Acceleron Pharma (XLRN) 10-Q 9 May 19 2019 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2019	Apr. 30, 2019
Document and Entity Information
Entity Registrant Name	ACCELERON PHARMA INC
Entity Central Index Key	0001280600
Document Type	10-Q
Document Period End Date	Mar. 31, 2019
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding (in shares)		52,685,740
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q1
Entity Emerging Growth Company	false
Entity Small Business	false

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 214,490	$ 144,052
Collaboration receivables (all amounts are with a related party)	2,780	7,039
Prepaid expenses and other current assets	7,897	7,662
Short-term investments	223,106	147,260
Total current assets	448,273	306,013
Property and equipment, net	7,027	7,106
Right-of-use - Operating leases	27,817
Restricted cash	1,597	1,597
Other assets	107	105
Long-term investments	75,509	0
Total assets	560,330	314,821
Current liabilities:
Accounts payable	3,930	419
Accrued expenses	13,803	18,209
Operating lease obligations, current portion	5,537
Deferred rent		284
Total current liabilities	23,270	18,912
Operating lease obligations, net of current portion	24,918
Deferred rent, net of current portion		2,381
Warrants to purchase common stock	1,607	1,491
Total liabilities	49,795	22,784
Commitments and contingencies (Note 14)
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; 52,542,118 and 46,260,747 shares issued and outstanding at March 31, 2019 and December 31, 2018, respectively	53	47
Additional paid-in capital	1,135,376	879,099
Accumulated deficit	(624,602)	(586,549)
Accumulated other comprehensive loss	(292)	(560)
Total stockholders’ equity	510,535	292,037
Total liabilities and stockholders’ equity	$ 560,330	$ 314,821

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Collaboration revenue:
Revenues	$ 2,780	$ 3,232
Costs and expenses:
Research and development	32,771	23,431
General and administrative	10,814	7,441
Total costs and expenses	43,585	30,872
Loss from operations	(40,805)	(27,640)
Other income, net	2,772	1,431
Loss before income taxes	(38,033)	(26,209)
Income tax provision	(20)	(10)
Net loss	$ (38,053)	$ (26,219)
Net loss per share- basic and diluted (in dollars per share)	$ (0.74)	$ (0.58)
Weighted-average number of common shares used in computing net loss per share- basic and diluted (in shares)	51,126	45,516
Other comprehensive loss:
Net loss	$ (38,053)	$ (26,219)
Net unrealized holding gains (losses) on short-term and long-term investments during the period, net of tax of $76 and zero for the three months ended March 31, 2019 and 2018, respectively	268	(429)
Comprehensive loss	(37,785)	(26,648)
Cost Sharing
Collaboration revenue:
Revenues	$ 2,780	$ 3,232

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Operations and Comprehensive Loss (Parenthetical) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Income Statement [Abstract]
Unrealized holding gain (loss) on securities, tax	$ 76	$ 0

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Mar. 31, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Preferred stock, par value (in usd per share)	$ 0.001	$ 0.001
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 usd per share)	$ 0.001	$ 0.001
Common stock, shares authorized	175,000,000	175,000,000
Common stock, shares issued	52,542,118	46,260,747
Common stock, shares outstanding	52,542,118	46,260,747

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Deficit	Comprehensive Loss
Stockholders' equity beginning balance (in shares) at Dec. 31, 2017		45,261,175
Stockholders' equity beginning balance at Dec. 31, 2017	$ 365,216	$ 46	$ 839,090	$ (473,024)	$ (895)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	5,696		5,696
Exercise of stock options (in shares)		358,685
Exercise of stock options	4,716		4,715
Vesting of restricted stock units (in shares)		65,183
Vesting of restricted stock units	(363)		(363)
Issuance of common stock related to ESPP (in shares)		19,556
Issuance of common stock related to ESPP	662		662
Net exercise of warrants to purchase common stock (in shares)		18,449
Net exercise of warrants to purchase common stock	797		797
Unrealized loss on available-for-sale securities	(429)				(429)
Net loss	(26,219)			(26,219)
Stockholders' equity ending balance (in shares) at Mar. 31, 2018		45,723,048
Stockholders' equity ending balance at Mar. 31, 2018	353,780	$ 46	850,597	(495,539)	(1,324)
Stockholders' equity beginning balance (in shares) at Dec. 31, 2018		46,260,747
Stockholders' equity beginning balance at Dec. 31, 2018	292,037	$ 47	879,099	(586,549)	(560)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	6,992		6,992
Issuance of common stock (in shares)		6,151,163
Issuance of common stock	$ 248,130	$ 6	248,124
Exercise of stock options (in shares)	35,519	35,919
Exercise of stock options	$ 766		766
Vesting of restricted stock units (in shares)		75,028
Vesting of restricted stock units	(393)		(393)
Issuance of common stock related to ESPP (in shares)		19,661
Issuance of common stock related to ESPP	788		788
Unrealized loss on available-for-sale securities	268				268
Net loss	(38,053)			(38,053)
Stockholders' equity ending balance (in shares) at Mar. 31, 2019		52,542,518
Stockholders' equity ending balance at Mar. 31, 2019	$ 510,535	$ 53	$ 1,135,376	$ (624,602)	$ (292)

Consolidated Statements of St_2

Consolidated Statements of Stockholders' Equity (Parenthetical) $ in Thousands	3 Months Ended
	Mar. 31, 2019USD ($)$ / shares
Statement of Stockholders' Equity [Abstract]
Issuance costs | $	$ 500
Common stock, par value (in usd per share) | $ / shares	$ 0.001

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Operating Activities
Net loss	$ (38,053)	$ (26,219)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	2,168	904
Stock-based compensation	6,992	5,696
Other non-cash items	959	88
Changes in assets and liabilities:
Prepaid expenses and other assets	(928)	(749)
Collaboration receivables (all amounts are with a related party)	4,259	338
Accounts payable	3,466	591
Accrued expenses	(4,544)	(4,233)
Operating lease obligations (Note 13)	(627)
Other changes in operating assets and liabilities	0	153
Net cash used in operating activities	(26,308)	(23,431)
Investing Activities
Purchases of investments	(216,116)	(35,947)
Proceeds from sales and maturities of investments	64,133	24,530
Purchases of property and equipment	(587)	(504)
Net cash used in investing activities	(152,570)	(11,921)
Financing Activities
Proceeds from issuance of common stock from public offering, net of issuance costs	248,155	0
Payments for capital lease expenditures	0	(78)
Net proceeds from exercises and vesting of stock awards, ESPP contributions, and exercise of warrants to purchase common stock	1,161	5,016
Net cash provided by financing activities	249,316	4,938
Net increase (decrease) in cash, cash equivalents and restricted cash	70,438	(30,414)
Cash, cash equivalents and restricted cash at beginning of period	145,649	101,282
Cash, cash equivalents and restricted cash at end of period	216,087	70,868
Supplemental Disclosure of Non-Cash Investing and Financing Activities:
Purchase of property and equipment included in accounts payable and accrued expenses	267	314
Reclassification of warrant liability to additional paid-in capital	0	797
Acquisition of capital lease	$ 0	$ 139

Nature of Business

Nature of Business	3 Months Ended
	Mar. 31, 2019
Nature of Business
Nature of Business	Nature of Business Acceleron Pharma Inc. (Acceleron or the Company) is a Cambridge, Massachusetts-based clinical stage biopharmaceutical company dedicated to the discovery, development, and commercialization of therapeutics to treat serious and rare diseases. The Company’s leadership in the understanding of TGF-beta biology and protein engineering generates innovative compounds that engage the body’s ability to regulate cellular growth and repair. The Company is subject to risks common to companies in the biotechnology industry, including, but not limited to, the risk that the Company never achieves profitability, the need for substantial additional financing, risk of relying on third parties, risks of clinical trial failures, dependence on key personnel, protection of proprietary technology and compliance with government regulations.

Basis of Presentation

Basis of Presentation	3 Months Ended
	Mar. 31, 2019
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 and reflect the application of certain significant accounting policies as described below and elsewhere in these notes to the financial statements. As of March 31, 2019 , 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, 2018 , have not changed, and 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, 2018 , except for the adoption of Accounting Standards Updates (ASU) No. 2016-02, Leases (Topic 842), as discussed further in Note 13. In the opinion of management, the accompanying interim condensed consolidated financial statements reflect all adjustments, consisting of normal recurring adjustments, necessary for the fair presentation of the Company’s financial position as of March 31, 2019 , the results of its operations for the three months ended March 31, 2019 and 2018 , and its cash flows for the three months ended March 31, 2019 and 2018 . The results for the three months ended March 31, 2019 are not necessarily indicative of the results to be expected for the year ending December 31, 2019 , 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, 2018 , 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, 2018 .

Use of Estimates

Use of Estimates	3 Months Ended
	Mar. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
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. 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 consolidated 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 consolidated 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 consolidated financial statements, management used significant estimates in the following areas, among others: revenue recognition and accrued clinical expenses.

Segment Information

Segment Information	3 Months Ended
	Mar. 31, 2019
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, which is the discovery, development and commercialization of highly innovative therapeutics to treat serious and rare diseases.

Cash Equivalents and Short-term

Cash Equivalents and Short-term and Long-term Investments	3 Months Ended
	Mar. 31, 2019
Cash and Cash Equivalents [Abstract]
Cash Equivalents and Short-term and Long-term Investments	Cash Equivalents and Short-term and Long-term Investments 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. The Company determines the appropriate classification of marketable securities at the time of purchase and reevaluates such designation at each balance sheet date. The Company has classified all of its marketable securities at March 31, 2019 as “available-for-sale” pursuant to ASC 320, Investments – Debt and Equity Securities. The Company records available-for-sale securities at fair value, with the unrealized gains and losses included in accumulated other comprehensive income (loss) in stockholders’ equity. There were no realized gains or losses on marketable securities for the three months ended March 31, 2019 and 2018 . Investments not classified as cash equivalents are presented as either short-term or long-term investments based on both their maturities as well as the time period the Company intends to hold such securities. The Company adjusts the cost of available-for-sale debt securities for amortization of premiums and accretion of discounts to maturity. The Company includes such amortization and accretion in interest income. The cost of securities sold is based on the specific identification method. The Company includes in interest income interest and dividends on securities classified as available-for-sale. The Company reviews marketable securities for other-than-temporary impairment whenever the fair value of a marketable security is less than the amortized cost and evidence indicates that a marketable security’s carrying amount is not recoverable within a reasonable period of time. Other-than-temporary impairments of investments are recognized in the consolidated statements of operations if the Company has experienced a credit loss, has the intent to sell the marketable security, or if it is more likely than not that the Company will be required to sell the marketable security before recovery of the amortized cost basis. Evidence considered in this assessment includes reasons for the impairment, compliance with the Company’s investment policy, the severity and the duration of the impairment and changes in value subsequent to the end of the period. In March 2017, the FASB issued issued Accounting Standards Update 2017-08, Receivables - Nonrefundable Fees and Other Costs (Subtopic 310-20): Premium Amortization on Purchased Callable Debt Securities (ASU 2017-08) . This standard amends the amortization period for certain purchased callable debt securities held at a premium by shortening the amortization period to the earliest call date. The Company adopted ASU 2017-08 effective January 1, 2019 with no material impact on its consolidated financial statements and related disclosures. The aggregate fair value of securities held by the Company in an unrealized loss position for less than twelve months as of March 31, 2019 and December 31, 2018 was $138.2 million and $51.2 million , respectively. The aggregate fair value of securities held by the Company in an unrealized loss position for more than twelve months as of March 31, 2019 and December 31, 2018 was $58.2 million and $94.3 million , respectively. The aggregate unrealized loss for those securities in an unrealized loss position for more than twelve months is $0.1 million and $0.4 million , respectively. The Company determined it did not hold any investments with any other-than-temporary impairment as of March 31, 2019 and December 31, 2018 . The following is a summary of cash, cash equivalents and available-for-sale securities as of March 31, 2019 and December 31, 2018 , (in thousands): March 31, 2019 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Cash and cash equivalents due in 90 days or less $ 214,497 $ — $ (7 ) $ 214,490 Available-for-sale securities: Corporate obligations due in one year or less 109,578 3 (117 ) 109,464 Corporate obligations due in more than one year 53,539 54 (6 ) 53,587 U.S. Treasury securities due in one year or less 85,044 13 (31 ) 85,026 U.S. Treasury securities due in more than one year 13,440 14 — 13,454 Certificates of deposit due in one year or less 1,715 — (2 ) 1,713 Certificates of deposit due in more than one year 490 1 — 491 Mortgage and other asset backed securities due in one year or less 26,953 — (50 ) 26,903 Mortgage and other asset backed securities due in more than one year 7,969 8 — 7,977 Total available-for-sale securities $ 298,728 $ 93 $ (206 ) $ 298,615 Total cash, cash equivalents and available-for-sale securities $ 513,225 $ 93 $ (213 ) $ 513,105 December 31, 2018 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Cash and cash equivalents due in 90 days or less $ 144,064 $ — $ (12 ) $ 144,052 Available-for-sale securities: Corporate obligations due in one year or less 73,671 — (267 ) 73,404 U.S. Treasury securities due in one year or less 45,346 — (79 ) 45,267 Certificates of deposit due in one year or less 1,715 — — 1,715 Mortgage and other asset backed securities due in one year or less 26,982 — (108 ) 26,874 Total available-for-sale securities $ 147,714 $ — $ (454 ) $ 147,260 Total cash, cash equivalents and available-for-sale securities $ 291,778 $ — $ (466 ) $ 291,312

Restricted Cash

Restricted Cash	3 Months Ended
	Mar. 31, 2019
Receivables [Abstract]
Restricted Cash	Restricted Cash The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the condensed consolidated balance sheet that sum to the total of the same such amounts shown in the statement of cash flows (in thousands): March 31, 2019 2018 Cash and cash equivalents $ 214,490 $ 69,736 Restricted cash 1,597 1,132 Total cash, cash equivalents and restricted cash shown in the statement of cash flows $ 216,087 $ 70,868 As of March 31, 2019 and December 31, 2018 , the Company maintained letters of credit totaling $1.6 million held in the form of certificates of deposit and money market funds as collateral for the Company's facility lease obligation and its credit cards.

Concentrations of Credit Risk a

Concentrations of Credit Risk and Off-Balance Sheet Risk	3 Months Ended
	Mar. 31, 2019
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, short-term and long-term investments and receivables. The Company maintains its cash and cash equivalent balances and short-term and long-term investments with financial institutions that management believes are creditworthy. Short-term and long-term investments consist of investment grade corporate obligations, treasury notes, asset backed securities, and certificates of deposit. 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 concentrations 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 collaboration receivables.

Fair Value Measurements

Fair Value Measurements	3 Months Ended
	Mar. 31, 2019
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 March 31, 2019 and December 31, 2018 (in thousands): March 31, 2019 Quoted Prices in Active Markets for Identical Items (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Total Assets: Money market funds $ 136,520 $ — $ — $ 136,520 Corporate obligations — 193,017 — 193,017 U.S. Treasury securities — 118,976 — 118,976 Certificates of deposit — 2,204 — 2,204 Mortgage and other asset backed securities — 34,881 — 34,881 Total assets $ 136,520 $ 349,078 $ — $ 485,598 Liabilities: Warrants to purchase common stock $ — $ — $ 1,607 $ 1,607 Total liabilities $ — $ — $ 1,607 $ 1,607 December 31, 2018 Quoted Prices in Active Markets for Identical Items (Level 1) Significant other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Total Assets: Money market funds $ 74,023 $ — $ — $ 74,023 Corporate obligations — 128,920 — 128,920 U.S. Treasury securities — 56,978 — 56,978 Certificates of deposit — 1,715 — 1,715 Mortgage and other asset backed securities — 26,874 — 26,874 Total assets $ 74,023 $ 214,487 $ — $ 288,510 Liabilities: Warrants to purchase common stock $ — $ — $ 1,491 $ 1,491 Total liabilities $ — $ — $ 1,491 $ 1,491 The money market funds noted above are included in cash and cash equivalents in the accompanying condensed consolidated 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 three months ended March 31, 2019 or the year ended December 31, 2018 . Items measured at fair value on a recurring basis include short-term and long-term investments (Note 5), 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 common stock warrant liabilities, which represent a recurring measurement that is classified within Level 3 of the fair value hierarchy, wherein fair value is estimated using significant unobservable inputs (in thousands): Three Months Ended March 31, 2019 2018 Beginning balance $ 1,491 $ 2,236 Change in fair value 116 (119 ) Exercises — (797 ) Ending balance $ 1,607 $ 1,320 The fair value of the warrants to purchase common stock on the date of issuance and on each re-measurement date for those warrants classified as liabilities was estimated using either the Monte Carlo simulation framework, which incorporates future financing events over the remaining life of the warrants to purchase common stock, or for certain re-measurement dates, due to the warrants being deeply in the money, the Black-Scholes option pricing model. Due to the nature of these inputs, the valuation of the warrants is considered a Level 3 measurement. At each reporting period, the Company evaluates the best valuation methodology. At March 31, 2019 , the Black-Scholes option pricing model was used. 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 re-measure 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 three months ended March 31, 2019 or the year ended December 31, 2018 .

Net Loss Per Share

Net Loss Per Share	3 Months Ended
	Mar. 31, 2019
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 their inclusion would have had an anti-dilutive effect (in thousands): Three Months Ended 2019 2018 Outstanding stock options 4,115 3,718 Common stock warrants 39 39 Shares issuable under employee stock purchase plan 13 13 Outstanding restricted stock units (1) 630 631 4,797 4,401 (1) This balance is comprised of both the restricted stock units and performance-based restricted stock units described in Note 17.

Comprehensive Loss

Comprehensive Loss	3 Months Ended
	Mar. 31, 2019
Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract]
Comprehensive Loss	Comprehensive Loss Comprehensive 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 loss consists of net loss and other comprehensive loss, which includes certain changes in equity that are excluded from net loss. Comprehensive loss has been disclosed in the accompanying consolidated statements of operations and comprehensive loss. Accumulated other comprehensive loss is presented separately on the consolidated balance sheets and consists entirely of unrealized holding gains and losses on investments as of March 31, 2019 and December 31, 2018 .

Recent Accounting Pronouncement

Recent Accounting Pronouncements - Not Yet Adopted	3 Months Ended
	Mar. 31, 2019
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Recent Accounting Pronouncements - Not Yet Adopted	Recent Accounting Pronouncements - Not Yet Adopted From time to time, new accounting pronouncements are issued by the Financial Accounting Standards Board (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	3 Months Ended
	Mar. 31, 2019
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- Average Exercise Balance Sheet Classification March 31, 2019 December 31, 2018 Price Per Share Expiration March 31, 2019 December 31, 2018 Warrants to purchase common stock 39 39 $ 5.88 June 10, 2020 - July 9, 2020 Liability Liability All warrants 39 39 $ 5.88

Leases

Leases	3 Months Ended
	Mar. 31, 2019
Leases [Abstract]
Leases	Leases In February 2016, the FASB issued Accounting Standards Codification Topic 842, Leases (ASC 842 ), which replaces the existing guidance for leases. ASC 842 requires lessees to recognize assets and liabilities on the balance sheet for the rights and obligations created by all leases with terms of more than 12 months. ASC 842 also requires certain qualitative and quantitative disclosures designed to give financial statement users information on the amount, timing, and uncertainty of cash flows arising from leases. The Company has adopted ASC 842 effective January 1, 2019. The Company has elected to employ the transitionary relief recently offered by the FASB under ASU 2018-11 and implement the new standard without the restatement of comparative periods' financial information. ASU-2018-11 also provides for recognizing the effects of applying ASC 842 as a cumulative-effect adjustment to retained earnings as of January 1, 2019; however, no such adjustment was recorded as of January 1, 2019. The Company has elected to employ the package of practical expedients offered under ASC 842, which allow the Company to not reassess the following: • the presence of a lease in any expired or existing contracts; • the lease classification for any existing or expired leases; and • the initial direct costs for any existing leases. The Company currently leases approximately 125,000 square feet of office and laboratory space in five adjacent buildings in Cambridge, Massachusetts (the Leases). The Leases were classified as operating leases under ASC 840. The Leases are also classified as operating leases under ASC 842 in accordance with the Company's election of the practical expedient under ASC 842. Pursuant to the package of practical expedients, the Company will also not reassess initial direct costs for the Leases. Additionally, the Company elected to account for the lease components and non-lease components as a single lease component. The Company occupied the premises of the Leases at various points in time prior to January 1, 2019 under non-cancelable agreements which expire at various dates through September 2023. Each of the Leases have options to renew for periods ranging from three to five years, which are not included in the measurement of these leases. All of the Company's leases contain escalating rent clauses, which require higher rent payments in future years. There are no variable payments, exercise purchase options, penalties, fees, or residual value guarantees under the Leases. The Company is also obligated to pay the Landlord for certain costs, taxes, and operating expenses related to the premises. However, the Company has concluded that these payments are not in-substance fixed payments and therefore are not included in the calculation of the related lease liability and asset under ASC 842. The Company recorded the liability associated with the Leases at the present value of the lease payments not yet paid, discounted using the discount rate for the Leases established at the adoption date. As the discount rate implicit in the Leases was typically not readily determinable, the Company utilized its incremental borrowing rate. In transition to ASC 842, the Company utilized the remaining lease term of its leases in determining the appropriate incremental borrowing rates. The IBR for the Leases was determined by establishing a credit rating of the Company using the Ordered Logit (oLogit) model. The oLogit Model is a quantitative way to assess the credit rating of a company. Based on the established credit rating, the Company determined a borrowing rate using regression analysis on selected financial ratios of publicly traded comparable companies and the companies' credit ratings, adjusted for the risk-free rate, which resulted in an IBR of approximately 10% . The Company recorded the liability associated with the Leases at the present value of the lease payments not yet paid, discounted using the incremental borrowing rate for the each lease established at the adoption date. On January 1, 2019, the Company recorded a right-of-use asset in the amount of $29.1 million , which represents a lease liability of $31.0 million , adjusted for previously recognized lease-related balances, including deferred rent of $2.7 million and prepaid rent of $0.7 million . This lease liability will be reduced over the remaining lease term based on cash payments made offset by accretion of monthly interest calculated on the lease liability. The right-of-use asset will be amortized over the remaining lease term in an amount equal to the difference between the calculated straight-line expense of the total lease payments less the monthly interest calculated on the remaining lease liability. The Company will recognize rent expense, calculated as the remaining cost of the lease allocated over the remaining lease term on a straight-line basis. Rent expense will be presented as part of continuing operations in the condensed statement of consolidated operations and comprehensive loss. For the three months ended March 31, 2019 , the Company recognized rent expense of $2.0 million . For the three months ended March 31, 2019 , the Company paid $2.1 million in rent relating to the Leases. As payments resulting from an operating lease, the $2.1 million is classified within operating activities in the condensed consolidated statements of cash flows. The following table contains supplemental balance sheet information pertaining to the Company's leases for the three months ended March 31, 2019 : Three Months Ended March 31, 2019 Weighted average remaining lease term 3.8 years Weighted average discount rate 10.58 % Future minimum lease payments under the Company's non-cancelable operating leases as of March 31, 2019 are as follows, (in thousands): As of March 31, 2019 2019 $ 6,292 2020 8,610 2021 8,336 2022 8,409 2023 6,450 Total Lease Payments $ 38,097 Less: imputed interest (7,642 ) Total $ 30,455

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 2019
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies Legal Proceedings The Company, from time to time, may be party to litigation arising in the ordinary course of its business. The Company was not subject to any material legal proceedings during the three months ended March 31, 2019 , and, to the best of its knowledge, no material legal proceedings are currently pending or threatened. Other The Company is also party to various agreements, principally relating to licensed technology, that require future payments relating to milestones not met at March 31, 2019 and December 31, 2018 , or royalties on future sales of specified products. No milestones or royalty payments under these agreements are expected to be payable in the immediate future. See Note 15 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	3 Months Ended
	Mar. 31, 2019
Significant Agreements [Abstract]
Significant Agreements	Significant Agreements Celgene Overview On February 20, 2008, the Company entered into an agreement with Celgene relating to sotatercept (the Original Sotatercept Agreement), which was amended on August 2, 2011 (as amended, the Amended Sotatercept Agreement). The Company further amended and restated the Original Sotatercept Agreement in its entirety on September 18, 2017, (the Restated Sotatercept Agreement). On August 2, 2011, the Company entered into a second agreement with Celgene for luspatercept, (the Luspatercept Agreement). Since December 31, 2018 , there have been no material changes to the key terms of the above agreements. For further information on the terms of the agreements, please see the notes to the consolidated financial statements included in the Company’s Form 10-K for the year ended December 31, 2018 . Restated Sotatercept Agreement The Restated Sotatercept Agreement provides Celgene with an exclusive license to sotatercept outside of the field of pulmonary hypertension, referred to as the PH field, and provides the Company with the worldwide rights to develop and commercialize sotatercept in the PH field. In connection with the Restated Sotatercept Agreement, Celgene agreed not to develop or commercialize in the PH field any compound developed under the Restated Sotatercept Agreement or the Luspatercept Agreement, and the Company agreed not to develop or commercialize any compound developed under the Restated Sotatercept Agreement or the Luspatercept Agreement in any field outside the PH field. The Company has the right to license, transfer, or sell its rights to develop and commercialize sotatercept in the PH field, subject to Celgene's first right of negotiation. 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 the Company files an Investigational New Drug application for the treatment of anemia. The Company retained responsibility for research and development through the end of Phase 1 and the Company's initial luspatercept beta-thalassemia and luspatercept MDS 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 and will be responsible for overseeing the manufacture of Phase 3 and commercial supplies by third party contract manufacturing organizations. Beginning in November 2013, the Company agreed to conduct certain extension studies for the benefit of the luspatercept program, which included certain clinical and non-clinical services. These studies were mutually agreed to by both parties and are directed by the Joint Development Committee. The Company is reimbursed for these services under the same terms and rates of the existing agreements. Both Agreements Under both agreements, Celgene is responsible for paying 100% of worldwide development costs and 100% of any commercialization costs worldwide for sotatercept (outside of the pulmonary hypertension field) and luspatercept. The Company has the right to co-promote sotatercept (outside of the pulmonary hypertension field), luspatercept and future products in North America, for which the commercialization costs provided in the commercialization plan and budget as approved by the Joint Commercialization Committee will be entirely funded by Celgene. Additionally, from time to time the Company may elect to conduct additional activities to support commercialization of luspatercept at its own expense. The Company will receive tiered royalties in the low-to-mid 20% range on net sales of sotatercept (outside of the pulmonary hypertension field) and luspatercept, and these royalty schedules are the same for both agreements. Accounting Analysis As of the ASC 606 adoption date, the only remaining undelivered element was participation in the JDC. The transaction price allocated to participation in the JDC based on the established standalone selling price of all performance obligations was de minimis as the sotatercept and luspatercept licenses carried the most significant portion of the value included in the agreements, and the Company's remaining effort on the JDC is minimal. Therefore, the Company recorded a cumulative-effect reduction to opening accumulated deficit of $3.7 million as the adoption date and a corresponding decrease to deferred revenue, of which $0.5 million was recorded to current deferred revenue and $3.2 million was recorded to long-term deferred revenue. The next likely milestone payment for luspatercept would be $25.0 million and result from U.S. Food and Drug Administration (FDA) or European Medicines Agency (EMA) acceptance of a Biologics Licensing Application or equivalent for luspatercept in either myelodysplastic syndromes or beta-thalassemia. The Company and Celgene submitted market applications for luspatercept in the United States and Europe in April 2019. Following application submission, the FDA will determine the acceptance of the application for "filing" by 60 days from the submission date. Similarly, the EMA will validate the application within 10 days of submission. In accordance with the Company's accounting policy regarding revenue recognition, the revenue associated with this milestone will be recognized once it is probable that the applications are accepted for review by either the FDA or EMA. Milestone payments that are not within the control of the Company or the licensee are not considered probable of being achieved until those approvals are received. The acceptance of the application by either the FDA or EMA could occur as early as the second quarter of 2019, which would result in the recognition of the milestone during that period. As of March 31, 2019 , the acceptance of the application is not within the control of the Company or the licensee, and therefore, the Company could not determine if it is probable that a regulatory agency will accept the application. Through March 31, 2019 , under all Celgene arrangements the Company has received net cost-share payments and milestones of $116.6 million and $44.6 million for luspatercept and sotatercept, respectively. The Company recorded net cost-sharing revenue of $2.8 million and $3.2 million during the three months ended March 31, 2019 and 2018 , respectively. Other Agreements 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 royalties ranging from 1.0% - 3.5% of net sales on any products under the licenses. During the three months ended March 31, 2019 and 2018 , 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 research fee of $0.3 million upon execution of the agreement. The Company also received an option to obtain a commercial license to the molecules developed during the collaboration. During the three months ended March 31, 2019 and 2018 , the Company expensed $0.2 million and zero , respectively, of milestones and fees defined under the agreement.

Stockholders' Equity

Stockholders' Equity	3 Months Ended
	Mar. 31, 2019
Equity [Abstract]
Stockholders' Equity	Stockholders' Equity On January 18, 2019, the Company completed the sale of its underwritten public offering of 5,348,838 shares of common stock at public offering price of $43.00 per share, resulting in net proceeds of $215.8 million . In connection with the January 2019 public offering, on February 12, 2019, the underwriters fully exercised their over-allotment option to purchase an additional 802,325 shares of the Company's common stock. The total net proceeds from the January 2019 public offering and the underwriters' exercise of their option to purchase additional shares of common stock was $248.2 million .

Stock-Based Compensation

Stock-Based Compensation	3 Months Ended
	Mar. 31, 2019
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock-Based Compensation	Stock-Based Compensation The Company recognized stock-based compensation expense related to the 2003 Stock Option and Restricted Stock Plan (the 2003 Plan), the 2013 Equity Incentive Plan (the 2013 Plan), and the 2013 Employee Stock Purchase Plan (the 2013 ESPP) in the consolidated statements of operations and comprehensive loss during the three months ended March 31, 2019 and 2018 , respectively, as follows (in thousands): Three Months Ended 2019 2018 Research and development $ 3,500 $ 2,869 General and administrative 3,492 2,827 $ 6,992 $ 5,696 Stock Options The fair value of each option issued to employees was estimated on the date of grant using the Black-Scholes option pricing model with the following weighted-average assumptions: Three Months Ended 2019 2018 Expected volatility 59.1 % 63.0 % Expected term (in years) 6.0 6.0 Risk-free interest rate 2.6 % 2.6 % Expected dividend yield — % — % The following table summarizes the stock option activity under the Company’s stock option plans during the three months ended March 31, 2019 (in thousands, except per share amounts and years): Number of Stock Options Weighted- Average Exercise Price Per Share Weighted- Average Contractual Life (in years) Aggregate Intrinsic Value(1) Outstanding at December 31, 2018 3,513 $ 34.40 7.31 Granted 663 $ 41.88 Exercised (36 ) $ 21.56 Canceled or forfeited (25 ) $ 41.60 Outstanding at March 31, 2019 4,115 $ 35.67 7.16 $ 45,481 Exercisable at March 31, 2019 2,202 $ 32.57 6.20 $ 30,869 (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 March 31, 2019 . During the three months ended March 31, 2019 , current and former employees of the Company exercised a total of 35,519 options, resulting in total proceeds of $0.8 million . The aggregate intrinsic value of options exercised during the three months ended March 31, 2019 was $0.9 million . As of March 31, 2019 , there was $39.9 million of unrecognized compensation expense related to unvested stock options that is expected to be recognized over a weighted-average period of 2.73 years. Restricted Stock Units The following table summarizes the restricted stock unit (RSU) activity under the 2013 Plan during the three months ended March 31, 2019 (in thousands, except per share amounts): Number Weighted- Unvested balance at December 31, 2018 372 $ 36.53 Granted 113 $ 41.70 Vested (84 ) $ 34.20 Forfeited (7 ) $ 35.99 Unvested balance at March 31, 2019 394 $ 38.51 As of March 31, 2019 , there was approximately $12.4 million of related unrecognized compensation cost, which the Company expects to recognize over a remaining weighted-average period of 1.96 years. Performance-Based Restricted Stock Units The Company has granted performance-based restricted stock units (PSU) whereby vesting accelerates upon the occurrence of certain milestone events. In September 2019, any of these PSUs that remain unvested will vest. When achievement of a milestone becomes probable, compensation cost is recognized from the grant date through the estimated date of achievement. If achievement is not considered probable the expense is recognized from the grant date through September 2019. The following table summarizes PSU activity under the 2013 Plan during the three months ended March 31, 2019 (in thousands, except per share amounts): Number Weighted- Unvested balance at December 31, 2018 236 $ 31.42 Granted — $ — Vested — $ — Forfeited — $ — Unvested balance at March 31, 2019 236 $ 31.42 As of March 31, 2019 , there was approximately $0.4 million of related unrecognized compensation cost, which the Company expects to recognize over a remaining weighted-average period of 0.19 years. Employee Stock Purchase Plan During the three months ended March 31, 2019 and 2018 , the Company recorded $0.1 million and $0.1 million , respectively, of stock-based compensation expense related to the 2013 ESPP.

Income Taxes

Income Taxes	3 Months Ended
	Mar. 31, 2019
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes Deferred tax assets and deferred tax liabilities are recognized based on temporary differences between the financial reporting and tax basis of assets and liabilities using statutory rates. A valuation allowance is recorded against deferred tax assets if it is more likely than not that some or all of the deferred tax assets will not be realized. Due to the uncertainty surrounding the realization of the favorable tax attributes in future tax returns, the Company has recorded a full valuation allowance against the Company’s otherwise recognizable net deferred tax assets.

Related Party Transactions

Related Party Transactions	3 Months Ended
	Mar. 31, 2019
Related Party Transactions [Abstract]
Related Party Transactions	Related Party Transactions Celgene Corporation In connection with the Company's January 2019 public offering, Celgene purchased 706,206 shares of common stock. In connection with this and prior transactions, Celgene owned 12.0% and 12.2% of the Company’s fully diluted equity as of March 31, 2019 and December 31, 2018 , respectively. Refer to Note 15 for additional information regarding this collaboration arrangement. During the three months ended March 31, 2019 and 2018 , all revenue recognized by the Company was recognized under the Celgene collaboration arrangement and, as of March 31, 2019 , the Company had no deferred revenue related to the Celgene collaboration arrangement.

Basis of Presentation (Policies

Basis of Presentation (Policies)	3 Months Ended
	Mar. 31, 2019
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 and reflect the application of certain significant accounting policies as described below and elsewhere in these notes to the financial statements. As of March 31, 2019 , 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, 2018 , have not changed, and 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, 2018 , except for the adoption of Accounting Standards Updates (ASU) No. 2016-02, Leases (Topic 842), as discussed further in Note 13. In the opinion of management, the accompanying interim condensed consolidated financial statements reflect all adjustments, consisting of normal recurring adjustments, necessary for the fair presentation of the Company’s financial position as of March 31, 2019 , the results of its operations for the three months ended March 31, 2019 and 2018 , and its cash flows for the three months ended March 31, 2019 and 2018 . The results for the three months ended March 31, 2019 are not necessarily indicative of the results to be expected for the year ending December 31, 2019 , 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, 2018 , 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, 2018 .
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. 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 consolidated 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 consolidated 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 consolidated financial statements, management used significant estimates in the following areas, among others: revenue recognition and accrued clinical expenses.
Recent Accounting Pronouncements	From time to time, new accounting pronouncements are issued by the Financial Accounting Standards Board (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.

Cash Equivalents and Short-te_2

Cash Equivalents and Short-term and Long-term Investments (Tables)	3 Months Ended
	Mar. 31, 2019
Cash and Cash Equivalents [Abstract]
Available-for-sale securities	The following is a summary of cash, cash equivalents and available-for-sale securities as of March 31, 2019 and December 31, 2018 , (in thousands): March 31, 2019 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Cash and cash equivalents due in 90 days or less $ 214,497 $ — $ (7 ) $ 214,490 Available-for-sale securities: Corporate obligations due in one year or less 109,578 3 (117 ) 109,464 Corporate obligations due in more than one year 53,539 54 (6 ) 53,587 U.S. Treasury securities due in one year or less 85,044 13 (31 ) 85,026 U.S. Treasury securities due in more than one year 13,440 14 — 13,454 Certificates of deposit due in one year or less 1,715 — (2 ) 1,713 Certificates of deposit due in more than one year 490 1 — 491 Mortgage and other asset backed securities due in one year or less 26,953 — (50 ) 26,903 Mortgage and other asset backed securities due in more than one year 7,969 8 — 7,977 Total available-for-sale securities $ 298,728 $ 93 $ (206 ) $ 298,615 Total cash, cash equivalents and available-for-sale securities $ 513,225 $ 93 $ (213 ) $ 513,105 December 31, 2018 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Cash and cash equivalents due in 90 days or less $ 144,064 $ — $ (12 ) $ 144,052 Available-for-sale securities: Corporate obligations due in one year or less 73,671 — (267 ) 73,404 U.S. Treasury securities due in one year or less 45,346 — (79 ) 45,267 Certificates of deposit due in one year or less 1,715 — — 1,715 Mortgage and other asset backed securities due in one year or less 26,982 — (108 ) 26,874 Total available-for-sale securities $ 147,714 $ — $ (454 ) $ 147,260 Total cash, cash equivalents and available-for-sale securities $ 291,778 $ — $ (466 ) $ 291,312

Restricted Cash (Tables)

Restricted Cash (Tables)	3 Months Ended
	Mar. 31, 2019
Receivables [Abstract]
Reconciliation of cash, cash equivalents and restricted cash	The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the condensed consolidated balance sheet that sum to the total of the same such amounts shown in the statement of cash flows (in thousands): March 31, 2019 2018 Cash and cash equivalents $ 214,490 $ 69,736 Restricted cash 1,597 1,132 Total cash, cash equivalents and restricted cash shown in the statement of cash flows $ 216,087 $ 70,868
Restrictions on cash and cash equivalents	The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the condensed consolidated balance sheet that sum to the total of the same such amounts shown in the statement of cash flows (in thousands): March 31, 2019 2018 Cash and cash equivalents $ 214,490 $ 69,736 Restricted cash 1,597 1,132 Total cash, cash equivalents and restricted cash shown in the statement of cash flows $ 216,087 $ 70,868

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	3 Months Ended
	Mar. 31, 2019
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 March 31, 2019 and December 31, 2018 (in thousands): March 31, 2019 Quoted Prices in Active Markets for Identical Items (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Total Assets: Money market funds $ 136,520 $ — $ — $ 136,520 Corporate obligations — 193,017 — 193,017 U.S. Treasury securities — 118,976 — 118,976 Certificates of deposit — 2,204 — 2,204 Mortgage and other asset backed securities — 34,881 — 34,881 Total assets $ 136,520 $ 349,078 $ — $ 485,598 Liabilities: Warrants to purchase common stock $ — $ — $ 1,607 $ 1,607 Total liabilities $ — $ — $ 1,607 $ 1,607 December 31, 2018 Quoted Prices in Active Markets for Identical Items (Level 1) Significant other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Total Assets: Money market funds $ 74,023 $ — $ — $ 74,023 Corporate obligations — 128,920 — 128,920 U.S. Treasury securities — 56,978 — 56,978 Certificates of deposit — 1,715 — 1,715 Mortgage and other asset backed securities — 26,874 — 26,874 Total assets $ 74,023 $ 214,487 $ — $ 288,510 Liabilities: Warrants to purchase common stock $ — $ — $ 1,491 $ 1,491 Total liabilities $ — $ — $ 1,491 $ 1,491
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 common stock warrant liabilities, which represent a recurring measurement that is classified within Level 3 of the fair value hierarchy, wherein fair value is estimated using significant unobservable inputs (in thousands): Three Months Ended March 31, 2019 2018 Beginning balance $ 1,491 $ 2,236 Change in fair value 116 (119 ) Exercises — (797 ) Ending balance $ 1,607 $ 1,320

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	3 Months Ended
	Mar. 31, 2019
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 their inclusion would have had an anti-dilutive effect (in thousands): Three Months Ended 2019 2018 Outstanding stock options 4,115 3,718 Common stock warrants 39 39 Shares issuable under employee stock purchase plan 13 13 Outstanding restricted stock units (1) 630 631 4,797 4,401 (1) This balance is comprised of both the restricted stock units and performance-based restricted stock units described in Note 17.

Warrants (Tables)

Warrants (Tables)	3 Months Ended
	Mar. 31, 2019
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- Average Exercise Balance Sheet Classification March 31, 2019 December 31, 2018 Price Per Share Expiration March 31, 2019 December 31, 2018 Warrants to purchase common stock 39 39 $ 5.88 June 10, 2020 - July 9, 2020 Liability Liability All warrants 39 39 $ 5.88

Leases (Tables)

Leases (Tables)	3 Months Ended
	Mar. 31, 2019
Leases [Abstract]
Lease, cost	The following table contains supplemental balance sheet information pertaining to the Company's leases for the three months ended March 31, 2019 : Three Months Ended March 31, 2019 Weighted average remaining lease term 3.8 years Weighted average discount rate 10.58 %
Lessee, operating lease, liability, maturity	Future minimum lease payments under the Company's non-cancelable operating leases as of March 31, 2019 are as follows, (in thousands): As of March 31, 2019 2019 $ 6,292 2020 8,610 2021 8,336 2022 8,409 2023 6,450 Total Lease Payments $ 38,097 Less: imputed interest (7,642 ) Total $ 30,455

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	3 Months Ended
	Mar. 31, 2019
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Schedule of compensation costs recognized	The Company recognized stock-based compensation expense related to the 2003 Stock Option and Restricted Stock Plan (the 2003 Plan), the 2013 Equity Incentive Plan (the 2013 Plan), and the 2013 Employee Stock Purchase Plan (the 2013 ESPP) in the consolidated statements of operations and comprehensive loss during the three months ended March 31, 2019 and 2018 , respectively, as follows (in thousands): Three Months Ended 2019 2018 Research and development $ 3,500 $ 2,869 General and administrative 3,492 2,827 $ 6,992 $ 5,696
Schedule of weighted-average assumptions used for estimating fair value	The fair value of each option issued to employees was estimated on the date of grant using the Black-Scholes option pricing model with the following weighted-average assumptions: Three Months Ended 2019 2018 Expected volatility 59.1 % 63.0 % Expected term (in years) 6.0 6.0 Risk-free interest rate 2.6 % 2.6 % Expected dividend yield — % — %
Summary of stock option activity	The following table summarizes the stock option activity under the Company’s stock option plans during the three months ended March 31, 2019 (in thousands, except per share amounts and years): Number of Stock Options Weighted- Average Exercise Price Per Share Weighted- Average Contractual Life (in years) Aggregate Intrinsic Value(1) Outstanding at December 31, 2018 3,513 $ 34.40 7.31 Granted 663 $ 41.88 Exercised (36 ) $ 21.56 Canceled or forfeited (25 ) $ 41.60 Outstanding at March 31, 2019 4,115 $ 35.67 7.16 $ 45,481 Exercisable at March 31, 2019 2,202 $ 32.57 6.20 $ 30,869 (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 March 31, 2019 .
Schedule of nonvested restricted stock units activity	The following table summarizes the restricted stock unit (RSU) activity under the 2013 Plan during the three months ended March 31, 2019 (in thousands, except per share amounts): Number Weighted- Unvested balance at December 31, 2018 372 $ 36.53 Granted 113 $ 41.70 Vested (84 ) $ 34.20 Forfeited (7 ) $ 35.99 Unvested balance at March 31, 2019 394 $ 38.51
Share-based compensation, performance shares award nonvested activity	The following table summarizes PSU activity under the 2013 Plan during the three months ended March 31, 2019 (in thousands, except per share amounts): Number Weighted- Unvested balance at December 31, 2018 236 $ 31.42 Granted — $ — Vested — $ — Forfeited — $ — Unvested balance at March 31, 2019 236 $ 31.42

Segment Information (Details)

Segment Information (Details)	3 Months Ended
	Mar. 31, 2019segment
Segment Information
Number of operating segments	1

Cash Equivalents and Short-te_3

Cash Equivalents and Short-term and Long-term Investments - Narrative (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Cash and Cash Equivalents [Abstract]
Unrealized loss position, less than twelve months, fair value	$ 138.2	$ 51.2
Continuous unrealized loss position, twelve months or longer, fair value	58.2	94.3
Continuous unrealized loss position, fair value	$ 0.1	$ 0.4

Cash Equivalents and Short-te_4

Cash Equivalents and Short-term and Long-term Investments - Summary of Cash, Cash Equivalents and Available-for-sale Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Cash and cash equivalents, amortized cost basis	$ 214,497	$ 144,064
Gross Unrealized Losses	(7)	(12)
Cash and cash equivalents due in 90 days or less	214,490	144,052	$ 69,736
Available-for-sale securities:
Amortized Cost	298,728	147,714
Gross Unrealized Gains	93	0
Gross Unrealized Losses	(206)	(454)
Estimated Fair Value	298,615	147,260
Cash, Cash Equivalents, And Available-For-Sale Securities [Abstract]
Amortized Cost	513,225	291,778
Gross Unrealized Gains	93	0
Gross Unrealized Losses	(213)	(466)
Estimated Fair Value	513,105	291,312
Corporate obligations due in one year or less
Available-for-sale securities:
Amortized Cost	109,578	73,671
Gross Unrealized Gains	3	0
Gross Unrealized Losses	(117)	(267)
Estimated Fair Value	109,464	73,404
Corporate obligations due in more than one year
Available-for-sale securities:
Amortized Cost	53,539
Gross Unrealized Gains	54
Gross Unrealized Losses	(6)
Estimated Fair Value	53,587
U.S. Treasury securities due in one year or less
Available-for-sale securities:
Amortized Cost	85,044	45,346
Gross Unrealized Gains	13	0
Gross Unrealized Losses	(31)	(79)
Estimated Fair Value	85,026	45,267
U.S. Treasury securities due in more than one year
Available-for-sale securities:
Amortized Cost	13,440
Gross Unrealized Gains	14
Gross Unrealized Losses	0
Estimated Fair Value	13,454
Certificates of deposit due in one year or less
Available-for-sale securities:
Amortized Cost	1,715	1,715
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(2)	0
Estimated Fair Value	1,713	1,715
Certificates of deposit due in more than one year
Available-for-sale securities:
Amortized Cost	490
Gross Unrealized Gains	1
Gross Unrealized Losses	0
Estimated Fair Value	491
Mortgage and other asset backed securities due in one year or less
Available-for-sale securities:
Amortized Cost	26,953	26,982
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(50)	(108)
Estimated Fair Value	26,903	$ 26,874
Mortgage and other asset backed securities due in more than one year
Available-for-sale securities:
Amortized Cost	7,969
Gross Unrealized Gains	8
Gross Unrealized Losses	0
Estimated Fair Value	$ 7,977

Restricted Cash (Details)

Restricted Cash (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Cash and cash equivalents	$ 214,490	$ 144,052	$ 69,736
Restricted cash	1,597	1,597	1,132
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	216,087	145,649	$ 70,868	$ 101,282
Certificates of Deposits and Money Market Funds | Letter of Credit
Accounts, Notes, Loans and Financing Receivable [Line Items]
Restricted cash	$ 1,600	$ 1,600

Fair Value Measurements - Finan

Fair Value Measurements - Financial Instrument at Fair Value Schedule (Details) - Recurring - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Assets:
Total assets	$ 485,598	$ 288,510
Liabilities:
Total liabilities	1,607	1,491
Common stock warrants
Liabilities:
Total liabilities	1,607	1,491
Money market funds
Assets:
Total assets	136,520	74,023
Corporate obligations
Assets:
Total assets	193,017	128,920
U.S. Treasury securities
Assets:
Total assets	118,976	56,978
Certificates of deposit
Assets:
Total assets	2,204	1,715
Mortgage and other asset backed securities
Assets:
Total assets	34,881	26,874
Quoted Prices in Active Markets for Identical Items (Level 1)
Assets:
Total assets	136,520	74,023
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	136,520	74,023
Quoted Prices in Active Markets for Identical Items (Level 1) | Corporate obligations
Assets:
Total assets	0	0
Quoted Prices in Active Markets for Identical Items (Level 1) | U.S. Treasury securities
Assets:
Total assets	0	0
Quoted Prices in Active Markets for Identical Items (Level 1) | Certificates of deposit
Assets:
Total assets	0	0
Quoted Prices in Active Markets for Identical Items (Level 1) | Mortgage and other asset backed securities
Assets:
Total assets	0	0
Significant Other Observable Inputs (Level 2)
Assets:
Total assets	349,078	214,487
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) | Corporate obligations
Assets:
Total assets	193,017	128,920
Significant Other Observable Inputs (Level 2) | U.S. Treasury securities
Assets:
Total assets	118,976	56,978
Significant Other Observable Inputs (Level 2) | Certificates of deposit
Assets:
Total assets	2,204	1,715
Significant Other Observable Inputs (Level 2) | Mortgage and other asset backed securities
Assets:
Total assets	34,881	26,874
Significant Unobservable Inputs (Level 3)
Assets:
Total assets	0	0
Liabilities:
Total liabilities	1,607	1,491
Significant Unobservable Inputs (Level 3) | Common stock warrants
Liabilities:
Total liabilities	1,607	1,491
Significant Unobservable Inputs (Level 3) | Money market funds
Assets:
Total assets	0	0
Significant Unobservable Inputs (Level 3) | Corporate obligations
Assets:
Total assets	0	0
Significant Unobservable Inputs (Level 3) | U.S. Treasury securities
Assets:
Total assets	0	0
Significant Unobservable Inputs (Level 3) | Certificates of deposit
Assets:
Total assets	0	0
Significant Unobservable Inputs (Level 3) | Mortgage and other asset backed securities
Assets:
Total assets	$ 0	$ 0

Fair Value Measurements - Prefe

Fair Value Measurements - Preferred and Common Stock Warrant Liability Rollforward (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
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	$ 1,491	$ 2,236
Change in fair value	116	(119)
Exercises	0	(797)
Ending balance	$ 1,607	$ 1,320

Net Loss Per Share (Details)

Net Loss Per Share (Details) - shares shares in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Net Income (Loss) Per Share
Anti-dilutive common stock equivalents excluded from the calculation of diluted net loss per share (in shares)	4,797	4,401
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)	4,115	3,718
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)	39	39
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)	13	13
Outstanding restricted stock units
Net Income (Loss) Per Share
Anti-dilutive common stock equivalents excluded from the calculation of diluted net loss per share (in shares)	630	631

Warrants (Details)

Warrants (Details) - $ / shares shares in Thousands	Mar. 31, 2019	Dec. 31, 2018
Warrants
Number of shares issuable upon exercise of outstanding warrants (in shares)	39	39
Weighted-average exercise price per share (in usd per share)	$ 5.88
Warrants to purchase Common stock expiring between June 10, 2020 to July 9, 2020
Warrants
Number of shares issuable upon exercise of outstanding warrants (in shares)	39	39
Weighted-average exercise price per share (in usd per share)	$ 5.88

Leases - Narrative (Details)

Leases - Narrative (Details) ft² in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2019USD ($)ft²building	Jan. 01, 2019USD ($)
Lessee, Lease, Description [Line Items]
Operating lease, leased area | ft²	125
Operating lease, number of buildings | building	5
IBR rate for operating leases	10.00%
Right-of-use - operating leases	$ 27,817
Operating lease, liability	30,455
Operating leases, rent expense	2,000
Operating lease, payments	$ 2,100
Minimum
Lessee, Lease, Description [Line Items]
Operating lease, extension term	3 years
Maximum
Lessee, Lease, Description [Line Items]
Operating lease, extension term	5 years
Accounting Standards Update 2016-02
Lessee, Lease, Description [Line Items]
Right-of-use - operating leases		$ 29,100
Operating lease, liability		31,000
Deferred rent		2,700
Prepaid rent		$ 700

Leases - Lease Costs (Details)

Leases - Lease Costs (Details)	Mar. 31, 2019
Leases [Abstract]
Weighted average remaining lease term	3 years 9 months 15 days
Weighted average discount rate	10.58%

Leases - Minimum Lease Payments

Leases - Minimum Lease Payments (Details) $ in Thousands	Mar. 31, 2019USD ($)
Leases [Abstract]
2019	$ 6,292
2020	8,610
2021	8,336
2022	8,409
2023	6,450
Total Lease Payments	38,097
Less: imputed interest	(7,642)
Total	$ 30,455

Significant Agreements - Celgen

Significant Agreements - Celgene, Both Agreements (Details) - Collaboration, License, and Option Agreement - Sotarcept and Luspatercept - Investor	3 Months Ended
	Mar. 31, 2019
Significant Agreements
Percentage of development costs for which collaborator is responsible	100.00%
Percentage of commercialization costs for which collaborator is responsible	100.00%
Weighted Average
Significant Agreements
Potential royalty rate	20.00%

Significant Agreements - Celg_2

Significant Agreements - Celgene, Accounting Analysis (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Jan. 01, 2018
Significant Agreements
Accumulated deficit	$ (624,602)		$ (586,549)
Revenue from contract with customer	2,780	$ 3,232
Cost Sharing
Significant Agreements
Revenue from contract with customer	2,780	3,232
Investor | Luspatercept | Collaboration, License, and Option Agreement
Significant Agreements
Payments received	116,600
Investor | Sotarcept | Collaboration, License, and Option Agreement
Significant Agreements
Payments received	44,600
Investor | Cost Sharing | Collaboration, License, and Option Agreement
Significant Agreements
Revenue from contract with customer	2,800	$ 3,200
Clinical Milestones | Investor | Luspatercept | Collaboration, License, and Option Agreement
Significant Agreements
Collaborative arrangement potential milestone payments on acceptance of regulatory application	$ 25,000
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Significant Agreements
Accumulated deficit				$ 3,700
Contract with customer, liability, current				500
Contract with customer, liability, noncurrent				$ 3,200

Significant Agreements - Other

Significant Agreements - Other Agreements (Details) - USD ($)	1 Months Ended	3 Months Ended		12 Months Ended
	May 31, 2014	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2004
Significant Agreements
Milestones and fees		$ 32,771,000	$ 23,431,000
License Agreement | License agreement with a non-profit institution | Minimum
Significant Agreements
Milestone fees payable as percentage of research and U.S. development progress and sublicensing revenue				10.00%
Royalty payable as percentage of net sales				1.00%
License Agreement | License agreement with a non-profit institution | Maximum
Significant Agreements
Milestone fees payable as percentage of research and U.S. development progress and sublicensing revenue				25.00%
Royalty payable as percentage of net sales				3.50%
License Agreement | License agreement with a non-profit institution | Sotarcept | Development milestone | Maximum
Significant Agreements
Total potential milestone payments (up to)				$ 2,000,000
License Agreement | License agreement with a non-profit institution | Luspatercept | Development milestone | Maximum
Significant Agreements
Total potential milestone payments (up to)				$ 700,000
License Agreement | Common Stock | License agreement with a non-profit institution
Significant Agreements
Number of shares issued as compensation for licenses (in shares)				62,500
Fair value of shares issued as compensation for licenses				$ 25,000
Collaborative Arrangement | License agreement with antibody technology company
Significant Agreements
Milestones and fees	$ 300,000
Non-collaborative Arrangement Transactions | License Agreement with Non Profit Institution
Significant Agreements
Milestones and fees		$ 200,000	$ 0

Stockholders' Equity (Details)

Stockholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands	Feb. 12, 2019	Jan. 18, 2019	Mar. 31, 2019	Mar. 31, 2018
Class of Stock [Line Items]
Price per share (in usd per share)		$ 43
Proceeds from issuance of common stock from public offering, net of issuance costs	$ 248,200	$ 215,800	$ 248,155	$ 0
Common Stock
Class of Stock [Line Items]
Number of shares issued in transaction (in shares)		5,348,838
Number of additional shares available for purchase (in shares)	802,325

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Stock-Based Compensation Costs Recognized (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost
Stock-based compensation expense	$ 6,992	$ 5,696
Research and development
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost
Stock-based compensation expense	3,500	2,869
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost
Stock-based compensation expense	$ 3,492	$ 2,827

Stock-Based Compensation - Sc_2

Stock-Based Compensation - Schedule of Weighted-Average Assumptions Used for Estimating Fair Value (Details) - Stock Option	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award
Expected volatility	59.10%	63.00%
Expected term (in years)	6 years	6 years
Risk-free interest rate	2.60%	2.60%
Expected dividend yield	0.00%	0.00%

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Number of Stock Options
Exercised (in shares)	(35,519)
Aggregate Intrinsic Value
Total proceeds from options exercised	$ 800
Aggregate intrinsic value of options exercised	$ 900
Stock Option
Number of Stock Options
Outstanding at the beginning of period (in shares)	3,513,000
Granted (in shares)	663,000
Exercised (in shares)	(36,000)
Canceled or forfeited (in shares)	(25,000)
Outstanding at the end of the period (in shares)	4,115,000	3,513,000
Exercisable at the end of the period (in shares)	2,202,000
Weighted- Average Exercise Price Per Share
Outstanding at the beginning of period (in usd per share)	$ 34.40
Granted (in usd per share)	41.88
Exercised (in usd per share)	21.56
Canceled or forfeited (in usd per share)	41.60
Outstanding at the end of the period (in usd per share)	35.67	$ 34.40
Exercisable at the end of the period (in usd per share)	$ 32.57
Weighted- Average Contractual Life (in years)
Outstanding at the beginning of the period	7 years 1 month 28 days	7 years 3 months 22 days
Outstanding at the end of the period	7 years 1 month 28 days	7 years 3 months 22 days
Exercisable at the end of the period	6 years 2 months 12 days
Aggregate Intrinsic Value
Outstanding at the end of the period	$ 45,481
Exercisable at the end of the period	30,869
Unrecognized compensation expense	$ 39,900
Weighted-average period for recognition	2 years 8 months 23 days

Stock-Based Compensation - Sc_3

Stock-Based Compensation - Schedule of Nonvested Restricted Stock Activity (Details) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended
	Mar. 31, 2019USD ($)$ / sharesshares
Restricted Stock Units
Number of Grants
Unvested balance at beginning of the period (in shares) | shares	372
Granted (in shares) | shares	113
Vested (in shares) | shares	(84)
Forfeited (in shares) | shares	(7)
Unvested balance at end of the period (in shares) | shares	394
Weighted- Average Grant Date Fair Value
Unvested balance at beginning of the period (in usd per share) | $ / shares	$ 36.53
Granted (in usd per share) | $ / shares	41.70
Vested (in usd per share) | $ / shares	34.20
Forfeited (in usd per share) | $ / shares	35.99
Unvested balance at end of the period (in usd per share) | $ / shares	$ 38.51
Unrecognized compensation cost | $	$ 12.4
Weighted-average period for recognition	1 year 11 months 16 days
Performance-Based Restricted Stock Units
Number of Grants
Unvested balance at beginning of the period (in shares) | shares	236
Granted (in shares) | shares	0
Vested (in shares) | shares	0
Forfeited (in shares) | shares	0
Unvested balance at end of the period (in shares) | shares	236
Weighted- Average Grant Date Fair Value
Unvested balance at beginning of the period (in usd per share) | $ / shares	$ 31.42
Granted (in usd per share) | $ / shares	0
Vested (in usd per share) | $ / shares	0
Forfeited (in usd per share) | $ / shares	0
Unvested balance at end of the period (in usd per share) | $ / shares	$ 31.42
Unrecognized compensation cost | $	$ 0.4
Weighted-average period for recognition	2 months 9 days

Stock-Based Compensation - Narr

Stock-Based Compensation - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award
Stock-based compensation expense	$ 6,992	$ 5,696
Employee Stock Purchase Plan 2013
Share-based Compensation Arrangement by Share-based Payment Award
Stock-based compensation expense	$ 100	$ 100

Related Party Transactions (Det

Related Party Transactions (Details) - Celgene - USD ($)	1 Months Ended
	Jan. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Related Party Transactions
Ownership percentage of entity's fully diluted equity		12.00%	12.20%
Deferred revenue		$ 0
Common Stock
Related Party Transactions
Issuance of common stock (in shares)	706,206

Uncategorized Items - xlrn-2019

Label	Element	Value
Cumulative Effect of New Accounting Principle in Period of Adoption	us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption	$ 3,704,000
Retained Earnings [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption	us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption	$ 3,704,000