Acceleron Pharma (XLRN) 10-Q 7 Nov 14 2014 Q3 Quarterly report Financial data

Document_and_Entity_Informatio

Document and Entity Information	9 Months Ended
	Sep. 30, 2014	Oct. 31, 2014
Document and Entity Information	'	'
Entity Registrant Name	'ACCELERON PHARMA INC	'
Entity Central Index Key	'0001280600	'
Document Type	'10-Q	'
Document Period End Date	30-Sep-14	'
Amendment Flag	'false	'
Current Fiscal Year End Date	'--12-31	'
Entity Current Reporting Status	'No	'
Entity Filer Category	'Non-accelerated Filer	'
Entity Common Stock, Shares Outstanding	'	32,253,193
Document Fiscal Year Focus	'2014	'
Document Fiscal Period Focus	'Q3	'

Condensed_Balance_Sheets

Condensed Balance Sheets (USD $)	Sep. 30, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Current assets:	'	'
Cash and cash equivalents	$189,256	$113,163
Collaboration receivables (all amounts are with related party)	3,228	3,616
Prepaid expenses and other current assets	3,947	2,243
Total current assets	196,431	119,022
Property and equipment, net	3,379	3,705
Restricted cash	902	913
Other assets	0	92
Total assets	200,712	123,732
Current liabilities:	'	'
Accounts payable	3,314	885
Accrued expenses	5,938	6,927
Deferred revenue	1,382	2,031
Deferred rent	478	499
Notes payable, net of discount	0	16,868
Total current liabilities	11,112	27,210
Deferred revenue, net of current portion	4,967	5,620
Deferred rent, net of current portion	1,984	2,337
Warrants to purchase common stock	15,028	30,753
Total liabilities	33,091	65,920
Commitments and contingencies (Note 13)	'	'
Stockholders’ equity:	'	'
Undesignated preferred stock, $0.001 par value: 25,000,000 shares authorized and no shares issued or outstanding	0	0
Common stock, $0.001 par value: 175,000,000 shares authorized; 32,062,884, and 28,348,630 shares issued and outstanding at September 30, 2014 and December 31, 2013, respectively	33	29
Additional paid-in capital	393,554	250,107
Accumulated deficit	-225,966	-192,324
Total stockholders’ equity	167,621	57,812
Total liabilities and stockholders’ equity	$200,712	$123,732

Condensed_Balance_Sheets_Paren

Condensed Balance Sheets (Parenthetical) (USD $)	Sep. 30, 2014	Dec. 31, 2013
Statement of Financial Position [Abstract]	'	'
Preferred stock, par value (in dollars per share)	$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 (in dollars per share)	$0.00	$0.00
Common stock, shares authorized	175,000,000	175,000,000
Common stock, shares issued	32,062,884	28,348,630
Common stock, shares outstanding	32,062,884	28,348,630

Condensed_Statements_of_Operat

Condensed Statements of Operations and Comprehensive (Loss) Income (USD $)	3 Months Ended				9 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013
Collaboration revenue:	'		'		'		'
License and milestone	$280		$638		$1,302		$36,044
Cost-sharing, net	3,228		3,632		9,592		9,666
Total revenue	3,508	[1]	4,270	[1]	10,894	[1]	45,710	[1]
Costs and expenses:	'		'		'		'
Research and development	11,876		8,143		36,318		25,834
Litigation settlement	0		0		5,000		0
General and administrative	3,023		3,011		10,485		9,472
Total costs and expenses	14,899		11,154		51,803		35,306
(Loss) income from operations	-11,391		-6,884		-40,909		10,404
Other income (expense):	'		'		'		'
Other income (expense), net	3,394		-11,149		8,130		-12,571
Interest income	25		5		59		25
Interest expense	0		-485		-922		-1,646
Total other income (expense), net	3,419		-11,629		7,267		-14,192
Net loss	-7,972		-18,513		-33,642		-3,788
Comprehensive loss	-7,972		-18,513		-33,642		-3,788
Reconciliation of net loss to net loss applicable to common stockholders:	'		'		'		'
Net loss	-7,972		-18,513		-33,642		-3,788
Accretion of dividends, interest, redemption value and issuance costs on redeemable convertible preferred stock	0		-6,272		0		-19,870
Gain on extinguishment of redeemable convertible preferred stock	0		0		0		2,765
Net loss applicable to participating securities	0		0		0		0
Net loss applicable to common stockholders—basic and diluted	-7,972		-24,785		-33,642		-20,893
Net loss applicable to participating securities	0		0		0		0
Net loss applicable to common stockholders—basic and diluted	($7,972)		($24,785)		($33,642)		($20,893)
Net loss per share applicable to common stockholders— basic and diluted: (Note 8)	'		'		'		'
Basic	($0.25)		($5.62)		($1.18)		($6.74)
Diluted	($0.25)		($5.62)		($1.18)		($6.74)
Weighted-average number of common shares used in computing net loss per share applicable to common stockholders:	'		'		'		'
Basic	31,855		4,406		28,594		3,100
Diluted	31,855		4,406		28,594		3,100
[1]	(1) Includes related party revenue (Note 18) $3,508 $4,270 $10,894 $20,682

Condensed_Statements_of_Operat1

Condensed Statements of Operations and Comprehensive (Loss) Income (Parenthetical) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Income Statement [Abstract]	'	'	'	'
Related party revenue	$3,508	$4,270	$10,894	$20,682

Condensed_Statements_of_Cash_F

Condensed Statements of Cash Flows (USD $)	9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013
Operating Activities	'	'
Net (loss) income	($33,642)	($3,788)
Adjustments to reconcile net loss to net cash used in operating activities:	'	'
Depreciation and amortization	832	681
Stock-based compensation	3,327	1,441
(Payment) / accretion of deferred interest	-536	257
Amortization of deferred debt issuance costs	36	182
Change in fair value of warrants	-8,124	12,649
Gain on retirement of warrants	0	-76
Forgiveness of related party receivable	0	237
Changes in assets and liabilities:	'	'
Prepaid expenses and other current assets	-1,723	-762
Collaboration receivables	388	-1,327
Related party receivable	0	-4
Accounts payable	2,429	243
Accrued expenses	-923	-1,602
Deferred revenue	-1,302	-26,044
Deferred rent	-374	-373
Restricted cash	11	0
Net cash used in operating activities	-39,601	-18,286
Investing Activities	'	'
Purchases of property and equipment	-498	-187
Net cash used in investing activities	-498	-187
Financing Activities	'	'
Proceeds from issuance of common stock from public offering, net issuance costs	129,174	87,406
Proceeds from issuance of common stock from private placement	0	10,000
Payments of long-term debt	-16,331	-1,815
Payments made to repurchase redeemable convertible preferred stock, common stock and warrants to purchase common stock	0	-300
Proceeds from exercise of stock options and warrants to purchase common stock	3,349	50
Net cash provided by financing activities	116,192	95,341
Net increase in cash and cash equivalents	76,093	76,868
Cash and cash equivalents at beginning of period	113,163	39,611
Cash and cash equivalents at end of period	189,256	116,479
Supplemental Disclosure of Cash Flow Information:	'	'
Cash paid for interest	1,574	1,262
Supplemental Disclosure of Non-Cash Investing and Financing Activities:	'	'
Accretion of dividends, interest, redemption value, and issuance costs on preferred stock	0	19,870
Cashless exercise of warrants	0	678
Follow-on offering costs included in accounts payable and accrued expense	0	582
Reclassification of warrant liability to additional paid-in capital	7,601	2,013
Purchase of property and equipment included in accounts payable and accrued expenses	7	0
Conversion of redeemable convertible preferred stock into common stock	$0	$286,094

Nature_of_Business

Nature of Business	9 Months Ended
	Sep. 30, 2014
Nature of Business	'
Nature of Business	'
	Nature of Business
	Acceleron Pharma Inc. (Acceleron or the Company) is a biopharmaceutical company focused on the discovery, development and commercialization of novel protein therapeutics for cancer and rare diseases. The Company's research focuses on the biology of the Transforming Growth Factor-Beta (TGF-β) protein superfamily, a large and diverse group of molecules that regulate the growth and repair of tissues throughout the human body. By coupling its discovery and development expertise, including its proprietary knowledge of the TGF-β superfamily, with internal protein engineering and manufacturing capabilities, the Company has built a highly productive research and development platform that has generated numerous innovative protein therapeutics with novel mechanisms of action. The Company has internally discovered three protein therapeutics that are currently being studied in numerous ongoing Phase 2 clinical trials, focused on the areas of cancer and rare diseases.
	The Company is headquartered in Cambridge, Massachusetts and has one wholly-owned subsidiary, Acceleron Securities Corp.
	The Company is subject to risks common to companies in the biotechnology industry, including, but not limited to, risk that the Company never achieves profitability, the need for substantial additional financing that may not be available on attractive terms, risk of relying on third parties, risk of clinical trial failures, dependence on key personnel, protection of proprietary technology and compliance with government regulations.

Basis_of_Presentation

Basis of Presentation	9 Months Ended
	Sep. 30, 2014
Organization, Consolidation and Presentation of Financial Statements [Abstract]	'
Basis of Presentation	'
	Basis of Presentation
	The accompanying interim condensed consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (GAAP). Any reference in these notes to applicable guidance is meant to refer to the authoritative United States generally accepted accounting principles as found in the Accounting Standards Codification (ASC) and Accounting Standards Update (ASU) of the Financial Accounting Standards Board (FASB).
	The accompanying interim condensed consolidated financial statements are unaudited. The unaudited interim financial statements have been prepared on the same basis as the audited annual financial statements as of and for the year ended December 31, 2013, and, in the opinion of management, reflect all adjustments, consisting of normal recurring adjustments, necessary for the fair presentation of the Company’s financial position as of September 30, 2014, and the results of its operations and its cash flows for the three and nine months ended September 30, 2014 and 2013.
	The results for the nine months ended September 30, 2014 are not necessarily indicative of the results to be expected for the year ending December 31, 2014, any other interim periods, or any future year or period. These interim financial statements should be read in conjunction with the audited financial statements as of and for the year ended December 31, 2013, and the notes thereto, together with Management’s Discussion and Analysis of Financial Condition and Results of Operations, contained in the Company’s Annual Report on Form 10-K for the year ended December 31, 2013.
	The accompanying interim condensed consolidated financial statements reflect the application of certain significant accounting policies as described below and elsewhere in these notes to the financial statements. As of September 30, 2014, the Company’s significant accounting policies and estimates, which are detailed in the Company’s Annual Report on Form 10-K for the year ended December 31, 2013, have not changed.

Use_of_Estimates

Use of Estimates	9 Months Ended
	Sep. 30, 2014
Use of Estimates	'
Use of Estimates	'
	Use of Estimates
	The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements, and the reported amounts expensed during the reporting period. Actual results could materially differ from those estimates.
	Management considers many factors in selecting appropriate financial accounting policies and controls, and in developing the estimates and assumptions that are used in the preparation of these financial statements. Management must apply significant judgment in this process. In addition, other factors may affect estimates, including: expected business and operational changes, sensitivity and volatility associated with the assumptions used in developing estimates, and whether historical trends are expected to be representative of future trends. The estimation process often may yield a range of potentially reasonable estimates of the ultimate future outcomes and management must select an amount that falls within that range of reasonable estimates. This process may result in actual results differing materially from those estimated amounts used in the preparation of the financial statements if these results differ from historical experience, or other assumptions do not turn out to be substantially accurate, even if such assumptions are reasonable when made. In preparing these financial statements, management used significant estimates in the following areas, among others: revenue recognition, stock-based compensation expense, the determination of the fair value of stock-based awards, the fair value of liability-classified warrants, accrued expenses, and the recoverability of the Company’s net deferred tax assets and related valuation allowance.

Segment_Information

Segment Information	9 Months Ended
	Sep. 30, 2014
Segment Reporting [Abstract]	'
Segment Information	'
	Segment 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 executive officer view the Company’s operations and manage its business as one operating segment. All material long-lived assets of the Company reside in the United States. The Company does use contract research organizations (CROs) and research institutions located outside the United States. Some of these expenses are subject to collaboration reimbursement which is presented as a component of cost sharing, net in the statement of operations and comprehensive loss.

Cash_and_Cash_Equivalents_and_

Cash and Cash Equivalents and Restricted Cash	9 Months Ended
	Sep. 30, 2014
Cash and Cash Equivalents [Abstract]	'
Cash and Cash Equivalents and Restricted Cash	'
	Cash and Cash Equivalents and Restricted Cash
	The Company considers all highly liquid investments purchased with original maturities of 90 days or less at acquisition to be cash equivalents. Cash and cash equivalents include cash held in banks and amounts held in interest-bearing money market accounts. Cash equivalents are carried at cost, which approximates their fair market value. As of September 30, 2014 and December 31, 2013, the Company maintained letters of credit totaling $0.9 million held in the form of a money market account as collateral for the Company’s facility lease obligations and its credit cards.

Concentrations_of_Credit_Risk_

Concentrations of Credit Risk and Off-Balance Sheet Risk	9 Months Ended
	Sep. 30, 2014
Risks and Uncertainties [Abstract]	'
Concentrations of Credit Risk and Off-Balance Sheet Risk	'
	Concentrations of Credit Risk and Off-Balance Sheet Risk
	The Company has no off-balance sheet risk, such as foreign exchange contracts, option contracts, or other foreign hedging arrangements. Financial instruments that potentially subject the Company to concentrations of credit risk are primarily cash, cash equivalents, restricted cash and collaboration receivables. The Company maintains its cash and cash equivalent balances in the form of money market accounts with financial institutions that management believes are creditworthy. The Company’s investment policy includes guidelines on the quality of the institutions and financial instruments and defines allowable investments that the Company believes minimizes the exposure to concentration of credit risk.
	The Company routinely assesses the creditworthiness of its customers and collaboration partners. The Company has not experienced any material losses related to receivables from individual customers and collaboration partners, or groups of customers. The Company does not require collateral. Due to these factors, no additional credit risk beyond amounts provided for collection losses is believed by management to be probable in the Company’s accounts receivable.

Fair_Value_Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2014
Fair Value Disclosures [Abstract]	'
Fair Value Measurements	'
	Fair Value Measurements
	The following tables set forth the Company’s financial instruments carried at fair value using the lowest level of input applicable to each financial instrument as of September 30, 2014 and December 31, 2013 (in thousands):
		30-Sep-14
		Quoted Prices			Significant Other			Significant		Total
		in Active Markets			Observable			Unobservable
		for Identical Items			Inputs			Inputs
		(Level 1)			(Level 2)			(Level 3)
	Assets:
	Money market funds	$	181,657		$	—		$	—	$	181,657
	Restricted cash	902			—			—		902
	Total assets	$	182,559		$	—		$	—	$	182,559
	Liabilities:
	Warrants to purchase common stock	$	—		$	—		$	15,028	$	15,028
	Total liabilities	$	—		$	—		$	15,028	15,028
		31-Dec-13
		Quoted Prices			Significant other			Significant		Total
		in Active Markets			Observable			Unobservable
		for Identical Items			Inputs			Inputs
		(Level 1)			(Level 2)			(Level 3)
	Assets:
	Money market funds	$	101,394		$	—		$	—	$	101,394
	Restricted cash	913			—			—		913
	Total assets	$	102,307		$	—		$	—	$	102,307
	Liabilities:
	Warrants to purchase common stock	$	—		$	—		$	30,753	$	30,753
	Total liabilities	$	—		$	—		$	30,753	$	30,753
	Items measured at fair value on a recurring basis include warrants to purchase redeemable convertible preferred stock and warrants to purchase common stock (Note 12). During the periods presented, the Company has not changed the manner in which it values assets and liabilities that are measured at fair value using Level 3 inputs.
	The following table sets forth a summary of changes in the fair value of the Company’s preferred and common stock warrant liability, which have been classified within Level 3 of the fair value hierarchy, wherein fair value is estimated using significant unobservable inputs (in thousands):
		Nine Months Ended September 30,
		2014			2013
	Beginning balance	$	30,753		$	6,651
	Change in fair value	(8,124		)	12,649
	Exercises	(7,601		)	(678		)
	Repurchases	—			(83		)
	Conversions	—			(2,013		)
	Ending balance	$	15,028		$	16,526
	The money market funds noted above are included in cash and cash equivalents in the accompanying balance sheets. The Company recognizes transfers between levels of the fair value hierarchy as of the end of the reporting period. There were no transfers within the hierarchy during the nine months ended September 30, 2014 or the year ended December 31, 2013 except for the transfer out of the warrants to purchase redeemable convertible preferred stock as described below.
	During the three months ended September 30, 2013, as a result of the closing of the Company's initial public offering (the IPO), the warrants to purchase preferred stock were converted to warrants to purchase common stock.  The resulting warrants to purchase common stock meet the criteria to be classified as permanent equity and are no longer required to be measured at fair value at each reporting period.
	The fair value of the warrants to purchase preferred stock that were classified as liabilities was estimated using the Black-Scholes option pricing model at the date of issuance and on each re-measurement date. This method of valuation involves using inputs such as the fair value of the Company’s various classes of preferred stock, stock price volatility, the contractual term of the warrants, risk free interest rates, and dividend yields. Due to the nature of these inputs, the valuation of the warrants is considered a Level 3 measurement. See Note 12 for further discussions of the accounting for the warrants, as well as for a summary of the significant inputs and assumptions used to determine the fair value of the warrants.
	The fair value of warrants to purchase common stock that are classified as liabilities is estimated using a Monte Carlo model.  This method of valuation involves using inputs such as the fair value of a share of common stock, stock price volatility, and the contractual term of the warrants.  Due to the nature of these inputs, the valuation for the warrants is considered a Level 3 measurement.
	The Company measures eligible assets and liabilities at fair value, with changes in value recognized in earnings. Fair value treatment may be elected either upon initial recognition of an eligible asset or liability or, for an existing asset or liability, if an event triggers a new basis of accounting. The Company did not elect to remeasure any of its existing financial
	assets or liabilities, and did not elect the fair value option for any financial assets and liabilities transacted in the nine months ended September 30, 2014 or the year ended December 31, 2013.
	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 September 30, 2014 and December 31, 2013, the Company did not have any significant uncertain tax positions.

Net_Loss_Per_Share

Net Loss Per Share	9 Months Ended
	Sep. 30, 2014
Earnings Per Share [Abstract]	'
Net Loss Per Share	'
	Net Loss 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 (in thousands):
		Three Months Ended		Nine Months Ended
		September 30,		September 30,
		2014	2013	2014	2013
	Outstanding stock options	3,459	3,667	3,459	3,690
	Common stock warrants	610	881	610	874
	Shares issuable under employee stock purchase plan	15	—	15	—
	Preferred stock	—	16,658	—	17,609
	Preferred stock warrants	—	130	—	152
		4,084	21,336	4,084	22,325

Comprehensive_Income_Loss

Comprehensive Income (Loss)	9 Months Ended
	Sep. 30, 2014
Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract]	'
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, other events, and circumstances from non-owner sources. Comprehensive income (loss) consists of net income (loss) and other comprehensive income (loss), which includes certain changes in equity that are excluded from net income (loss). Comprehensive income (loss) has been disclosed in the accompanying condensed consolidated statements of operations and comprehensive (loss) income and equals the Company’s net (loss) income for all periods presented.

Subsequent_Events

Subsequent Events	9 Months Ended
	Sep. 30, 2014
Subsequent Events [Abstract]	'
Subsequent Events	'
	Subsequent Events
	The Company considers events or transactions that occur after the balance sheet date but prior to the issuance of the financial statements to provide additional evidence relative to certain estimates or to identify matters that require additional disclosure. The Company has evaluated all subsequent events and determined that there are no material recognized or unrecognized subsequent events requiring disclosure, other than those disclosed in this Report on Form 10-Q and as discussed below in Note 13.

Recently_Adopted_Accounting_Pr

Recently Adopted Accounting Pronouncements	9 Months Ended
	Sep. 30, 2014
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]	'
Recently Adopted Accounting Pronouncements	'
	Recently Adopted Accounting Pronouncements
	From time to time, new accounting pronouncements are issued by the FASB or other standard setting bodies and adopted by the Company as of the specified effective date. Unless otherwise discussed, the Company believes that the impact of recently issued standards that are not yet effective will not have a material impact on its financial position or results of operations upon adoption.

Warrants

Warrants	9 Months Ended
	Sep. 30, 2014
Warrants and Rights Note Disclosure [Abstract]	'
Warrants	'
	Warrants
	Below is a summary of the number of shares issuable upon exercise of outstanding warrants and the terms and accounting treatment for the outstanding warrants (in thousands, except per share data):
		Warrants as of
				Weighted-			Balance Sheet
				Average			Classification
				Exercise
		30-Sep-14	31-Dec-13	Price Per		Expiration	30-Sep-14	31-Dec-13
				Share
	Warrants to purchase common stock	14	46	$	10.92	June 25, 2019	Equity(1)	Equity(1)
	Warrants to purchase common stock	19	64	12.56		March 18, 2020	Equity(2) (3)	Equity(2) (3)
	Warrants to purchase common stock	564	857	5.88		June 10, 2020 - July 9, 2020	Liability(4) (5) (6) (7) (8) (9)	Liability(4) (5) (6) (7) (8) (9)
	Warrants to purchase common stock	13	13	4.00 - 7.40		March 31, 2015 - December 31, 2017	Equity(10)	Equity(10)
	All warrants	610	980	$	6.18
	-1	In March 2014, the warrant holders exercised warrants to purchase 32,050 shares of Common Stock on a net basis, resulting in the issuance of 22,955 shares of Common Stock.
	-2	In March 2014, the warrant holders exercised warrants to purchase 12,738 shares of Common Stock on a net basis, resulting in the issuance of 9,202 shares of Common Stock.
	-3	In April 2014, the warrant holders exercised warrants to purchase 31,847 shares of common stock on a net basis, resulting in the issuance of 21,082 shares of Common Stock.
	-4	In March 2014, the warrant holders exercised warrants to purchase 543 shares of Common Stock on a net basis, resulting in the issuance of 456 shares of Common Stock.
	-5	In March 2014, the warrant holders exercised warrants to purchase 23,445 shares of Common Stock on a cash basis, resulting in the issuance of 23,445 shares of Common Stock.
	-6	In May and June 2014, the warrant holders exercised warrants to purchase 114,103 shares of common stock on a net basis, resulting in the issuance of 92,173 shares of Common Stock.
	-7	In April 2014, the warrant holders exercised warrants to purchase 16,956 shares of common stock on a cash basis, resulting in the issuance of 16,956 shares of Common Stock.
	-8	In August 2014, the warrant holders exercised warrants to purchase 11,611 shares of common stock on a net basis, resulting in the issuance of 9,332 shares of Common Stock.
	-9	In September 2014, the warrant holders exercised warrants to purchase 126,283 shares of common stock on a cash basis, resulting in the issuance of 126,283 shares of Common Stock.
	-10	Warrants to purchase common stock were issued in connection with various debt financing transactions that were consummated in periods prior to December 31, 2012. See discussion below for further details.
	In connection with various financing transactions that were consummated in periods prior to December 31, 2013, the Company issued warrants for the purchase of up to 106,500 shares of the Company’s Series A redeemable convertible preferred stock (Series A Preferred Stock), 31,891 shares of the Company’s Series B redeemable convertible preferred stock (Series B Preferred Stock), 45,786 shares of the Company’s Series C-1 redeemable convertible preferred stock (Series C-1 Preferred Stock), and 63,693 shares of the Company’s Series D-1 redeemable convertible preferred stock (Series D-1 Preferred Stock). Each warrant was immediately exercisable. The warrants to purchase Series A and Series B Preferred Stock expire seven years from the original date of issuance, while the warrants to purchase Series C-1 and Series D-1 Preferred Stock expire ten years from the original date of issuance. The warrants to purchase shares of the Company’s preferred stock have an exercise price equal to the original issuance price of the underlying instrument. Each warrant is exercisable on either a physical settlement or net share settlement basis and the redemption provisions are outside the control of the Company.  In connection with the closing of the Company’s IPO on September 24, 2013, the outstanding warrants to purchase Series B Preferred Stock, Series C-1 Preferred Stock, and Series D-1 Preferred Stock were converted into warrants to purchase the same number of shares of common stock   The exercise prices for each of these warrants remained unchanged.
	The Company follows the provisions of ASC Topic 480, Issuer’s Accounting for Freestanding Warrants and Other Similar Instruments on Shares that Are Redeemable, which requires that warrants to purchase redeemable preferred stock be classified as liabilities. In addition, the value of the warrants is remeasured to the then-current fair value at each reporting date. Changes in fair value are recorded to other income (expense), net. For the nine months ended September 30, 2013 using current assumptions, the remeasurement resulted in an increase in fair value of $1.3 million, which was recorded in other expense, net in the accompanying consolidated statements of operations and comprehensive loss. As a result of the closing of the IPO and the resulting conversion of the warrants to purchase preferred shares into warrants to purchase common stock, the fair value of the warrant liability at September 24, 2013 was reclassified to permanent equity and therefore, is no longer subject to remeasurement.
	In December 2012, the Company modified the warrant to purchase 106,500 shares of Series A Preferred Stock and extended the expiration date from December 21, 2012 to February 28, 2013. During the nine months ended September 30, 2013, the holder of the warrant exercised the warrant on a net basis, resulting in the issuance of 46,668 shares of Series A Preferred Stock. Upon exercise, the Company re-measured the fair value of the warrant and recorded the resulting increase in fair value of $0.1 million as other expense in the accompanying consolidated statement of operations and comprehensive loss for the nine months ended September 30, 2013.
	In connection with the Series E redeemable convertible preferred stock (Series E Preferred Stock) financing transactions that took place in June 2010 and July 2010, the Company issued warrants to purchase up to 871,580 shares of common stock. Each warrant was immediately exercisable and expires ten years from the original date of issuance. The warrants to purchase shares of the Company’s common stock have an exercise price equal to the estimated fair value of the underlying instrument as of the initial date such warrants were issued. Each warrant is exercisable on either a physical settlement or net share settlement basis from the date of issuance. The warrant agreement contains a provision requiring an adjustment to the number of shares in the event the Company issues common stock, or securities convertible into or exercisable for common stock, at a price per share lower than the warrant exercise price. The Company concluded the anti-dilution feature required the warrants to be classified as liabilities under ASC Topic 815, Derivatives and Hedging—Contracts in Entity’s Own Equity (ASC 815). The warrants are measured at fair value, with changes in fair value recognized as a gain or loss to other income (expense) in the statements of operations and comprehensive income (loss) for each reporting period thereafter. The fair value of the common stock warrants were recorded as a discount to the preferred stock issued of $3.0 million, and the preferred stock was being accreted to the redemption value. At the end of each reporting period or through the life of the instrument, the Company remeasured the fair value of the outstanding warrants, using current assumptions, resulting in a (decrease) increase in fair value of $(3.4) million, $10.1 million, $(8.1) million, and $11.3 million, respectively, which was recorded in other expense in the accompanying consolidated statements of operations and comprehensive loss for the three months September 30, 2014 and 2013 and the nine months ended September 30, 2014 and 2013. The Company will continue to re-measure the fair value of the liability associated with the warrants to purchase common stock at the end of each reporting period until the earlier of the exercise or the expiration of the applicable warrants. On March 31, 2013, the Company retired 13,994 warrants to purchase common stock as a consequence of a repurchase of shares from an investor. All remaining outstanding warrants were fully vested and exercisable as of September 30, 2014 and December 31, 2013.

Commitments_and_Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2014
Commitments and Contingencies Disclosure [Abstract]	'
Commitments and Contingencies	'
	Commitments and Contingencies
	Legal Proceedings
	On October 18, 2012, the Salk Institute for Biological Studies (Salk) filed a complaint in the Massachusetts Superior Court for Suffolk County, alleging that the Company breached one of the Company’s two licensing agreements with Salk. The licensing agreement in dispute provides the Company with a license with respect to certain of Salk’s U.S. patents related to the ActRIIB activin receptor proteins. Salk contended that, under the licensing agreement, the Company owed Salk a greater share of the upfront payment that it received under its now-terminated agreement with Shire AG regarding ACE-031 and a share of the upfront payment and development milestone payments that the Company has received under its ongoing collaboration agreement with Celgene regarding luspatercept (also known as ACE-536). Salk was seeking a total of approximately $10.5 million plus interest and a 15% share of future development milestone payments received under the agreement with Celgene regarding luspatercept. The Company contended that no additional amounts were due to Salk and that it had complied with all of its payment obligations under the applicable Salk license agreement.
	The Company moved to dismiss the complaint on December 3, 2012. The Court denied the Company’s motion on February 28, 2013. On March 14, 2013, Acceleron answered the complaint and asserted patent invalidity counterclaims. On the basis of those counterclaims, Acceleron removed the action on March 28, 2013 to the United States District Court for the District of Massachusetts. The parties reached an agreement on a stipulation as to certain patent issues raised in the action, and Acceleron dismissed its counterclaims. The Court held an initial scheduling conference on May 30, 2013, and the fact discovery was closed.
	On July 25, 2014, the Company and the Salk Institute for Biological Studies entered into an amendment (the Amendment) to that certain Exclusive License Agreement between Salk and the Company regarding Activin Receptors (Type IIB) and Related Subject Matter for Therapeutic and Diagnostic Purposes, dated August 11, 2010 (the License Agreement). The License Agreement provides the Company with a license with respect to certain of Salk's U.S. patents related to the ActRIIB activin receptor proteins.
	The Amendment was entered into as a condition to the settlement with Salk that provides for the release of all claims in the lawsuit. Pursuant to the settlement, the Company has made a one-time total payment of $5 million, inclusive of interest, to Salk and the Company has agreed to pay Salk 6% of future development milestone payments received under the agreement with Celgene relating to luspatercept. Finally, the Company and Salk have further agreed that the royalty percentage on net sales of luspatercept will remain at 1% as provided in the original license agreement with Salk, and that such royalty will be payable until June 2022.
	The Company recorded the impact of the settlement as a charge to operations in the accompanying unaudited condensed consolidated statements of operations and comprehensive loss for the nine months ended September 30, 2014.
	Other
	The Company is also party to various agreements, principally relating to licensed technology, that require future payments relating to milestones not met at September 30, 2014 and December 31, 2013, or royalties on future sales of specified products. No milestone or royalty payments under these agreements are expected to be payable in the immediate future. See Note 14 for discussion of these arrangements.
	The Company enters into standard indemnification agreements in the ordinary course of business. Pursuant to the agreements, the Company indemnifies, holds harmless, and agrees to reimburse the indemnified party for losses suffered or incurred by the indemnified party, generally the Company’s business partners or customers, in connection with any U.S. patent or any copyright or other intellectual property infringement claim by any third party with respect to the Company’s products. The term of these indemnification agreements is generally perpetual any time after execution of the agreement. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is unlimited. The Company has never incurred costs to defend lawsuits or settle claims related to these indemnification agreements.

Significant_Agreements

Significant Agreements	9 Months Ended
	Sep. 30, 2014
Significant Agreements [Abstract]	'
Significant Agreements	'
	Significant Agreements
	Celgene
	Overview
	On February 20, 2008 the Company entered into a collaboration, license, and option agreement (the Sotatercept Agreement) with Celgene Corporation (Celgene) relating to sotatercept. On August 2, 2011, the Company entered into a second collaboration, license and option agreement with Celgene for luspatercept (the Luspatercept Agreement), and also amended certain terms of the Sotatercept Agreement. These agreements provide Celgene exclusive licenses for sotatercept and luspatercept in all indications, as well as exclusive rights to obtain a license to certain future compounds. Celgene is a global biopharmaceutical company primarily engaged in the discovery, development and commercialization of innovative therapies designed to treat cancer and immune-inflammatory related diseases.
	Sotatercept Agreement
	Under the terms of the Sotatercept Agreement, the Company and Celgene collaborate worldwide for the joint development and commercialization of sotatercept. The Company also granted Celgene an option to license three discovery stage compounds. Under the terms of the agreement, the Company and Celgene will jointly develop, manufacture and commercialize sotatercept. Celgene paid $45.0 million of nonrefundable, upfront license and option payments to the Company upon the closing of the Sotatercept Agreement.
	The Company retained responsibility for research and development through the end of Phase 2a clinical trials, as well as manufacturing the clinical supplies for these trials. These activities were substantially completed in 2011. Celgene is conducting the ongoing Phase 2 trials for myelodysplastic syndromes (MDS), chronic kidney disease, and β-thalassemia and will be responsible for any Phase 3 clinical trials, as well as additional Phase 2 clinical trials, and will be responsible for overseeing the manufacture of Phase 3 and commercial supplies by third party contract manufacturing organizations. Under the agreement, the Company was eligible to receive clinical milestones of up to $88.0 million, regulatory milestones of up to $272.0 million, and commercial milestones of up to $150.0 million for sotatercept. Clinical milestone payments are triggered upon initiation of a defined phase of clinical research for a product candidate. Regulatory milestone payments are triggered upon the acceptance of the marketing application and upon the approval to market a product candidate by the Food and Drug Administration (FDA) or other global regulatory authorities. Commercial milestone payments are triggered when an approved pharmaceutical product reaches certain defined levels of net sales by Celgene in countries outside of North America. In addition, to the extent sotatercept is commercialized, the Company would be entitled to receive tiered royalty payments in the low-to-mid twenty percent range of net sales from sales generated from all geographies. Royalty payments are subject to certain reductions, including for entry of a generic product onto the market.
	Additionally, for three named discovery stage option programs the Company was eligible to receive option fees of up to $30.0 million, clinical milestones of up to $53.3 million, regulatory milestones of up to $204.0 million, and commercial milestones of up to $150.0 million for each option program. Clinical milestone payments are triggered upon initiation of a defined phase of clinical research for a product candidate. Regulatory milestone payments are triggered upon the acceptance of the marketing application and upon the approval to market a product candidate by the FDA or other global regulatory authorities. Commercial milestone payments are triggered when an approved pharmaceutical product reaches certain defined levels of net sales by Celgene in countries outside of North America. Option fee payments are triggered upon license of any of the option programs by Celgene. In addition, to the extent an option compound is commercialized, the Company would be entitled to receive tiered royalty payments in the low-to-mid twenty percent range of net sales from sales generated from all geographies. Royalty payments are subject to certain reductions, including for entry of a generic product onto the market. None of the three discovery stage programs has advanced to the stage to achieve payment of a milestone.
	In connection with entering into the Sotatercept Agreement, Celgene purchased 457,875 shares of Series C-1 Preferred Stock at the aggregate purchase price of $5.0 million. The Series C-1 Preferred Stock was purchased at an amount that was deemed to represent fair value at the time of purchase. Per our agreement and concurrent with the IPO, Celgene purchased 666,667 shares of Common Stock at the IPO offer price of $15.00 per share for $10.0 million.
	Commensurate with the execution of the Luspatercept Agreement described below, the Company and Celgene agreed to modify the terms of the Sotatercept Agreement. The modified terms included: (1) a change to the responsibility for development costs to align with the Luspatercept Agreement, with Celgene responsible for more than half of the worldwide costs through December 31, 2012, and 100% of the development costs thereafter, (2) future contingent development milestones for sotatercept were amended to a two-category (oncology and non-oncology) structure with potential future clinical milestones of $27.0 million and regulatory milestones of $190.0 million from a four-category (various cancer indications) structure, (3) future contingent development milestones for option compounds were amended to a two-category (oncology and non-oncology) structure with potential future clinical milestones of $25.5 million and regulatory milestones of $142.5 million from a four-category (various cancer indications) structure, and (4) an option to buy down tiered royalty payments on both sotatercept and luspatercept with a one-time $25.0 million payment on or prior to January 1, 2013. The potential commercial milestones remained unchanged. To date, the Company has received $42.1 million in research and development funding and milestone payments for sotatercept under the original and modified agreements. The next likely clinical milestone payment would be $10.0 million and result from Celgene's start of a Phase 3 study in MDS or β-thalassemia.
	The Sotatercept Agreement will expire on a country-by-country basis on the occurrence of both of the following: (1) the expiration of the royalty term with respect to all license products in such country, and (2) the exercise or forfeiture by Celgene of its option with regard to each option compound. The royalty term for each licensed product in each country outside North America is the period commencing with first commercial sale of the applicable licensed product in the applicable country and ending on the latest of expiration of specified patent coverage or a specified period of years. The royalty term for each licensed product in North America is the period commencing with the first commercial sale in North America and ending, on a licensed product and country-by-country basis on the date which commercialization of such licensed product has ceased. The term for each option compound runs for a specified period of years unless Celgene exercises its option, in which case the compound becomes a licensed product, or forfeits its option by failing to make certain payments following the achievement of certain milestones in early clinical development of the option compound.
	Celgene has the right to terminate the agreement with respect to one or more licensed targets or in its entirety, upon 180 days’ notice (or 45 days’ notice if the licensed product has failed to meet certain end point criteria with respect to clinical trials or other development activities). The agreement may also be terminated in its entirety by either Celgene or the Company in the event of a material breach by the other party or in the event of a bankruptcy filing of the other party. There are no cancellation, termination or refund provisions in this arrangement that contain material financial consequences to the Company.
	Luspatercept Agreement
	Under the terms of the Luspatercept Agreement, the Company and Celgene collaborate worldwide for the joint development and commercialization of luspatercept. The Company also granted Celgene an option for future products for which Acceleron files an Investigational New Drug application for the treatment of anemia. Celgene paid $25.0 million on the closing of the Luspatercept Agreement in August, 2011.
	The Company retains responsibility for research and development through the end of Phase 1 and initial Phase 2 clinical trials, as well as manufacturing the clinical supplies for these studies. Celgene will conduct subsequent Phase 2 and Phase 3 clinical studies. Acceleron will manufacture luspatercept for the Phase 1 and Phase 2 clinical trials and Celgene will be responsible for overseeing the manufacture of Phase 3 and commercial supplies by third party contract manufacturing organizations. The Company is eligible to receive clinical milestones of up to $32.5 million, regulatory milestones of up to $105.0 million and commercial milestones of up to $80.0 million for luspatercept. The Company will receive additional, lower development, regulatory, and commercial milestones for any additional products for the treatment of anemia on which Celgene exercises an option. Clinical milestone payments are triggered upon initiation of a defined phase of clinical research for a product candidate. Regulatory milestone payments are triggered upon the acceptance of the marketing application and upon approval to market a protein therapeutic candidate by the FDA or other global regulatory authorities. Commercial milestone payments are triggered when an approved pharmaceutical product reaches certain defined levels of net sales by Celgene in countries outside of North America. In addition, to the extent luspatercept is commercialized, the Company would be entitled to receive tiered royalty payments in the low-to-mid twenty percent range of net sales from sales generated from all geographies. Royalty payments are subject to certain reductions, including for entry of a generic product onto the market.
	Through September 30, 2014, the Company has received $41.4 million in research and development funding and milestone payments for luspatercept. The next likely clinical milestone payment would be $15.0 million and result from the start of a Phase 3 study in MDS or β-thalassemia. The Company has not yet identified additional compounds for the treatment of anemia. Accordingly, there is no assurance that the Company will generate future value from additional programs.
	The Luspatercept Agreement will expire on a country-by-country basis on the occurrence of both of the following: (1) the expiration of the royalty term with respect to all license products in such country, and (2) the end of the option term. The royalty term for each licensed product in each country outside North America is the period commencing with first commercial sale of the applicable licensed product in the applicable country and ending on the latest of expiration of specified patent coverage or a specified period of years. The royalty term for each licensed product in North America is the period commencing with the first commercial sale in North America and ending, on a licensed product and country-by-country basis on the date which commercialization of such licensed product has ceased. The option term runs until the later of (1) the date on which no development or commercialization activities are ongoing or are expected to commence for any licensed products under the Luspatercept Agreement; (2) the date on which no development or commercialization activities are ongoing or are expected to commence for any licensed products under the Sotatercept Agreement and all option rights under the Sotatercept Agreement have been forfeited with respect to each option compound where Celgene has made a payment with respect to such compound; and (3) the royalty term for all licensed products under the Luspatercept Agreement and the Sotatercept Agreement has ended; provided that if at the time the option term would otherwise end any option compounds under the Luspatercept Agreement are in clinical development the option term shall continue until Celgene’s rights to such compound are either exercised or forfeited.
	Celgene has the right to terminate the Luspatercept Agreement with respect to one or more licensed targets or in its entirety, upon 180 days’ notice (or 45 days’ notice if the licensed product has failed to meet certain end point criteria with respect to clinical trials or other development activities), provided that Celgene may not terminate the Luspatercept Agreement prior to the completion of the on-going Luspatercept β-thalassemia and Luspatercept MDS Phase 2 clinical trials, except under certain conditions. The agreement may also be terminated in its entirety by either Celgene or the Company in the event of a material breach by the other party or in the event of a bankruptcy filing of the other party. There are no cancellation, termination or refund provisions in this arrangement that contain material financial consequences to the Company.
	Both Agreements
	The Company and Celgene shared development costs under the Sotatercept and Luspatercept Agreements through December 31, 2012. As of January 1, 2013, Celgene is responsible for paying 100% of worldwide development costs under both agreements. Celgene will be responsible for all commercialization costs worldwide. The Company has the right to co-promote sotatercept, luspatercept and future products under both agreements in North America. Celgene’s option to buy down royalty rates for sotatercept and luspatercept expired unexercised and, therefore, the Company will receive tiered royalties in the low-to-mid twenty percent range on net sales of sotatercept and luspatercept. The royalty schedules for sotatercept and luspatercept are the same.
	Accounting Analysis
	Prior to 2011, the Company accounted for the Sotatercept Agreement, as a multiple element arrangement under ASC 605-25 (prior to the amendments of ASU 2009-13). The Company identified the following deliverables under the arrangement; (1) the license to the ActRIIA compound, (2) right to license option program compounds, (3) participation in the joint development committee, (4) participation in the joint commercialization committee and (5) research and development activities. Under the provisions of ASC 605-25 applicable to the arrangement, since the Company could not establish vendor-specific objective evidence (VSOE) for the undelivered elements, the Company was required to recognize the initial consideration, consisting of the $45.0 million of nonrefundable upfront license and option payments, over the period the undelivered elements were to be delivered, which was initially estimated to be 15 years. As of the date of the modification of the agreement, there was approximately $34.7 million of deferred revenue under the arrangement.
	As a result of the material modifications to the cost sharing obligations, milestone payments structure and royalty payment structure, the Company concluded the modification represented a significant modification under ASU 2009-13, which required the Company to apply the updated provisions of ASU 2009-13 subsequent to the modification.
	Because the Luspatercept Agreement and the amendment to the Sotatercept Agreement were negotiated in contemplation of each other, and the Company had not yet completed all of its obligations pursuant to the Sotatercept Agreement, the agreements were considered one arrangement for accounting purposes. The deliverables under the combined arrangement include: (1) licenses to develop and commercialize sotatercept and luspatercept, (2) performance of research and
	development services, (3) participation on the joint development committees, and (4) the performance of manufacturing services to provide clinical material to Celgene. The Company has determined the option to future products related to the treatment of anemia represents a substantive option. The Company is under no obligation to discover, develop or deliver any new compounds that modulate anemia and Celgene is not contractually obligated to exercise the option. As a result, the Company is at risk as to whether Celgene will exercise the option.
	All of these deliverables identified in the arrangement were deemed to have stand-alone value and to meet the criteria to be accounted for as separate units of accounting under ASC 605-25. Factors considered in making this determination included, among other things, the subject of the licenses, the nature of the research and development services, and the capabilities of Celgene.
	The total arrangement consideration of $77.7 million under the Luspatercept Agreement and amended Sotatercept Agreement consisted of (1) the $25.0 million up-front payment for the license of luspatercept, (2) the remaining deferred revenue from the Sotatercept Agreement of $34.7 million, and (3) estimated payments for development activities and manufacturing services of $18.0 million. The Company used its best estimate of selling price (BESP) for each of the undelivered elements as the Company did not have VSOE or third-party evidence (TPE) of selling price for each deliverable. The Company’s BESP considered its development plan for the compounds, expected manufacturing services, and reimbursement from Celgene (reimbursement of more than half of development expenses through December 31, 2012 and 100% thereafter). The Company determined its BESP for each of the undelivered elements under the arrangements as of the arrangement execution date as follows:
	•	$18.8 million for research and development services
	•	$2.9 million for the sotatercept joint development committee
	•	$3.7 million for the luspatercept joint development committee
	•	$2.8 million for the manufacturing services
	After determining the BESP of the undelivered elements, the remaining consideration of $49.5 million was recognized upon execution of the arrangements. The difference between the estimated payments of $18.0 million and the estimated selling prices which totaled $28.2 million, using BESP, for undelivered elements was $10.2 million. This amount was deferred at inception and will be recognized as the undelivered elements are delivered, using the proportional performance method, or ratably in the case of performance on the Joint Development Committee.
	As noted above, the total arrangement consideration includes estimated payments for development activities and manufacturing services identified at the outset of the Luspatercept Agreement and amended Sotatercept Agreement. At the end of each reporting period, the Company reassesses the estimated payments to be received related to these services and the BESP of the undelivered elements based upon the Company’s current estimates. The Company accounts for such changes as a change in accounting estimate and the cumulative impact of any change is reflected in the period of change.
	During 2011, the Company achieved a $7.5 million clinical milestone under its Luspatercept Agreement, related to the dosing of the first patient in a multiple-dose clinical trial. The Company evaluated the milestone and determined that it was not substantive, as there was no significant uncertainty to achieving the milestone upon execution of the Luspatercept Agreement. As such, the Company allocated the $7.5 million payment based on the allocation of arrangement consideration determined at the execution date of the Luspatercept Agreement and amended Sotatercept Agreement. Based on this allocation, the Company recognized $4.8 million of the payment upon achievement, with the remaining $2.7 million recognized as revenue as the undelivered elements are delivered, consistent with the treatment of the up-front payment. During January 2013, the Company achieved a $10.0 million clinical milestone under its Luspatercept Agreement, related to the dosing of the first patient for a Phase 2 clinical trial. The Company evaluated the milestone and deemed it to be substantive and consistent with the definition of a milestone included in ASU 2010-17 and, accordingly, recognized the $10.0 million payment in revenue during the nine months ended September 30, 2013. The remaining development milestones under the Luspatercept and Sotatercept Agreements were deemed to be substantive and consistent with the definition of a milestone included in ASU 2010-17 and, accordingly, the Company will recognize payments related to the achievement of such milestones, if any, when such milestone is achieved. Factors considered in this determination included scientific and regulatory risks that must be overcome to achieve the milestones, the level of effort and investment required to achieve each milestone, and the monetary value attributed to each milestone. During the three months ended September 30, 2014 and 2013, and the nine months ended September 30, 2014 and 2013, the Company recognized $0.3 million, $0.6 million, $1.3 million, and $1.7 million, respectively, of the total deferred revenue as license and milestone revenue in the accompanying consolidated statements of operations and comprehensive loss.
	As noted above, under the terms of the Luspatercept Agreement the Company retained responsibility for certain research and development activities through the completion of Phase 1 and initial Phase 2 clinical trials, as well as manufacturing the clinical supplies for these studies. Celgene is responsible for the conduct of subsequent Phase 2 and Phase 3 clinical studies. In November, 2013, the Company agreed to conduct additional activities for the benefit of the luspatercept program including certain clinical and non-clinical services such as multiple toxicology studies and associated assay development and sample testing, clinical extension studies, and market development work. These activities will be reimbursed under the same terms and rates of the existing Agreements. The Company evaluated the additional services to be provided and determined that as the Company is under no obligation to conduct these additional activities, these services do not represent a deliverable under or modification to the Luspatercept Agreement, but rather, represent a separate services arrangement which should be accounted for as the services are delivered.
	Pursuant to the terms of the agreement, Celgene and the Company shared development costs, with Celgene responsible for substantially more than half of the costs for sotatercept and luspatercept until December 31, 2012 and 100% of the costs from January 1, 2013 and thereafter. Payments from Celgene with respect to research and development costs incurred by the Company are recorded as cost-sharing revenue. Payments by the Company to Celgene for research and development costs incurred by Celgene are recorded as a reduction to cost-sharing revenue. During the three months ended September 30, 2014 and 2013, and the nine months ended September 30, 2014 and 2013, the Company recorded net cost-sharing revenue of $3.2 million, $3.6 million, $9.6 million, and $9.0 million, respectively.
	Other Agreements
	Shire License
	In September 2010, the Company entered into a license and collaboration agreement granting Shire AG the exclusive right to develop, manufacture and commercialize ActRIIB compounds in territories outside North America. Shire also received the right to conduct research and manufacture commercial supplies in North America for ActRIIB compounds. The lead ActRIIB compound was designated ACE-031. Under the initial development plan, the Company and Shire share the costs associated with developing and commercializing ACE-031 in Duchenne Muscular Dystrophy. In September 2010, Shire made a nonrefundable, up-front license payment to the Company of $45.0 million. In accordance with the Company’s revenue recognition policy prior to the adoption of ASU 2009-13, the up-front license payment of $45.0 million was deferred, and was to be recognized as revenue ratably over three years, which represented the original period over which the Company expected to perform and deliver research and development and manufacturing services. On February 8, 2011, the FDA placed ACE-031 on clinical hold. The Company re-assessed the duration of its deliverables under the license agreement and estimated the new term to be approximately five years. The adjustment was treated as a change in accounting estimate with the remaining deferred revenue of $38.8 million at February 8, 2011, recognized prospectively over the new period of research and development and manufacturing services. In April 2013, the Company and Shire determined not to further pursue development of ACE-031 and Shire sent the Company a notice of termination for the ACE-31 collaboration. The collaboration terminated effective June 30, 2013. Upon the effectiveness of the termination of the Shire Agreement in the second quarter of 2013, the Company accelerated the recognition of $22.4 million of remaining deferred revenue from upfront non-refundable payments received under the Shire Agreement as it had no further obligation for deliverables under the Shire Agreement. During the nine months ended September 30, 2013, the Company recognized $24.3 million of the up-front, non-refundable payments as license and milestone revenue in the accompanying consolidated statements of operations and comprehensive loss. No amounts we recognized subsequent to the June 30, 2013 termination.
	The agreement also included contingent milestone payments, based on the achievement of development milestones totaling $223.8 million and commercial milestones of $228.8 million for ActRIIB compounds. The milestones under the Shire Agreement were deemed to be substantive and consistent with the definition of a milestone included in ASU 2010-17 and, accordingly, the Company recognized payments related to the achievement of such milestones, if any, when such milestone was achieved. Factors considered in this determination included scientific and regulatory risks that must be overcome to achieve the milestones, the level of effort and investment required to achieve each milestone, and the monetary value attributed to each milestone.
	Pursuant to the terms of the agreement, Shire and the Company shared development costs, with Shire responsible for 65% of the costs for ACE-031 and 55% of the costs for licensed compounds other than ACE-031. Payments from Shire with respect to research and development costs incurred by the Company are recorded as cost-sharing revenue. Payments by the Company to Shire for research and development costs incurred by Shire are recorded as a reduction to cost-sharing revenue. During the nine months ended September 30, 2013, the Company recorded net cost-sharing revenue of $0.6 million, which includes payments to Shire of $0.2 million, which are recorded as contra-revenue in the accompanying consolidated statements of operations and comprehensive loss. No amounts we recognized subsequent to the June 30, 2013 termination.
	Other
	In 2004, the Company entered into a license agreement with a non-profit institution for an exclusive, sublicensable, worldwide, royalty-bearing license to certain patents developed by the institution (Primary Licensed Products). In addition, the Company was granted a non-exclusive, non-sub-licensable license for Secondary Licensed Products. As compensation for the licenses, the Company issued 62,500 shares of its common stock to the institution, the fair value of which was $25,000, and was expensed during 2004 to research and development expense. The Company also agreed to pay specified development milestone payments totaling up to $2.0 million for sotatercept and $0.7 million for luspatercept. In addition, the Company is obligated to pay milestone fees based on the Company’s research and development progress, and U.S. sublicensing revenue ranging from 10%-25%, as well as a royalty ranging from 1.0%-3.5% of net sales on any products under the licenses. During the three months ended September 30, 2014 and 2013, and the nine months ended September 30, 2014 and 2013, the Company expensed milestones and fees of $0.1 million, $0.1 million, $0.1 million, and $0.1 million defined under the agreement, which is recorded as research and development expense.
	In 2004, the Company entered into another license agreement with certain individuals for an exclusive, sublicensable, worldwide, royalty-bearing license to certain patents developed by the individuals. The Company agreed to pay specified development and sales milestone payments aggregating up to $1.0 million relating to the development and commercialization of dalantercept. In addition, the Company is required to pay royalties in the low single-digits on worldwide net product sales of dalantercept, with royalty obligations continuing at a 50% reduced rate for a period of time after patent expiration. If the Company sublicenses its patent rights, it will owe a percentage of sublicensing revenue, excluding payments based on the level of sales, profits or other levels of commercialization. During the nine months ended September 30, 2014 and 2013, the Company did not reach any milestones defined under the agreement and, therefore, no amounts have been paid or expensed.
	During 2012, the Company executed a license agreement with a research institution for an exclusive, sublicensable, worldwide, royalty-bearing license. The Company is obligated to pay development milestones and commercial milestone fees totaling up to $1.0 million. Under the agreement, if the Company engages the inventors in the clinical research, the development milestones are waived and commercial milestones shall change to $0.8 million plus any waived milestones. The Company will also pay $25,000 annually upon first commercial sale as well as royalties of 1.5% of net sales on any products developed under the patents. During the nine months ended September 30, 2014 and 2013, the Company did not reach any milestones defined under the agreement and, therefore, no amounts have been paid or expensed.
	In May 2014, the Company executed a collaboration agreement with a research technology company. The Company paid an upfront and research fee of $0.3 million upon execution of the agreement and the Company is obligated to pay additional research fees of approximately $1.0 million over approximately the next year, depending on the success of the research program. The Company also received an option to obtain a commercial license to the molecules developed during the collaboration. During the three and nine months ended September 30, 2014, the Company expensed milestones and fees of $0.3 million and $0.5 million, respectively, which is recorded as research and development expense.

StockBased_Compensation

Stock-Based Compensation	9 Months Ended
	Sep. 30, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]	'
Stock-Based Compensation	'
	Stock-Based Compensation
	The Company recognized stock-based compensation expense related to stock options and the employee stock purchase plan totaling $1.2 million and $0.5 million during the three months ended September 30, 2014 and 2013, respectively, and $3.3 million and $1.4 million during the nine months ended September 30, 2014 and 2013, respectively.
	Total compensation cost recognized for all stock-based compensation awards in the consolidated statements of operations and comprehensive loss is as follows (in thousands):
		Three Months Ended							Nine Months Ended
		September 30,							September 30,
		2014			2013				2014			2013
	Research and development	$	486		$	149			$	1,339		$	460
	General and administrative	708			344				1,988			981
		$	1,194		$	493			$	3,327		$	1,441
	The fair value of each stock option issued to employees was estimated at the date of grant using the Black-Scholes option pricing model with the following weighted-average assumptions:
		Three Months Ended					Nine Months Ended
		September 30,					September 30,
		2014		2013(1)			2014			2013
	Expected volatility	70.7	%	—	%		71.2	%		70.3	%
	Expected term (in years)	6		0			6			6
	Risk-free interest rate	1.9	%	—	%		1.8	%		1.4	%
	Expected dividend yield	—	%	—	%		—	%		—	%
	-1	No grants were made in the three months ended September 30, 2013.
	The following table summarizes the stock option activity under the 2003 and 2013 Plans during the nine months ended September 30, 2014 (in thousands):
		Number		Weighted-				Weighted-		Aggregate
		of Grants		Average				Average		Intrinsic
				Exercise				Contractual		Value(1)
				Price				Life (in years)
				Per Share
	Outstanding at December 31, 2013	3,942		$	7.05			6.43
	Granted	192		$	38.56
	Exercised	(627	)	$	3.78
	Canceled or forfeited	(48	)	$	21.5
	Outstanding at September 30, 2014	3,459		$	9.19			6.15		$	74,526
	Exercisable at September 30, 2014	2,361		$	5.34			5.03		$	58,971
	Vested and expected to vest at September 30, 2014(2)	3,400		$	9			6.1		$	73,832
	-1	The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the estimated fair value of the common stock for the options that were in the money at September 30, 2014.
	-2	This represents the number of vested options at September 30, 2014, plus the number of unvested options expected to vest at September 30, 2014, based on the unvested options outstanding at September 30, 2014, adjusted for the estimated forfeiture rate.
	During the nine months ended September 30, 2014, the Company granted stock options to purchase an aggregate of 191,950 shares of its common stock, with a weighted-average grant date fair value of options granted of $24.58.
	During the nine months ended September 30, 2014, current and former employees of the Company exercised a total of 626,835 options, resulting in total proceeds of $2.4 million.
	The aggregate intrinsic value of options exercised during the nine months ended September 30, 2014 was $19.0 million.
	As of September 30, 2014, there was $11.1 million of unrecognized compensation expense related to unvested stock options that is expected to be recognized over a weighted-average period of 2.4 years.

Income_Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2014
Income Tax Disclosure [Abstract]	'
Income Taxes	'
	Income Taxes
	For the three and nine months ended September 30, 2014 and 2013, the Company did not record a current or deferred income tax expense or benefit.
	The Company has evaluated the positive and negative evidence bearing upon the realizability of its deferred tax assets. Based on the Company’s history of operating losses, the Company has concluded that it is more likely than not that the benefit of its deferred tax assets will not be realized. Accordingly, the Company has provided a full valuation allowance for deferred tax assets as of September 30, 2014 and December 31, 2013.
	The Company files income tax returns in the United States, and various state and foreign jurisdictions. The federal, state and foreign income tax returns are generally subject to tax examinations for the tax years ended December 31, 2009 through December 31, 2012. To the extent the Company has tax attribute carryforwards, the tax years in which the attribute was generated may still be adjusted upon examination by the Internal Revenue Service, state or foreign tax authorities to the extent utilized in a future period.

LongTerm_Debt

Long-Term Debt	9 Months Ended
	Sep. 30, 2014
Debt Disclosure [Abstract]	'
Long-Term Debt	'
	Long-Term Debt
	On June 7, 2012, the Company entered into a loan and security agreement (the Loan Agreement) with three lenders, pursuant to which the Company received a loan in the aggregate principal amount of $20.0 million. The Company was required to repay the aggregate principal balance under the Loan Agreement in 42 months. The first 12 payments were interest only and the remaining 30 payments were equal monthly installments of principal plus interest. The Loan Agreement provided that the interest only period could be extended under certain circumstances. The Company did not trigger the requirements and began paying principal in July 2013.
	Per annum interest was payable at 8.5%. The Loan Agreement also included a closing fee of $0.2 million. The Company amortized the cost over the 42 months of loan. The Loan Agreement was also subject to an additional deferred payment of $1.2 million due with the final payment. The Company recorded the deferred payment to interest expense over the term of the Loan Agreement. The resulting effective interest rate is approximately 11.8%. The Company was not subject to any financial covenants and the Loan Agreement was secured by a lien on all of the Company’s personal property as of, or acquired after, the date of the Loan Agreement, except for intellectual property.
	On March 12, 2014, the Company repaid the outstanding balance of the Loan Agreement. At the time of repayment the Company recognized interest expense related to the remaining $0.6 million of the $1.2 million deferred payment due with the final payment. The Company also recognized $0.3 million in prepayment fees as additional expense.

Related_Party_Transactions

Related Party Transactions	9 Months Ended
	Sep. 30, 2014
Related Party Transactions [Abstract]	'
Related Party Transactions	'
	Related Party Transactions
	Celgene Corporation (Celgene)
	In connection with its entry into the collaboration agreement with Celgene, on February 2008, the Company sold Celgene 457,875 shares of its Series C-1 Preferred Stock. As part of the Company’s June 2010 Series E financing, Celgene purchased 36,496 shares of Series E Preferred Stock and received warrants to purchase 38,979 shares of common stock. As part of the Company’s December 2011 Series F financing, Celgene purchased 1,990,446 shares of Series F Preferred Stock. In connection with the Company’s September 2013 initial public offering, Celgene purchased 666,667 shares of common stock. In connection with the Company's January 2014 public offering, Celgene purchased 300,000 shares of common stock. In May 2014 Celgene purchased 1,100,000 shares of common stock from five current shareholders of the Company. As a result of these transactions, Celgene owned 12.8% and 9.7% of the Company’s fully diluted equity as of September 30, 2014 and December 31, 2013, respectively. Refer to Note 14 for additional information regarding this collaboration agreement.
	During the nine months ended September 30, 2014, the Company recognized $10.9 million in collaboration revenue under the Celgene collaboration arrangement and, as of September 30, 2014, had $6.3 million of deferred revenue related to the Celgene collaboration arrangement.
	The Company recognized revenue from Celgene during the three and nine months ended September 30, 2014 and 2013 as follows (in thousands):
		Three Months Ended				Nine Months Ended
		September 30,				September 30,
		2014		2013		2014		2013
	License and milestone	$	280	$	638	$	1,302	$	11,721
	Cost sharing, net	3,228		3,632		9,592		8,961
		$	3,508	$	4,270	$	10,894	$	20,682

Basis_of_Presentation_Policies

Basis of Presentation (Policies)	9 Months Ended
	Sep. 30, 2014
Organization, Consolidation and Presentation of Financial Statements [Abstract]	'
Basis of Presentation	'
	The accompanying interim condensed consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (GAAP). Any reference in these notes to applicable guidance is meant to refer to the authoritative United States generally accepted accounting principles as found in the Accounting Standards Codification (ASC) and Accounting Standards Update (ASU) of the Financial Accounting Standards Board (FASB).
	The accompanying interim condensed consolidated financial statements are unaudited. The unaudited interim financial statements have been prepared on the same basis as the audited annual financial statements as of and for the year ended December 31, 2013, and, in the opinion of management, reflect all adjustments, consisting of normal recurring adjustments, necessary for the fair presentation of the Company’s financial position as of September 30, 2014, and the results of its operations and its cash flows for the three and nine months ended September 30, 2014 and 2013.
	The results for the nine months ended September 30, 2014 are not necessarily indicative of the results to be expected for the year ending December 31, 2014, any other interim periods, or any future year or period. These interim financial statements should be read in conjunction with the audited financial statements as of and for the year ended December 31, 2013, and the notes thereto, together with Management’s Discussion and Analysis of Financial Condition and Results of Operations, contained in the Company’s Annual Report on Form 10-K for the year ended December 31, 2013.
	The accompanying interim condensed consolidated financial statements reflect the application of certain significant accounting policies as described below and elsewhere in these notes to the financial statements. As of September 30, 2014, the Company’s significant accounting policies and estimates, which are detailed in the Company’s Annual Report on Form 10-K for the year ended December 31, 2013, have not changed.

Fair_Value_Measurements_Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2014
Fair Value Disclosures [Abstract]	'
Schedule of financial instruments carried at fair value	'
	The following tables set forth the Company’s financial instruments carried at fair value using the lowest level of input applicable to each financial instrument as of September 30, 2014 and December 31, 2013 (in thousands):
		30-Sep-14
		Quoted Prices			Significant Other			Significant		Total
		in Active Markets			Observable			Unobservable
		for Identical Items			Inputs			Inputs
		(Level 1)			(Level 2)			(Level 3)
	Assets:
	Money market funds	$	181,657		$	—		$	—	$	181,657
	Restricted cash	902			—			—		902
	Total assets	$	182,559		$	—		$	—	$	182,559
	Liabilities:
	Warrants to purchase common stock	$	—		$	—		$	15,028	$	15,028
	Total liabilities	$	—		$	—		$	15,028	15,028
		31-Dec-13
		Quoted Prices			Significant other			Significant		Total
		in Active Markets			Observable			Unobservable
		for Identical Items			Inputs			Inputs
		(Level 1)			(Level 2)			(Level 3)
	Assets:
	Money market funds	$	101,394		$	—		$	—	$	101,394
	Restricted cash	913			—			—		913
	Total assets	$	102,307		$	—		$	—	$	102,307
	Liabilities:
	Warrants to purchase common stock	$	—		$	—		$	30,753	$	30,753
	Total liabilities	$	—		$	—		$	30,753	$	30,753
Summary of changes in the fair value of the preferred and common stock warrant liability	'
	The following table sets forth a summary of changes in the fair value of the Company’s preferred and common stock warrant liability, which have been classified within Level 3 of the fair value hierarchy, wherein fair value is estimated using significant unobservable inputs (in thousands):
		Nine Months Ended September 30,
		2014			2013
	Beginning balance	$	30,753		$	6,651
	Change in fair value	(8,124		)	12,649
	Exercises	(7,601		)	(678		)
	Repurchases	—			(83		)
	Conversions	—			(2,013		)
	Ending balance	$	15,028		$	16,526

Net_Loss_Per_Share_Tables

Net Loss Per Share (Tables)	9 Months Ended
	Sep. 30, 2014
Earnings Per Share [Abstract]	'
Schedule of anti-dilutive common stock equivalents excluded from the calculation of diluted net loss 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 (in thousands):
		Three Months Ended		Nine Months Ended
		September 30,		September 30,
		2014	2013	2014	2013
	Outstanding stock options	3,459	3,667	3,459	3,690
	Common stock warrants	610	881	610	874
	Shares issuable under employee stock purchase plan	15	—	15	—
	Preferred stock	—	16,658	—	17,609
	Preferred stock warrants	—	130	—	152
		4,084	21,336	4,084	22,325

Warrants_Tables

Warrants (Tables)	9 Months Ended
	Sep. 30, 2014
Warrants and Rights Note Disclosure [Abstract]	'
Summary of the number of shares issuable upon exercise of outstanding warrants and the terms and accounting treatment for the outstanding warrants	'
	Below is a summary of the number of shares issuable upon exercise of outstanding warrants and the terms and accounting treatment for the outstanding warrants (in thousands, except per share data):
		Warrants as of
				Weighted-			Balance Sheet
				Average			Classification
				Exercise
		30-Sep-14	31-Dec-13	Price Per		Expiration	30-Sep-14	31-Dec-13
				Share
	Warrants to purchase common stock	14	46	$	10.92	June 25, 2019	Equity(1)	Equity(1)
	Warrants to purchase common stock	19	64	12.56		March 18, 2020	Equity(2) (3)	Equity(2) (3)
	Warrants to purchase common stock	564	857	5.88		June 10, 2020 - July 9, 2020	Liability(4) (5) (6) (7) (8) (9)	Liability(4) (5) (6) (7) (8) (9)
	Warrants to purchase common stock	13	13	4.00 - 7.40		March 31, 2015 - December 31, 2017	Equity(10)	Equity(10)
	All warrants	610	980	$	6.18
	-1	In March 2014, the warrant holders exercised warrants to purchase 32,050 shares of Common Stock on a net basis, resulting in the issuance of 22,955 shares of Common Stock.
	-2	In March 2014, the warrant holders exercised warrants to purchase 12,738 shares of Common Stock on a net basis, resulting in the issuance of 9,202 shares of Common Stock.
	-3	In April 2014, the warrant holders exercised warrants to purchase 31,847 shares of common stock on a net basis, resulting in the issuance of 21,082 shares of Common Stock.
	-4	In March 2014, the warrant holders exercised warrants to purchase 543 shares of Common Stock on a net basis, resulting in the issuance of 456 shares of Common Stock.
	-5	In March 2014, the warrant holders exercised warrants to purchase 23,445 shares of Common Stock on a cash basis, resulting in the issuance of 23,445 shares of Common Stock.
	-6	In May and June 2014, the warrant holders exercised warrants to purchase 114,103 shares of common stock on a net basis, resulting in the issuance of 92,173 shares of Common Stock.
	-7	In April 2014, the warrant holders exercised warrants to purchase 16,956 shares of common stock on a cash basis, resulting in the issuance of 16,956 shares of Common Stock.
	-8	In August 2014, the warrant holders exercised warrants to purchase 11,611 shares of common stock on a net basis, resulting in the issuance of 9,332 shares of Common Stock.
	-9	In September 2014, the warrant holders exercised warrants to purchase 126,283 shares of common stock on a cash basis, resulting in the issuance of 126,283 shares of Common Stock.
	-10	Warrants to purchase common stock were issued in connection with various debt financing transactions that were consummated in periods prior to December 31, 2012. See discussion below for further details.

StockBased_Compensation_Tables

Stock-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]	'
Schedule of total compensation cost recognized for all stock-based compensation awards in the statements of operations and comprehensive income (loss)	'
	Total compensation cost recognized for all stock-based compensation awards in the consolidated statements of operations and comprehensive loss is as follows (in thousands):
		Three Months Ended							Nine Months Ended
		September 30,							September 30,
		2014			2013				2014			2013
	Research and development	$	486		$	149			$	1,339		$	460
	General and administrative	708			344				1,988			981
		$	1,194		$	493			$	3,327		$	1,441
Schedule of weighted-average assumptions used for estimating fair value of each option issued to employees at the date of grant using the Black-Scholes option pricing model	'
	The fair value of each stock option issued to employees was estimated at the date of grant using the Black-Scholes option pricing model with the following weighted-average assumptions:
		Three Months Ended					Nine Months Ended
		September 30,					September 30,
		2014		2013(1)			2014			2013
	Expected volatility	70.7	%	—	%		71.2	%		70.3	%
	Expected term (in years)	6		0			6			6
	Risk-free interest rate	1.9	%	—	%		1.8	%		1.4	%
	Expected dividend yield	—	%	—	%		—	%		—	%
Summary of the stock option activity under the 2003 Plan	'
	The following table summarizes the stock option activity under the 2003 and 2013 Plans during the nine months ended September 30, 2014 (in thousands):
		Number		Weighted-				Weighted-		Aggregate
		of Grants		Average				Average		Intrinsic
				Exercise				Contractual		Value(1)
				Price				Life (in years)
				Per Share
	Outstanding at December 31, 2013	3,942		$	7.05			6.43
	Granted	192		$	38.56
	Exercised	(627	)	$	3.78
	Canceled or forfeited	(48	)	$	21.5
	Outstanding at September 30, 2014	3,459		$	9.19			6.15		$	74,526
	Exercisable at September 30, 2014	2,361		$	5.34			5.03		$	58,971
	Vested and expected to vest at September 30, 2014(2)	3,400		$	9			6.1		$	73,832
	-1	The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the estimated fair value of the common stock for the options that were in the money at September 30, 2014.
	-2	This represents the number of vested options at September 30, 2014, plus the number of unvested options expected to vest at September 30, 2014, based on the unvested options outstanding at September 30, 2014, adjusted for the estimated forfeiture rate.

Related_Party_Transactions_Tab

Related Party Transactions (Tables)	9 Months Ended
	Sep. 30, 2014
Related Party Transactions [Abstract]	'
Schedule of revenues from related party	'
	The Company recognized revenue from Celgene during the three and nine months ended September 30, 2014 and 2013 as follows (in thousands):
		Three Months Ended				Nine Months Ended
		September 30,				September 30,
		2014		2013		2014		2013
	License and milestone	$	280	$	638	$	1,302	$	11,721
	Cost sharing, net	3,228		3,632		9,592		8,961
		$	3,508	$	4,270	$	10,894	$	20,682

Nature_of_Business_Details

Nature of Business (Details)	9 Months Ended
	Sep. 30, 2014
	protein_therapeutics
Nature of Business	'
Number of protein therapeutics internally discovered	3

Segment_Information_Details

Segment Information (Details)	9 Months Ended
	Sep. 30, 2014
	segment
Segment Information	'
Number of operating segments	1

Cash_and_Cash_Equivalents_and_1

Cash and Cash Equivalents and Restricted Cash (Details) (Money market account, Letters of credit, USD $)	Sep. 30, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Money market account | Letters of credit	'	'
Cash and Cash Equivalents and Restricted cash	'	'
Letters of credit held in the form of a money market account as collateral for lease obligations and credit cards	$0.90	$0.90

Fair_Value_Measurements_Detail

Fair Value Measurements (Details) (Recurring, USD $)	Sep. 30, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Quoted Prices in Active Markets for Identical Items (Level 1)	'	'
Assets:	'	'
Total assets	$182,559	$102,307
Liabilities:	'	'
Total liabilities	0	0
Quoted Prices in Active Markets for Identical Items (Level 1) | Common stock warrants	'	'
Liabilities:	'	'
Total liabilities	0	0
Quoted Prices in Active Markets for Identical Items (Level 1) | Money market funds	'	'
Assets:	'	'
Total assets	181,657	101,394
Quoted Prices in Active Markets for Identical Items (Level 1) | Restricted cash	'	'
Assets:	'	'
Total assets	902	913
Significant Other Observable Inputs (Level 2)	'	'
Assets:	'	'
Total assets	0	0
Liabilities:	'	'
Total liabilities	0	0
Significant Other Observable Inputs (Level 2) | Common stock warrants	'	'
Liabilities:	'	'
Total liabilities	0	0
Significant Other Observable Inputs (Level 2) | Money market funds	'	'
Assets:	'	'
Total assets	0	0
Significant Other Observable Inputs (Level 2) | Restricted cash	'	'
Assets:	'	'
Total assets	0	0
Significant Unobservable Inputs (Level 3)	'	'
Assets:	'	'
Total assets	0	0
Liabilities:	'	'
Total liabilities	15,028	30,753
Significant Unobservable Inputs (Level 3) | Common stock warrants	'	'
Liabilities:	'	'
Total liabilities	15,028	30,753
Significant Unobservable Inputs (Level 3) | Money market funds	'	'
Assets:	'	'
Total assets	0	0
Significant Unobservable Inputs (Level 3) | Restricted cash	'	'
Assets:	'	'
Total assets	0	0
Total	'	'
Assets:	'	'
Total assets	182,559	102,307
Liabilities:	'	'
Total liabilities	15,028	30,753
Total | Common stock warrants	'	'
Liabilities:	'	'
Total liabilities	15,028	30,753
Total | Money market funds	'	'
Assets:	'	'
Total assets	181,657	101,394
Total | Restricted cash	'	'
Assets:	'	'
Total assets	$902	$913

Fair_Value_Measurements_Detail1

Fair Value Measurements (Details 2) (USD $)	9 Months Ended		12 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Summary of changes in the fair value of the preferred and common stock warrant liability classified within Level 3 of the fair value hierarchy	'	'	'
Beginning balance	$30,753	$6,651	$6,651
Change in fair value	-8,124	12,649	'
Exercises	-7,601	-678	'
Repurchases	0	-83	'
Conversions	0	-2,013	'
Ending balance	15,028	16,526	30,753
Transfers within the hierarchy	$0	'	$0

Net_Loss_Per_Share_Details

Net Loss Per Share (Details)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Net Income (Loss) Per Share	'	'	'	'
Anti-dilutive common stock equivalents excluded from the calculation of diluted net loss per share (in shares)	4,084	21,336	4,084	22,325
Outstanding stock options	'	'	'	'
Net Income (Loss) Per Share	'	'	'	'
Anti-dilutive common stock equivalents excluded from the calculation of diluted net loss per share (in shares)	3,459	3,667	3,459	3,690
Common stock warrants	'	'	'	'
Net Income (Loss) Per Share	'	'	'	'
Anti-dilutive common stock equivalents excluded from the calculation of diluted net loss per share (in shares)	610	881	610	874
Shares issuable under employee stock purchase plan	'	'	'	'
Net Income (Loss) Per Share	'	'	'	'
Anti-dilutive common stock equivalents excluded from the calculation of diluted net loss per share (in shares)	15	0	15	0
Preferred stock	'	'	'	'
Net Income (Loss) Per Share	'	'	'	'
Anti-dilutive common stock equivalents excluded from the calculation of diluted net loss per share (in shares)	0	16,658	0	17,609
Preferred stock warrants	'	'	'	'
Net Income (Loss) Per Share	'	'	'	'
Anti-dilutive common stock equivalents excluded from the calculation of diluted net loss per share (in shares)	0	130	0	152

Warrants_Details

Warrants (Details) (USD $)

9 Months Ended

9 Months Ended

9 Months Ended

9 Months Ended

1 Months Ended

1 Months Ended

0 Months Ended

1 Months Ended

2 Months Ended

3 Months Ended

9 Months Ended

0 Months Ended

Sep. 30, 2014

Sep. 30, 2013

Dec. 31, 2013

Sep. 30, 2013

Sep. 30, 2014

Dec. 31, 2013

Dec. 31, 2012

Sep. 30, 2014

Sep. 30, 2014

Dec. 31, 2013

Dec. 31, 2013

Sep. 30, 2014

Dec. 31, 2013

Mar. 31, 2014

Sep. 30, 2014

Dec. 31, 2013

Apr. 30, 2014

Mar. 31, 2014

Sep. 30, 2014

Dec. 31, 2013

Mar. 31, 2013

Sep. 30, 2014

Aug. 31, 2014

Apr. 30, 2014

Mar. 31, 2014

Jun. 30, 2014

Sep. 30, 2014

Sep. 30, 2013

Sep. 30, 2014

Sep. 30, 2013

Dec. 31, 2013

Jul. 31, 2010

Jul. 31, 2010

Sep. 30, 2014

Dec. 31, 2013

Sep. 30, 2014

Sep. 30, 2014

Warrants to purchase preferred stock

Warrant to purchase Series A preferred stock

Warrant to purchase Series A preferred stock

Warrant to purchase Series A preferred stock

Warrant to purchase Series A preferred stock

Warrant to purchase Series B preferred stock

Warrant to purchase Series B preferred stock

Warrant to purchase Series C-1 preferred stock

Warrant to purchase Series D-1 preferred stock

Warrant to purchase Series D-1 preferred stock

Warrants to purchase Common stock having expiration date as of June 25, 2019

Warrants to purchase Common stock having expiration date as of June 25, 2019

Warrants to purchase Common stock having expiration date as of June 25, 2019

Warrants to purchase Common stock expiring March 18, 2020

Warrants to purchase Common stock expiring March 18, 2020

Warrants to purchase Common stock expiring March 18, 2020

Warrants to purchase Common stock expiring March 18, 2020

Warrants to purchase Common stock expiring between June 10, 2020 to July 9, 2020

Warrants to purchase Common stock expiring between June 10, 2020 to July 9, 2020

Warrants to purchase Common stock expiring between June 10, 2020 to July 9, 2020

Warrants to purchase Common stock expiring between June 10, 2020 to July 9, 2020

Warrants to purchase Common stock expiring between June 10, 2020 to July 9, 2020

Warrants to purchase Common stock expiring between June 10, 2020 to July 9, 2020

Warrants to purchase Common stock expiring between June 10, 2020 to July 9, 2020

Warrants to purchase Common stock expiring between June 10, 2020 to July 9, 2020

Warrants to purchase Common stock expiring between June 10, 2020 to July 9, 2020

Warrants to purchase Common stock expiring between June 10, 2020 to July 9, 2020

Warrants to purchase Common stock expiring between June 10, 2020 to July 9, 2020

Warrants to purchase Common stock expiring between June 10, 2020 to July 9, 2020

Warrants to purchase Common stock expiring between June 10, 2020 to July 9, 2020

Warrants to purchase Common stock expiring between March 31, 2015 to December 31, 2017

Warrants to purchase Common stock expiring between March 31, 2015 to December 31, 2017

Minimum

Maximum

Warrants issued in connection with financing transactions

Warrants issued in connection with financing transactions

Series A Preferred Stock

Warrants issued in connection with financing transactions

Warrants issued in connection with financing transactions

Warrants issued in connection with financing transactions

Warrants issued in connection with financing transactions

Series E Preferred Stock

Warrants to purchase Common stock expiring between March 31, 2015 to December 31, 2017

Warrants to purchase Common stock expiring between March 31, 2015 to December 31, 2017

Warrants

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Number of shares issuable upon exercise of outstanding warrants

610,000

'

980,000

'

'

106,500

106,500

'

'

31,891

45,786

'

63,693

'

14,000

[1]

46,000

[1]

'

'

19,000

[2],[3]

64,000

[2],[3]

'

564,000

[4],[5],[6],[7],[8],[9]

'

'

'

'

564,000

[4],[5],[6],[7],[8],[9]

'

564,000

[4],[5],[6],[7],[8],[9]

'

857,000

[4],[5],[6],[7],[8],[9]

871,580

'

13,000

[10]

13,000

[10]

'

'

Weighted-Average Exercise Price Per Share (in dollars per share)

$6.18

'

'

'

'

'

'

'

'

'

'

'

'

'

$10.92

[1]

'

'

'

$12.56

[2],[3]

'

'

$5.88

[4],[5],[6],[7],[8],[9]

'

'

'

'

$5.88

[4],[5],[6],[7],[8],[9]

'

$5.88

[4],[5],[6],[7],[8],[9]

'

'

'

'

'

'

$4

[10]

$7.40

[10]

Class of Warrant or Right Number of Warrants Exercised on Cash Basis

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

126,283

'

16,956

23,445

'

'

'

'

'

'

'

'

'

'

'

'

Stock Issued During Period Shares Warrants Exercised on Cash Basis

'

'

'

'

'

'

'

'

'

'

'

'

'

22,955

'

'

21,082

9,202

'

'

'

126,283

'

16,956

23,445

'

'

'

'

'

'

'

'

'

'

'

'

Class of Warrant or Right Number of Warrants Exercised on Net Basis

'

'

'

'

'

'

'

'

'

'

'

'

'

32,050

'

'

31,847

12,738

'

'

'

'

11,611

'

543

114,103

'

'

'

'

'

'

'

'

'

'

'

Issuance of stock on exercise of warrant, net basis (in shares)

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

456

'

'

'

'

'

'

'

'

'

'

'

'

Issuance of stock on exercise of warrant, cash basis (in shares)

'

'

'

'

'

'

'

46,668

'

'

'

'

'

'

'

'

'

'

'

'

'

'

9,332

'

'

92,173

'

'

'

'

'

'

'

'

'

'

'

Expiration period

'

'

'

'

'7 years

'

'

'

'7 years

'

'

'10 years

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'10 years

'

'

'

'

'

Increase (decrease) in fair value of warrants

($8,124,000)

$12,649,000

'

$1,300,000

$100,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

($3,400,000)

$10,100,000

($8,100,000)

$11,300,000

'

'

'

'

'

'

'

Discount recorded to the preferred stock issued

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

$3,000,000

'

'

'

'

Warrants retired (in shares)

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

13,994

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

[1]

In March 2014, the warrant holders exercised warrants to purchase 32,050 shares of Common Stock on a net basis, resulting in the issuance of 22,955 shares of Common Stock.

[2]

In March 2014, the warrant holders exercised warrants to purchase 12,738 shares of Common Stock on a net basis, resulting in the issuance of 9,202 shares of Common Stock.

[3]

In April 2014, the warrant holders exercised warrants to purchase 31,847 shares of common stock on a net basis, resulting in the issuance of 21,082 shares of Common Stock.

[4]

In March 2014, the warrant holders exercised warrants to purchase 23,445 shares of Common Stock on a cash basis, resulting in the issuance of 23,445 shares of Common Stock.

[5]

In May and June 2014, the warrant holders exercised warrants to purchase 114,103 shares of common stock on a net basis, resulting in the issuance of 92,173 shares of Common Stock.

[6]

In August 2014, the warrant holders exercised warrants to purchase 11,611 shares of common stock on a net basis, resulting in the issuance of 9,332 shares of Common Stock.

[7]

In April 2014, the warrant holders exercised warrants to purchase 16,956 shares of common stock on a cash basis, resulting in the issuance of 16,956 shares of Common Stock.

[8]

In September 2014, the warrant holders exercised warrants to purchase 126,283 shares of common stock on a cash basis, resulting in the issuance of 126,283 shares of Common Stock.

[9]

In March 2014, the warrant holders exercised warrants to purchase 543 shares of Common Stock on a net basis, resulting in the issuance of 456 shares of Common Stock.

[10]

Warrants to purchase common stock were issued in connection with various debt financing transactions that were consummated in periods prior to December 31, 2012. See discussion below for further details.

Commitments_and_Contingencies_

Commitments and Contingencies (Details) (USD $)	Sep. 30, 2014	Oct. 18, 2012	Jul. 25, 2014
In Millions, unless otherwise specified	Licensed Technology and Other Agreements	Salk Litigation	Subsequent Event
		licensing_agreement	Salk Litigation
Commitments and Contingencies	'	'	'
Number of license agreements allegedly breached	'	1	'
Number of licensing agreements	'	2	'
Total amount sought	'	$10.50	'
Total amount paid	'	'	5
Percentage of future development milestone payments received under the agreement with Celgene regarding ACE-536 sought	'	15.00%	6.00%
Royalty payable as percentage of net sales	'	'	1.00%
Amounts due	$0	$0	'

Significant_Agreements_Sotater

Significant Agreements (Sotatercept Agreement) (Details) (USD $)

1 Months Ended

9 Months Ended

0 Months Ended

9 Months Ended

1 Months Ended

0 Months Ended

1 Months Ended

0 Months Ended

26 Months Ended

68 Months Ended

9 Months Ended

0 Months Ended

In Millions, except Share data, unless otherwise specified

Aug. 31, 2011

Sep. 30, 2014

Sep. 30, 2014

Feb. 20, 2008

Sep. 30, 2014

Aug. 31, 2011

Feb. 20, 2008

Feb. 20, 2008

Feb. 20, 2008

Aug. 31, 2011

Feb. 20, 2008

Sep. 30, 2013

Sep. 30, 2013

Aug. 31, 2011

Sep. 30, 2014

Aug. 31, 2011

Aug. 31, 2011

Aug. 31, 2011

Aug. 31, 2011

Feb. 20, 2008

Feb. 20, 2008

Feb. 20, 2008

Feb. 20, 2008

Feb. 20, 2008

Sep. 30, 2014

Feb. 20, 2008

Sep. 30, 2014

Feb. 20, 2008

Feb. 20, 2008

Sotatercept agreements

Sotatercept agreements

Modified Sotatercept agreement

Modified Sotatercept agreement

Original Sotatercept agreement

Original Sotatercept agreement

Original Sotatercept agreement

Nonrefundable, upfront payments

Nonrefundable, upfront payments

Research and development funding and milestones

Research and development funding and milestones

Clinical milestones

Clinical milestones

Clinical milestones

Clinical milestones

Regulatory milestones

Regulatory milestones

Regulatory milestones

Maximum

Maximum

Maximum

Maximum

Maximum

Maximum

Minimum

Series C-1 Preferred Stock

Common Stock

Celgene

Celgene

Celgene

Celgene

Discovery stage compounds

Celgene

Celgene

Sotatercept agreements

Sotatercept agreements

Sotatercept agreements

Modified Sotatercept agreement

Modified Sotatercept agreement

Modified Sotatercept agreement

Modified Sotatercept agreement

Original Sotatercept agreement

Original Sotatercept agreement

Clinical milestones

Clinical milestones

Regulatory milestones

Commercial milestones

Commercial milestones

Sotatercept agreements

Sotatercept agreements

Sotatercept agreements

Discovery stage compounds

Sotatercept and ACE-536

item

Discovery stage compounds

Sotarcept

Celgene

Celgene

Celgene

Celgene

Celgene

Celgene

Celgene

Celgene

Celgene

Original Sotatercept agreement

Original Sotatercept agreement

Original Sotatercept agreement

Original Sotatercept agreement

Original Sotatercept agreement

Celgene

Celgene

Celgene

discovery_stage_compound

Sotarcept

Sotarcept

Discovery stage compounds

Sotarcept

Discovery stage compounds

Sotarcept

Discovery stage compounds

Discovery stage compounds

Celgene

Celgene

Celgene

Celgene

Celgene

licensed_target

Discovery stage compounds

Sotarcept

Sotarcept

Discovery stage compounds

Discovery stage compounds

Significant Agreements

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Number of license options granted

'

'

'

3

'

'

3

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Potential option fees

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

$30

'

'

'

'

'

'

'

'

Payments received

'

'

'

'

'

'

'

'

'

25

45

41.4

42.1

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Potential milestone payments receivable

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

25.5

27

142.5

190

204

'

53.3

88

272

150

150

'

'

'

Optional one-time royalty payment

'

'

'

'

'

25

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Potential royalty rate (as a percent)

20.00%

'

'

'

'

'

'

20.00%

20.00%

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Temporary Equity, Shares Issued

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

457,875

'

Aggregate purchase price

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

5

10

Shares purchased by collaborators

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

666,667

Offer price (in dollars per share)

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

$15

Percentage of development costs for which collaborator is responsible

'

'

'

'

100.00%

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Milestone payment receivable on commencement of Phase 2b clinical trial

'

'

'

'

'

'

'

'

'

'

'

'

'

$15

$10

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Number of licenses which may be terminated

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

1

'

'

Period of notice for termination of agreement

'

'180 days

'180 days

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Period of notice for termination of agreement on failure to meet certain criteria of licensed product

'

'45 days

'45 days

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Significant_Agreements_ACE536_

Significant Agreements (ACE-536 Agreement) (Details) (USD $)	1 Months Ended	9 Months Ended	1 Months Ended	26 Months Ended					9 Months Ended
In Millions, unless otherwise specified	Aug. 31, 2011	Sep. 30, 2014	Aug. 31, 2011	Sep. 30, 2013	Aug. 31, 2011	Aug. 31, 2011	Aug. 31, 2011	Aug. 31, 2011	Sep. 30, 2014
			Nonrefundable, upfront payments	Research and development funding and milestones	Clinical milestones	Maximum	Maximum	Maximum	Minimum
						Clinical milestones	Regulatory milestones	Commercial milestones	licensed_target
Significant Agreements	'	'	'	'	'	'	'	'	'
Payments received	'	'	$25	$41.40	'	'	'	'	'
Potential milestone payments receivable	'	'	'	'	'	32.5	105	80	'
Potential royalty rate (as a percent)	20.00%	'	'	'	'	'	'	'	'
Milestone payment receivable on commencement of a Phase 3 study in MDS or B-thalassemia	'	'	'	'	$15	'	'	'	'
Number of licensed targets	'	'	'	'	'	'	'	'	1
Period of notice for termination of agreement	'	'180 days	'	'	'	'	'	'	'
Period of notice for termination of agreement on failure to meet certain criteria of licensed product	'	'45 days	'	'	'	'	'	'	'

Significant_Agreements_Both_Ag

Significant Agreements (Both Agreements and Accounting Analysis) (Details) (USD $)

1 Months Ended

3 Months Ended

9 Months Ended

3 Months Ended

9 Months Ended

0 Months Ended

9 Months Ended

1 Months Ended

9 Months Ended

1 Months Ended

9 Months Ended

12 Months Ended

Aug. 31, 2011

Sep. 30, 2014

Sep. 30, 2013

Sep. 30, 2014

Sep. 30, 2013

Sep. 30, 2014

Sep. 30, 2013

Sep. 30, 2014

Sep. 30, 2013

Feb. 20, 2008

Sep. 30, 2014

Aug. 31, 2011

Sep. 30, 2014

Aug. 31, 2011

Aug. 31, 2011

Aug. 31, 2011

Aug. 31, 2011

Aug. 31, 2011

Aug. 31, 2011

Jan. 31, 2013

Sep. 30, 2014

Dec. 31, 2012

ACE-536 Agreement and amended Sotatercept Agreement

ACE-536 Agreement and amended Sotatercept Agreement

ACE-536 Agreement and amended Sotatercept Agreement

ACE-536 Agreement and amended Sotatercept Agreement

Original Sotatercept agreement

Modified Sotatercept agreement

Modified Sotatercept agreement

Modified Sotatercept agreement

Nonrefundable, upfront payments

Nonrefundable, upfront payments

Research and development services

Sotatercept joint development committee

ACE 536 joint development committee

Manufacturing services

Clinical milestones

Clinical milestones

Clinical milestones

Celgene

Celgene

Celgene

Celgene

Celgene

Celgene

Celgene

Celgene

ACE-536 agreement

ACE-536 Agreement and amended Sotatercept Agreement

ACE-536 Agreement and amended Sotatercept Agreement

ACE-536 Agreement and amended Sotatercept Agreement

ACE-536 Agreement and amended Sotatercept Agreement

ACE-536 agreement

ACE-536 agreement

ACE-536 agreement

Collaboration arrangement

Collaboration arrangement

Collaboration arrangement

Collaboration arrangement

Collaboration arrangement

Collaboration arrangement

Collaboration arrangement

Celgene

Celgene

Celgene

Celgene

Celgene

Celgene

Celgene

Celgene

Collaboration arrangement

Collaboration arrangement

Collaboration arrangement

Collaboration arrangement

Collaboration arrangement

Collaboration arrangement

Collaboration arrangement

Collaboration arrangement

Significant Agreements

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Percentage of worldwide development costs for which collaborator is responsible

'

'

'

'

'

'

'

100.00%

'

'

100.00%

'

100.00%

'

'

'

'

'

'

'

'

'

Potential royalty rate (as a percent)

20.00%

'

'

'

'

'

'

20.00%

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Deferred revenue

'

'

'

'

'

'

'

'

'

$45,000,000

'

$34,700,000

'

'

'

'

'

'

'

'

'

$2,700,000

Delivery period of contract

'

'

'

'

'

'

'

'

'

'15 years

'

'

'

'

'

'

'

'

'

'

'

'

Estimated payments for development activities and manufacturing services

'

'

'

'

'

'

'

'

'

'

'

18,000,000

'

'

'

'

'

'

'

'

'

'

Total arrangement consideration

'

'

'

'

'

'

'

'

'

'

'

77,700,000

'

'

'

'

'

'

'

'

'

'

Payments received

'

'

'

'

'

'

'

'

'

'

'

'

'

25,000,000

25,000,000

'

'

'

'

10,000,000

'

7,500,000

Deferred revenue recognized

'

'

'

'

'

300,000

600,000

1,300,000

1,700,000

'

'

'

'

'

'

'

'

'

'

'

10,000,000

4,800,000

BESP of the undelivered elements

'

'

'

'

'

'

'

'

'

'

'

28,200,000

'

'

'

18,800,000

2,900,000

3,700,000

2,800,000

'

'

'

Difference between the estimated payments and the estimated selling prices

'

'

'

'

'

'

'

'

'

'

'

10,200,000

'

'

'

'

'

'

'

'

'

'

Residual consideration recognized

'

'

'

'

'

'

'

'

'

'

'

49,500,000

'

'

'

'

'

'

'

'

'

'

Cost-sharing, net

'

$3,228,000

$3,632,000

$9,592,000

$9,666,000

$3,200,000

$3,600,000

$9,600,000

$9,000,000

'

'

'

'

'

'

'

'

'

'

'

'

'

Significant_Agreements_Shire_L

Significant Agreements (Shire License) (Details) (USD $)

3 Months Ended

9 Months Ended

1 Months Ended

9 Months Ended

0 Months Ended

3 Months Ended

Sep. 30, 2014

Sep. 30, 2013

Sep. 30, 2014

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2014

Sep. 30, 2014

Aug. 31, 2011

Sep. 30, 2010

Sep. 30, 2013

Feb. 08, 2011

Sep. 30, 2013

Sep. 30, 2010

Sep. 30, 2010

Shire AG

Shire AG

Shire AG

Nonrefundable, upfront payments

Nonrefundable, upfront payments

Nonrefundable, upfront payments

Change in accounting estimate adjustments

Change in accounting estimate adjustments

Development milestones

Commercial milestones

License and collaboration agreement

ACE-031

Licensed compounds other than ACE-031

Shire AG

Shire AG

Nonrefundable, upfront payments

Nonrefundable, upfront payments

Shire AG

Shire AG

License and collaboration agreement

License and collaboration agreement

ACE-031

ACE-031

Shire AG

Shire AG

ActRIIB compounds

ActRIIB compounds

License and collaboration agreement

License and collaboration agreement

ACE-031

ACE-031

License and collaboration agreement

License and collaboration agreement

License and collaboration agreement

License and collaboration agreement

Significant Agreements

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Payments received

'

'

'

'

'

'

'

$25,000,000

$45,000,000

'

'

'

'

'

Deferred revenue

'

'

'

'

'

'

'

'

45,000,000

'

38,800,000

'

'

'

Estimated revenue recognition period

'

'

'

'

'

'

'

'

'3 years

'

'5 years

'

'

'

Deferred revenue recognized

'

'

'

'

'

'

'

'

'

24,300,000

'

22,400,000

'

'

Contingent milestone payments

'

'

'

'

'

'

'

'

'

'

'

'

223,800,000

228,800,000

Percentage of development costs for which collaborator is responsible

'

'

'

'

'

65.00%

55.00%

'

'

'

'

'

'

'

Cost-sharing, net

3,228,000

3,632,000

9,592,000

9,666,000

600,000

'

'

'

'

'

'

'

'

'

Payment for research and development costs

'

'

'

'

$200,000

'

'

'

'

'

'

'

'

'

Significant_Agreements_Other_D

Significant Agreements (Other) (Details) (USD $)

3 Months Ended

9 Months Ended

3 Months Ended

9 Months Ended

12 Months Ended

3 Months Ended

9 Months Ended

12 Months Ended

3 Months Ended

9 Months Ended

12 Months Ended

1 Months Ended

12 Months Ended

Sep. 30, 2014

Sep. 30, 2013

Sep. 30, 2014

Sep. 30, 2013

Sep. 30, 2014

Sep. 30, 2013

Sep. 30, 2014

Sep. 30, 2013

Dec. 31, 2004

Sep. 30, 2014

Sep. 30, 2013

Sep. 30, 2014

Sep. 30, 2013

Dec. 31, 2013

Sep. 30, 2014

Sep. 30, 2014

Dec. 31, 2004

Dec. 31, 2004

Dec. 31, 2004

Dec. 31, 2004

Dec. 31, 2004

Dec. 31, 2004

Dec. 31, 2013

31-May-14

Dec. 31, 2013

License agreement with a non-profit institution

License agreement with a non-profit institution

License agreement with a non-profit institution

License agreement with a non-profit institution

License agreement with certain individuals

License agreement with certain individuals

License agreement with certain individuals

License agreement with certain individuals

License agreement with certain individuals

License agreement with research institution

License agreement with research institution

License agreement with research institution

License agreement with research institution

License agreement with research institution

License agreement with antibody technology company

License agreement with antibody technology company

Common Stock

Maximum

Minimum

Development milestone

Development milestone

Development and sales milestone

Development and commercial milestone

Additional research fees

Commercial milestones

License agreement with a non-profit institution

License agreement with a non-profit institution

License agreement with a non-profit institution

Sotarcept

ACE-536

Dalantercept

Maximum

License agreement with antibody technology company

License agreement with research institution

Maximum

Maximum

Maximum

License agreement with research institution

License agreement with certain individuals

Significant Agreements

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Number of shares issued as compensation for licenses

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

62,500

'

'

'

'

'

'

'

'

Fair value of shares issued as compensation for licenses

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

$25,000

'

'

'

'

'

'

'

'

Total potential milestone payments

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

2,000,000

700,000

1,000,000

1,000,000

1,000,000

800,000

Annual payment upon first commercial sale

'

'

'

'

'

'

'

'

'

'

'

'

'

25,000

'

'

'

'

'

'

'

'

'

'

'

Percentage of reduction in royalty rate for a period of time after patent expiration

'

'

'

'

'

'

'

'

50.00%

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Milestone fees payable as percentage of research and U.S. development progress and sublicensing revenue

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

25.00%

10.00%

'

'

'

'

'

'

Royalty payable as percentage of net sales

'

'

'

'

'

'

'

'

'

'

'

'

'

1.50%

'

'

'

3.50%

1.00%

'

'

'

'

'

'

Milestones and fees expensed

$100,000

$100,000

$100,000

$100,000

$0

$0

$0

$0

'

$0

$0

$0

$0

'

$300,000

$500,000

'

'

'

'

'

'

'

$300,000

'

StockBased_Compensation_Detail

Stock-Based Compensation (Details) (USD $)	3 Months Ended				9 Months Ended			12 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013	Dec. 31, 2013
Compensation cost recognized	'		'		'		'	'
Stock-based compensation	$1,200,000		$500,000		$3,327,000		$1,441,000	'
2003 Plan	'		'		'		'	'
Compensation cost recognized	'		'		'		'	'
Total compensation cost recognized	1,194,000		493,000		3,327,000		1,441,000	'
Number of Grants	'		'		'		'	'
Outstanding at the beginning of period (in shares)	'		'		3,942,304		'	'
Granted (in shares)	'		'		191,950		'	'
Exercised (in shares)	'		'		-626,835		'	'
Canceled or forfeited (in shares)	'		'		-48,462		'	'
Outstanding at the end of the period (in shares)	3,458,857		'		3,458,857		'	3,942,304
Exercisable at the end of the period (in shares)	2,361,396		'		2,361,396		'	'
Vested and expected to vest at the end of the period (in shares)	3,400,383	[1]	'		3,400,383	[1]	'	'
Weighted-Average Exercise Price Per Share	'		'		'		'	'
Outstanding at the beginning of period (in dollars per share)	'		'		$7.05		'	'
Granted (in dollars per share)	'		'		$38.56		'	'
Exercised (in dollars per share)	'		'		$3.78		'	'
Canceled or forfeited (in dollars per share)	'		'		$21.50		'	'
Outstanding at the end of the period (in dollars per share)	$9.19		'		$9.19		'	$7.05
Exercisable at the end of the period (in dollars per share)	$5.34		'		$5.34		'	'
Vested and expected to vest at the end of the period (in dollars per share)	$9	[1]	'		$9	[1]	'	'
Weighted-Average Contractual Life	'		'		'		'	'
Outstanding at the end of the period	'		'		'6 years 1 month 25 days		'	'6 years 5 months 5 days
Exercisable at the end of the period	'		'		'5 years 0 months 10 days		'	'
Vested and expected to vest at the end of the period	'		'		'6 years 1 month 7 days	[1]	'	'
Aggregate Intrinsic Value	'		'		'		'	'
Outstanding at the end of the period	74,526,000	[2]	'		74,526,000	[2]	'	'
Exercisable at the end of the period	58,971,000	[2]	'		58,971,000	[2]	'	'
Vested and expected to vest at the end of the period	73,832,000	[1],[2]	'		73,832,000	[1],[2]	'	'
Additional disclosure	'		'		'		'	'
Weighted average grant date fair value	'		'		$24.58		'	'
Total proceeds from options exercised	'		'		2,400,000		'	'
Intrinsic value of options exercised	'		'		19,000,000		'	'
Unrecognized compensation expense related to unvested stock options	11,100,000		'		11,100,000		'	'
2003 Plan | Options	'		'		'		'	'
Weighted-average assumptions for estimating fair value of each option issued to employees at the date of grant using the Black-Scholes option pricing model	'		'		'		'	'
Expected volatility (as a percent)	70.70%		0.00%	[3]	71.20%		70.30%	'
Expected term	'6 years		'0 years	[3]	'6 years		'6 years	'
Risk-free interest rate (as a percent)	1.90%		0.00%	[3]	1.80%		1.40%	'
Expected dividend yield (as a percent)	0.00%		0.00%	[3]	0.00%		0.00%	'
Weighted-average period over which unrecognized compensation expense related to unvested stock options is expected to be recognized	'		'		'2 years 4 months 24 days		'	'
2003 Plan | Research and development	'		'		'		'	'
Compensation cost recognized	'		'		'		'	'
Total compensation cost recognized	486,000		149,000		1,339,000		460,000	'
2003 Plan | General and administrative	'		'		'		'	'
Compensation cost recognized	'		'		'		'	'
Total compensation cost recognized	$708,000		$344,000		$1,988,000		$981,000	'
[1]	This represents the number of vested options at September 30, 2014, plus the number of unvested options expected to vest at September 30, 2014, based on the unvested options outstanding at September 30, 2014, adjusted for the estimated forfeiture rate.
[2]	The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the estimated fair value of the common stock for the options that were in the money at September 30, 2014.
[3]	No grants were made in the three months ended September 30, 2013.

Income_Taxes_Details

Income Taxes (Details) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Income Tax Disclosure [Abstract]	'	'	'	'
Provision for (benefit from) income taxes	$0	$0	$0	$0

LongTerm_Debt_Details

Long-Term Debt (Details) (USD $)	Mar. 12, 2014	Jun. 07, 2012	Jun. 07, 2012
In Millions, unless otherwise specified			Loan Agreement
			payment
			lender
Long-term debt	'	'	'
Number of lenders	'	'	3
Aggregate principal amount	'	'	$20
Payment period of debt	'	'	'42 months
Number of interest only payments	'	'	12
Number of principal and interest payments	'	'	30
Interest rate (as a percent)	'	'	8.50%
Closing fee	'	'	0.2
Period of amortization of financing costs	'	'	'42 months
Additional deferred payment	0.6	1.2	1.2
Effective interest rate (as a percent)	'	'	11.80%
Prepayment fees recognized as additional expense	$0.30	'	'

Related_Party_Transactions_Det

Related Party Transactions (Details) (USD $)

1 Months Ended

3 Months Ended

9 Months Ended

3 Months Ended

9 Months Ended

1 Months Ended

1 Months Ended

31-May-14

Sep. 30, 2014

Sep. 30, 2013

Sep. 30, 2014

Sep. 30, 2013

Dec. 31, 2013

Sep. 30, 2014

Sep. 30, 2013

Sep. 30, 2014

Sep. 30, 2013

Dec. 31, 2013

Jun. 30, 2010

Sep. 30, 2013

Feb. 29, 2008

Jun. 30, 2010

Dec. 31, 2012

31-May-14

Jan. 31, 2014

shareholder

Celgene

Celgene

Celgene

Celgene

Celgene

Celgene

Celgene

Celgene

Celgene

Celgene

Stock Purchase Agreements

Stock Purchase Agreements

Common Stock

Common Stock

Series C-1 Preferred Stock

Series E Redeemable Convertible Preferred Stock

Series F Redeemable Convertible Preferred Stock

Celgene

Celgene

Initial Public Offering

Common Stock

Common Stock

Related Party Transactions

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Shares sold

'

'

'

'

'

'

'

'

'

'

'

'

'

457,875

36,496

1,990,446

'

'

Number of shares that can be purchased through warrants

'

610,000

'

610,000

'

980,000

'

'

'

'

'

38,979

'

'

'

'

'

'

Shares purchased by collaborators

'

'

'

'

'

'

'

'

'

'

'

'

666,667

'

'

'

1,100,000

300,000

Number of stockholders of the company

5

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Ownership percentage of entity's fully diluted equity

'

'

'

'

'

'

12.80%

'

12.80%

'

9.70%

'

'

'

'

'

'

'

Deferred revenue

'

'

'

'

'

'

$6,300,000

'

$6,300,000

'

'

'

'

'

'

'

'

'

Revenue from related party

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

License and milestone

'

-280,000

-638,000

-1,302,000

-36,044,000

'

280,000

638,000

1,302,000

11,721,000

'

'

'

'

'

'

'

'

Cost-sharing, net

'

-3,228,000

-3,632,000

-9,592,000

-9,666,000

'

3,228,000

3,632,000

9,592,000

8,961,000

'

'

'

'

'

'

'

'

Collaboration revenue

'

'

'

'

'

'

$3,508,000

$4,270,000

$10,894,000

$20,682,000

'

'

'

'

'

'

'

'