recorded stock-based compensation expense related to RSUs of $0.1 million for the three months ended March 31, 2015. No compensation expense related to RSUs was recorded during the three months ended March 31, 2014. These amounts are included in the total stock-based compensation expense disclosed below. As of March 31, 2015, there was approximately $0.2 million of total unrecognized compensation expense related to RSUs, which is expected to be recognized over a period of six months.
	The following table presents unvested RSU activity for the three months ended March 31, 2015:
		Three Months Ended
		March 31, 2015
	Unvested shares beginning of period		10,000
	Granted		—
	Vested		—
	Unvested shares end of period		10,000
	Restricted Stock and Early Exercise of Stock Options
	At March 31, 2015, there were 5,681 shares of unvested restricted stock which remain subject to the Company’s right of repurchase.
	Unvested restricted stock activity for the three months ended March 31, 2015 is summarized as follows:
		Three Months Ended
		March 31, 2015
	Unvested shares beginning of period		8,522
	Vested		(2,841	)
	Unvested shares end of period		5,681
	Performance-Based Stock Option Grants
	During the three months ended March 31, 2015 and 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. Performance-based vesting criteria for 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 March 31, 2015, certain of the performance-based milestones had been achieved. The achievement of certain other milestones have been deemed probable and therefore the related expense either has 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 March 31, 2015 and therefore no expense has been recognized related to these awards. During each of the three months ended March 31, 2015 and 2014, the Company recognized stock-based compensation expense of $0.1 million related to stock options with performance-based vesting criteria.
	Stock-Based Compensation Expense
	During the three months ended March 31, 2015 and 2014, the Company recorded stock-based compensation expense for employee and non-employee stock options, the employee stock purchase plan, restricted stock units and restricted stock, which was allocated as follows in the condensed consolidated interim statements of operations (in thousands):
		Three Months Ended March 31,
		2015			2014
	Research and development expense	$	2,611		$	1,064
	General and administrative expense		2,449			441
		$	5,060		$	1,505
	The fair value of each stock option granted to employees is estimated on the date of grant using the Black-Scholes option-pricing model. For non-employees, the fair value of each stock option is estimated on each vesting and reporting date 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:
		Three Months Ended March 31,
		2015			2014
	Risk-free interest rate		1.71	%		1.82	%
	Expected dividend yield		—			—
	Expected term (in years)		6.08			6.08
	Expected volatility		70.35	%		84.18	%
	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 Company issued 10,664 shares during the three months ended March 31, 2015 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 less than $0.1 million of stock-based compensation expense for the three months ended March 31, 2015 related to the 2013 ESPP. No stock-based compensation expense related to the 2013 ESPP was recorded during the three months ended March 31, 2014.

Income_Taxes

Income Taxes	3 Months Ended
	Mar. 31, 2015
Income Tax Disclosure [Abstract]
Income Taxes	8. Income Taxes
	In January 2014, the Company paid $6.0 million as payment in full of its U.S. federal income tax liability related to the year ended December 31, 2011, including $1.5 million of interest and penalties accrued. The Company filed a carryback claim to apply the net losses incurred during the year ended December 31, 2013 against the previous taxable income. The amount to be refunded by the Internal Revenue Service (“IRS”) was recorded as refundable income taxes as of December 31, 2014. During the three months ended March 31, 2015, the Company received the balance of the refundable income tax. There was no (benefit) provision for income taxes during the three months ended March 31, 2015 and 2014.

Loss_per_Share

Loss per Share	3 Months Ended
	Mar. 31, 2015
Earnings Per Share [Abstract]
Loss per Share	9. Loss per Share
	Basic net loss per share is calculated by dividing net loss by the weighted average shares outstanding during the period, without consideration for common stock equivalents. Diluted net loss per share 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 calculation, stock options, unvested restricted stock and employee stock purchase plan options are considered to be common stock equivalents but are excluded from the calculation of diluted net loss per share as their effect would be anti-dilutive; therefore, basic and diluted net loss per share were the same for all periods presented.
	The following common stock equivalents were excluded from the calculation of diluted net loss per share for the periods indicated because including them would have had an anti-dilutive effect:
		Three Months Ended March 31,
		2015		2014
	Stock options and restricted stock units		4,553,028		3,958,495
	Unvested restricted stock		5,681		18,750
	Employee stock purchase plan options		1,125		—
			4,559,834		3,977,245

Subsequent_Events

Subsequent Events	3 Months Ended
	Mar. 31, 2015
Subsequent Events [Abstract]
Subsequent Events	10. Subsequent Events
	Subsequent to March 31, 2015, the Company selected its third novel IDH mutant inhibitor, AG-881, for clinical development. On April 27, 2015, the Company entered into a new joint worldwide development and profit share collaboration and license agreement with Celgene (the “AG-881 US Agreement”), and the Company’s wholly owned subsidiary, Agios International Sarl, entered into a collaboration and license agreement with Celgene International II Sarl (the “AG-881 ROW Agreement” and, together with the AG-881 US Agreement, the “AG-881 Agreements”). The AG-881 Agreements establish a worldwide collaboration focused on the development and commercialization of AG-881 products. Under the terms of the AG-881 Agreements, the Company will receive an initial payment of $10.0 million and is eligible to receive up to $70.0 million in milestone-based payments. The Company will equally split all worldwide development costs, subject to specified exceptions, as well as any profits from any net sales of, or commercialization losses related to, licensed AG-881 products.

Summary_of_Significant_Account1

Summary of Significant Accounting Policies and Recent Accounting Pronouncements (Policies)	3 Months Ended
	Mar. 31, 2015
Accounting Policies [Abstract]
Significant Accounting Policies	Significant Accounting Policies
	There have been no material changes to the significant accounting policies previously disclosed in the Annual Report on Form 10-K for the year ended December 31, 2014.
Recent Accounting Pronouncements	Recent Accounting Pronouncements
	In May 2014, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“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. In April 2015, the FASB proposed a one year deferral of the effective date of this accounting update to annual periods beginning after December 15, 2017, along with an option to permit early adoption. 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.
Fair Value Measurements and Disclosures	The Company records cash equivalents and marketable securities at fair value. Accounting Standards Codification (“ASC”) Topic 820, Fair Value Measurements and Disclosures, established 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, 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.
Investments - Debt and Equity Securities	Marketable securities at March 31, 2015 and December 31, 2014 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 condensed consolidated interim statements of comprehensive loss, until realized. 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 three months ended March 31, 2015 and 2014.
	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 condensed consolidated interim 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.

Fair_Value_Measurements_Tables

Fair Value Measurements (Tables)	3 Months Ended
	Mar. 31, 2015
Fair Value Disclosures [Abstract]
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 March 31, 2015 (in thousands):
		Level 1		Level 2		Level 3		Total
	Cash equivalents	$	32,562	$	720	$	—	$	33,282
	Marketable securities:
	Certificates of deposit		—		11,480		—		11,480
	U.S. Treasuries		393,228		—		—		393,228
		$	425,790	$	12,200	$	—	$	437,990
	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

Marketable_Securities_Tables

Marketable Securities (Tables)	3 Months Ended
	Mar. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Marketable Securities	Marketable securities at March 31, 2015 consist of the following (in thousands):
		Amortized Cost		Unrealized		Unrealized			Fair
				Gains		Losses			Value
	Current:
	Certificates of deposit	$	11,480	$	1	$	(1	)	$	11,480
	U.S. Treasuries		297,883		90		(3	)		297,970
	Non-current:
	U.S. Treasuries		95,154		104		—			95,258
		$	404,517	$	195	$	(4	)	$	404,708
	Marketable securities at December 31, 2014 consist of the following (in thousands):
		Amortized Cost		Unrealized		Unrealized			Fair
				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

Accrued_Expenses_Tables

Accrued Expenses (Tables)	3 Months Ended
	Mar. 31, 2015
Payables and Accruals [Abstract]
Summary of Accrued Expenses	Accrued expenses consist of the following (in thousands):
		March 31,		December 31,
		2015		2014
	Accrued compensation	$	2,370	$	5,689
	Accrued contracted research and development costs		11,829		7,340
	Accrued professional fees		764		549
	Accrued other		641		442
	Total	$	15,604	$	14,020

ShareBased_Payments_Tables

Share-Based Payments (Tables)	3 Months Ended
	Mar. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Summary of Stock Option Activity of All Stock Incentive Plans	The following table summarizes the activity of all stock incentive plans for the three months ended March 31, 2015:
		Number of			Weighted-			Weighted-		Aggregate
		Stock Options			Average			Average		Intrinsic
					Exercise			Remaining		Value
					Price			Contractual		(in thousands)
								Term
								(in years)
	Outstanding at December 31, 2014		3,805,420		$	17.19			7.58	$	360,935
	Granted		922,642			108.21
	Exercised		(175,034	)		9.42
	Forfeited/expired		—			—
	Outstanding at March 31, 2015		4,553,028		$	35.94			7.88	$	279,052
	Exercisable at March 31, 2015		1,664,398		$	5.42			5.91	$	147,926
	Vested and expected to vest at March 31, 2015		4,203,171		$	35.39			7.8	$	259,635
Unvested Stock Unit Activity	The following table presents unvested RSU activity for the three months ended March 31, 2015:
		Three Months Ended
		March 31, 2015
	Unvested shares beginning of period		10,000
	Granted		—
	Vested		—
	Unvested shares end of period		10,000
Unvested Stock Unit Activity	Unvested restricted stock activity for the three months ended March 31, 2015 is summarized as follows:
		Three Months Ended
		March 31, 2015
	Unvested shares beginning of period		8,522
	Vested		(2,841	)
	Unvested shares end of period		5,681
Recorded Stock-Based Compensation Expense for Employee and Non-Employee Stock Options, Restricted Stock Units and Restricted Stock	During the three months ended March 31, 2015 and 2014, the Company recorded stock-based compensation expense for employee and non-employee stock options, the employee stock purchase plan, restricted stock units and restricted stock, which was allocated as follows in the condensed consolidated interim statements of operations (in thousands):
		Three Months Ended March 31,
		2015			2014
	Research and development expense	$	2,611		$	1,064
	General and administrative expense		2,449			441
		$	5,060		$	1,505
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:
		Three Months Ended March 31,
		2015			2014
	Risk-free interest rate		1.71	%		1.82	%
	Expected dividend yield		—			—
	Expected term (in years)		6.08			6.08
	Expected volatility		70.35	%		84.18	%

Loss_per_Share_Tables

Loss per Share (Tables)	3 Months Ended
	Mar. 31, 2015
Earnings Per Share [Abstract]
Common Stock Excluded from Calculation of Diluted Earnings Per Share	The following common stock equivalents were excluded from the calculation of diluted net loss per share for the periods indicated because including them would have had an anti-dilutive effect:
		Three Months Ended March 31,
		2015		2014
	Stock options and restricted stock units		4,553,028		3,958,495
	Unvested restricted stock		5,681		18,750
	Employee stock purchase plan options		1,125		—
			4,559,834		3,977,245

Overview_and_Basis_of_Presenta1

Overview and Basis of Presentation - Additional Information (Detail) (USD $)	12 Months Ended	1 Months Ended		3 Months Ended
In Millions, except Share data, unless otherwise specified	Dec. 31, 2014	Apr. 30, 2014	31-May-14	Mar. 31, 2015
				Diseases
Underwriter [Member]
Basis Of Presentation [Line Items]
Common stock shares purchased	297,968
Private Placement [Member]
Basis Of Presentation [Line Items]
Proceeds from public offering of common stock	31
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		82.3
Public Offering [Member] | Underwriter [Member]
Basis Of Presentation [Line Items]
Common stock shares purchased			300,000
Proceeds from public offering of common stock			12.4
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
Public offering price	111
Proceeds from public offering of common stock	206.9
Minimum [Member]
Basis Of Presentation [Line Items]
Number of rare genetic disease under entity therapeutic group				600

Fair_Value_Measurements_Cash_E

Fair Value Measurements - Cash Equivalents and Marketable Securities Measured at Fair Value on a Recurring Basis (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents and marketable securities, total	$404,708	$453,416
Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents	33,282	12,370
Cash equivalents and marketable securities, total	437,990	465,786
Fair Value, Measurements, Recurring [Member] | Certificates of Deposit [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents and marketable securities, total	11,480	13,155
Fair Value, Measurements, Recurring [Member] | U.S. Treasuries [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents and marketable securities, total	393,228	440,261
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents	32,562	11,410
Cash equivalents and marketable securities, total	425,790	451,671
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | U.S. Treasuries [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents and marketable securities, total	393,228	440,261
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents	720	960
Cash equivalents and marketable securities, total	12,200	14,115
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Certificates of Deposit [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents and marketable securities, total	$11,480	$13,155

Fair_Value_Measurements_Additi

Fair Value Measurements - Additional Information (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
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
Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets classified as Level 3	0	0
Financial liabilities classified as Level 3	$0	$0

Marketable_Securities_Addition

Marketable Securities - Additional Information (Detail) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
	Securities		Securities
Schedule of Available-for-sale Securities [Line Items]
Realized gains or losses on marketable securities	$0	$0
Minimum period to liquidate	12 months		12 months
Debt securities in an unrealized loss position	23		92
Aggregate fair value of debt securities in an unrealized loss position	$42,700,000		$236,900,000
Maximum [Member]
Schedule of Available-for-sale Securities [Line Items]
Investment maturity period, non-current	2 years		2 years
Minimum [Member]
Schedule of Available-for-sale Securities [Line Items]
Investment maturity period, non-current	1 year		1 year

Marketable_Securities_Marketab

Marketable Securities - Marketable Securities (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$404,517	$453,473
Unrealized Gains	195	50
Unrealized Losses	-4	-107
Fair Value	404,708	453,416
Current: [Member] | Certificates of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	11,480	13,160
Unrealized Gains	1
Unrealized Losses	-1	-5
Fair Value	11,480	13,155
Current: [Member] | U.S. Treasuries [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	297,883	314,866
Unrealized Gains	90	45
Unrealized Losses	-3	-32
Fair Value	297,970	314,879
Non-Current: [Member] | U.S. Treasuries [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	95,154	125,447
Unrealized Gains	104	5
Unrealized Losses		-70
Fair Value	$95,258	$125,382

Collaboration_Agreements_Addit

Collaboration Agreements - Additional Information (Detail) (USD $)	1 Months Ended	3 Months Ended		12 Months Ended		1 Months Ended	0 Months Ended
	Apr. 30, 2010	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014	Dec. 31, 2013	Jul. 31, 2014	Jul. 31, 2014
		Candidate
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Agreement amended date	2011-10
Amended collaboration expiration date	2014-04			2016-04	2015-04
Discovery phase years	2 years
Term of the initial discovery period before amendment to extend				5 years	4 years
Term of extended period after amendment of the initial discovery period				6 years	5 years
Eligible payment to be received from extension of the discovery phase				$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
Payment made in pursuant to the amendment				20,000,000
Recognized revenue to extension		0
Increase in revenue		34,202,000	8,411,000
Achievement of an early clinical development milestone event		22,500,000
Dosing of the last patient in a Company-sponsored		25,000,000
Development cost reimbursed		33,100,000
Collaboration receivable		7,000,000		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
Milestone payment reimbursement, percentage		50.00%				50.00%
Reduction in research and development expenditure		-3,800,000
Increase in revenue		5,100,000
Reduction in net loss		8,900,000
Decrease in net loss per share		$0.24
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
Revenue recognized		15,800,000
Celgene [Member] | In Process Research and Development [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue recognition, selling price							13,600,000
Celgene [Member] | Participation Service [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue recognition, selling price							200,000
Celgene [Member] | Planning Services [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Reduction in research and development expenditure		600,000
Revenue		100,000
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
Phase 1 [Member] | Celgene [Member] | In Process Research and Development [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue recognition, selling price							50,800,000
Maximum [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payment upon achievement of a specified clinical development milestone event		25,000,000
Receipts of potential milestone payments		120,000,000
Milestone payments upon achievement of specified regulatory milestone events		70,000,000
Milestone payment upon achievement of a specified commercial milestone event		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

Accrued_Expenses_Summary_of_Ac

Accrued Expenses - Summary of Accrued Expenses (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified
Payables and Accruals [Abstract]
Accrued compensation	$2,370	$5,689
Accrued contracted research and development costs	11,829	7,340
Accrued professional fees	764	549
Accrued other	641	442
Total	$15,604	$14,020

ShareBased_Payments_Additional

Share-Based Payments - Additional Information (Detail) (USD $)	3 Months Ended		0 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Jan. 01, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$5,060,000	$1,505,000
Stock options granted	922,642
Number of shares issued	37,289,050			37,100,513
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]
Unrecognized compensation expense related to stock option	200,000
Total number of unvested stock option	10,000			10,000
Fair value granted amount	0	0
Stock-based compensation expense	100,000	0
Stock based compensation associated with restricted stock, recognition period	6 months
2013 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance	5,754,381
Shares available for future issuance	1,201,353
Shares authorized for issuance			1,484,020
Weighted-average grant date fair value of options	$68.60	$21.81
Intrinsic value of options exercised	18,200,000	24,100,000
Unrecognized compensation expense related to stock option	84,800,000
Weighted-average period to recognize compensation expense	3 years 6 months
Total number of unvested stock option	5,681
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	100,000	100,000
Stock options granted	0	0
2013 Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares issued	10,664
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 $)	3 Months Ended	12 Months Ended
In Thousands, except Share data, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Number of Stock Options, Outstanding, Beginning balance	3,805,420
Number of Stock Options, Granted	922,642
Number of Stock Options, Exercised	-175,034
Number of Stock Options, Forfeited/cancelled	0
Number of Stock Options, Outstanding, Ending balance	4,553,028	3,805,420
Number of Stock Options, Exercisable	1,664,398
Number of Stock Options, Vested and expected to vest	4,203,171
Weighted-Average Exercise Price, Outstanding, Beginning balance	$17.19
Weighted-Average Exercise Price, Granted	$108.21
Weighted-Average Exercise Price, Exercised	$9.42
Weighted-Average Exercise Price, Forfeited/cancelled	$0
Weighted-Average Exercise Price, Outstanding, Ending balance	$35.94	$17.19
Weighted-Average Exercise Price, Exercisable	$5.42
Weighted-Average Exercise Price, Vested and expected to vest	$35.39
Weighted-Average Remaining Contractual Term (in years), Outstanding	7 years 10 months 17 days	7 years 6 months 29 days
Weighted-Average Remaining Contractual Term (in years), Exercisable	5 years 10 months 28 days
Weighted-Average Remaining Contractual Term (in years), Vested and expected to vest	7 years 9 months 18 days
Aggregate Intrinsic Value, Outstanding, Beginning balance	$360,935
Aggregate Intrinsic Value, Outstanding, Ending balance	279,052	360,935
Aggregate Intrinsic Value, Exercisable	147,926
Aggregate Intrinsic Value, Vested and expected to vest	$259,635

ShareBased_Payments_Summary_of1

Share-Based Payments - Summary of Unvested RSUs Activity (Detail) (Restricted Stock Units (RSUs) [Member])	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unvested shares beginning of period	10,000
Granted	0	0
Vested	0
Unvested shares end of period	10,000

ShareBased_Payments_Unvested_R

Share-Based Payments - Unvested Restricted Stock Activity (Detail) (Restricted Stock Units (RSUs) [Member])	3 Months Ended
	Mar. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unvested shares beginning of period	10,000
Vested	0
Unvested shares end of period	10,000
Unvested [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unvested shares beginning of period	8,522
Vested	-2,841
Unvested shares end of period	5,681

ShareBased_Payments_StockBased

Share-Based Payments - Stock-Based Compensation Expense for Employee and Non-Employee Stock Options, Employee Stock Purchase Plan and Restricted Stock (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated share based compensation expense	$5,060,000	$1,505,000
Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated share based compensation expense	2,611,000	1,064,000
General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated share based compensation expense	$2,449,000	$441,000

ShareBased_Payments_Schedule_o

Share-Based Payments - Schedule of Fair Value Option Award Weighted Average Assumptions Used (Detail)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Risk-free interest rate	1.71%	1.82%
Expected dividend yield	0.00%	0.00%
Expected term (in years)	6 years 29 days	6 years 29 days
Expected volatility	70.35%	84.18%

Income_Taxes_Additional_Inform

Income Taxes - Additional Information (Detail) (USD $)	1 Months Ended	3 Months Ended
	Jan. 31, 2014	Mar. 31, 2015	Mar. 31, 2014
Income Tax Disclosure [Abstract]
Income tax liability paid	$6,000,000		$5,958,000
Interest and penalties paid	1,500,000
(Benefit) provision for income taxes		$0	$0

Loss_per_Share_Common_Stock_Ex

Loss per Share - Common Stock Excluded from Calculation of Diluted Net Loss Per Share (Detail)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	4,559,834	3,977,245
Stock Options and Restricted Stock Units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	4,553,028	3,958,495
Unvested Restricted Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	5,681	18,750
Employee Stock Purchase Plan Options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	1,125

Subsequent_Events_Additional_I

Subsequent Events - Additional Information (Detail) (Subsequent Event [Member], USD $)	0 Months Ended
In Millions, unless otherwise specified	Apr. 27, 2015	Apr. 27, 2015
Subsequent Event [Member]
Subsequent Event [Line Items]
Milestone-based payments	$70	$70
Initial payment received	$10