Acceleron Pharma (XLRN) 10-Q 7 Nov 17 2017 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2017	Oct. 31, 2017
Document and Entity Information
Entity Registrant Name	ACCELERON PHARMA INC
Entity Central Index Key	1,280,600
Document Type	10-Q
Document Period End Date	Sep. 30, 2017
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)		45,235,024
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	Q3

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 238,959	$ 20,950
Collaboration receivables (all amounts are with related party)	2,879	3,234
Prepaid expenses and other current assets	3,334	3,862
Short-term investments	98,266	118,740
Total current assets	343,438	146,786
Property and equipment, net	7,211	5,201
Restricted cash	1,132	946
Other assets	124	22
Long-term investments	29,372	94,692
Total assets	381,277	247,647
Current liabilities:
Accounts payable	1,024	1,590
Accrued expenses	14,141	13,249
Deferred revenue	541	541
Deferred rent	178	769
Total current liabilities	15,884	16,149
Deferred revenue, net of current portion	3,297	3,704
Deferred rent, net of current portion	1,628	953
Warrants to purchase common stock	1,927	1,244
Total liabilities	22,736	22,050
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; 44,414,694 and 38,251,826 shares issued and outstanding at September 30, 2017 and December 31, 2016, respectively	45	39
Additional paid-in capital	803,907	590,474
Accumulated deficit	(445,097)	(364,491)
Accumulated other comprehensive loss	(314)	(425)
Total stockholders’ equity	358,541	225,597
Total liabilities and stockholders’ equity	$ 381,277	$ 247,647

Condensed Consolidated Balance3

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Preferred stock, par value (in dollars 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 dollars per share)	$ 0.001	$ 0.001
Common stock, shares authorized	175,000,000	175,000,000
Common stock, shares issued	44,414,694	38,251,826
Common stock, shares outstanding	44,414,694	38,251,826

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Collaboration revenue:
License and milestone	$ 135	$ 135	$ 406	$ 15,415
Cost-sharing, net	2,879	2,870	9,370	8,987
Total revenue (all amounts are with related party)	3,014	3,005	9,776	24,402
Costs and expenses:
Research and development	21,059	17,102	64,387	49,492
General and administrative	7,533	6,411	26,735	19,029
Total costs and expenses	28,592	23,513	91,122	68,521
Loss from operations	(25,578)	(20,508)	(81,346)	(44,119)
Other (expense) income, net	(410)	(794)	(683)	5,026
Interest income	496	512	1,474	1,348
Total other income (expense) net	86	(282)	791	6,374
Loss before income taxes	(25,492)	(20,790)	(80,555)	(37,745)
Income tax benefit	41	20	29	20
Net loss applicable to common stockholders	$ (25,451)	$ (20,770)	$ (80,526)	$ (37,725)
Net loss per share applicable to common stockholders-basic and diluted (Note 9) (in dollars per share)	$ (0.65)	$ (0.55)	$ (2.08)	$ (1.01)
Weighted-average number of common shares used in computing net loss per share applicable to common stockholders (in shares)	39,361	37,616	38,804	37,268
Other comprehensive loss:
Net loss	$ (25,451)	$ (20,770)	$ (80,526)	$ (37,725)
Net unrealized holding gains (losses) on short-term and long-term investments during the period, net of tax of $59 thousand, $59 thousand, $53 thousand and $53 thousand for the three and nine months ended September 30, 2017 and 2016, respectively	100	(374)	170	98
Comprehensive loss	$ (25,351)	$ (21,144)	$ (80,356)	$ (37,627)

Condensed Consolidated Stateme5

Condensed Consolidated Statements of Operations and Comprehensive Loss (Parenthetical) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income Statement [Abstract]
Unrealized holding gain (loss) on securities, tax	$ 59	$ 53	$ 59	$ 53

Condensed Consolidated Stateme6

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Operating Activities
Net loss	$ (80,526)	$ (37,725)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	2,023	1,158
Stock-based compensation	21,877	13,387
Change in fair value of warrants	683	(5,026)
Other non-cash items	211	(262)
Changes in assets and liabilities:
Prepaid expenses and other assets	426	(1,211)
Collaboration receivables	355	609
Accounts payable	(566)	1,196
Accrued expenses	160	389
Restricted cash	(186)	(150)
Deferred revenue	(407)	(414)
Deferred rent	22	94
Net cash used in operating activities	(55,928)	(27,955)
Investing Activities
Purchases of investments	(245)	(132,565)
Proceeds from sales and maturities of investments	86,001	21,120
Purchases of property and equipment	(3,639)	(2,246)
Net cash provided by (used in) investing activities	82,117	(113,691)
Financing Activities
Proceeds from issuance of common stock from public offering, net of issuance costs	187,986	140,697
Payments for withholding taxes on restricted stock units	(226)	0
Proceeds from exercise of stock options and warrants to purchase common stock	3,233	3,524
Proceeds from issuances of common stock related to employee stock purchase plan	827	658
Net cash provided by financing activities	191,820	144,879
Net increase in cash and cash equivalents	218,009	3,233
Cash and cash equivalents at beginning of period	20,950	27,783
Cash and cash equivalents at end of period	238,959	31,016
Supplemental Disclosure of Non-Cash Investing and Financing Activities:
Purchase of property and equipment included in accounts payable and accrued expenses	395	25
Capitalized follow-on public offering costs included in accrued expenses	$ 337	$ 0

Nature of Business

Nature of Business	9 Months Ended
	Sep. 30, 2017
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, 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	9 Months Ended
	Sep. 30, 2017
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, 2016 except for the adoption of Accounting Standards Update (ASU) No. 2016-09, Compensation - Stock Compensation (Topic 718), Improvements to Employee Share-Based Payment Accounting, which did not have a material impact , 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, 2017 , the results of its operations for the three and nine months ended September 30, 2017 and 2016 , and its cash flows for the nine months ended September 30, 2017 and 2016 . The results for the three and nine months ended September 30, 2017 are not necessarily indicative of the results to be expected for the year ending December 31, 2017 , 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, 2016 , 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, 2016 . 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 for certain estimates or to identify matters that require additional disclosure. Subsequent events have been evaluated as required. The Company has evaluated all subsequent events and determined that there are no material recognized or unrecognized events requiring disclosure, other than those disclosed in this Report on Form 10-Q. 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, 2017 , 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, 2016 , have not changed, except for the adoption of Accounting Standards Update (ASU) No. 2016-09, Compensation - Stock Compensation (Topic 718), Improvements to Employee Share-Based Payment Accounting, which is discussed further in Note 16.

Use of Estimates

Use of Estimates	9 Months Ended
	Sep. 30, 2017
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, 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, 2017
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. 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 consolidated statements of operations and comprehensive loss.

Cash Equivalents and Short-term

Cash Equivalents and Short-term and Long-term Investments	9 Months Ended
	Sep. 30, 2017
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 September 30, 2017 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 and nine months ended September 30, 2017 and 2016 . 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. The aggregate fair value of securities held by the Company in an unrealized loss position for less than twelve months as of September 30, 2017 and December 31, 2016 was $53.1 million and $172.2 million , respectively. The aggregate fair value of securities held by the Company in an unrealized loss position for more than twelve months as of September 30, 2017 and December 31, 2016 was $55.8 million and $5.5 million , respectively. The aggregate unrealized loss for those securities in an unrealized loss position for more than twelve months is $0.1 million and $2 thousand , respectively. As a result, the Company determined it did not hold any investments with any other-than-temporary impairment as of September 30, 2017 and December 31, 2016 . The following is a summary of cash, cash equivalents and available-for-sale securities as of September 30, 2017 and December 31, 2016 (in thousands): September 30, 2017 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Cash and cash equivalents due in 90 days or less $ 238,959 $ — $ — $ 238,959 Available-for-sale securities: Corporate obligations due in one year or less 44,789 1 (47 ) 44,743 Corporate obligations due in more than one year 11,560 — (47 ) 11,513 U.S. Treasury securities due in one year or less 10,004 — (31 ) 9,973 U.S. Treasury securities due in more than one year 4,021 — (27 ) 3,994 Certificates of deposit due in one year or less 9,092 — — 9,092 Certificates of deposit due in more than one year 1,904 — — 1,904 Mortgage and other asset backed securities due in one year or less 34,497 — (39 ) 34,458 Mortgage and other asset backed securities due in more than one year 12,026 — (65 ) 11,961 Total available-for-sale securities $ 127,893 $ 1 $ (256 ) $ 127,638 Total cash, cash equivalents and available-for-sale securities $ 366,852 $ 1 $ (256 ) $ 366,597 December 31, 2016 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Cash and cash equivalents due in 90 days or less $ 20,950 $ — $ — $ 20,950 Available-for-sale securities: Corporate obligations due in one year or less 45,839 1 (58 ) 45,782 Corporate obligations due in more than one year 42,895 — (185 ) 42,710 U.S. Treasury securities due in one year or less 22,490 — (10 ) 22,480 U.S. Treasury securities due in more than one year 11,541 — (53 ) 11,488 Certificates of deposit due in one year or less 13,562 — — 13,562 Certificates of deposit due in more than one year 9,811 — — 9,811 Mortgage and other asset backed securities due in one year or less 36,948 — (32 ) 36,916 Mortgage and other asset backed securities due in more than one year 30,771 — (88 ) 30,683 Total available-for-sale securities $ 213,857 $ 1 $ (426 ) $ 213,432 Total cash, cash equivalents and available-for-sale securities $ 234,807 $ 1 $ (426 ) $ 234,382

Restricted Cash

Restricted Cash	9 Months Ended
	Sep. 30, 2017
Receivables [Abstract]
Restricted Cash	Restricted Cash As of September 30, 2017 and December 31, 2016 , the Company maintained letters of credit totaling $1.1 million and $0.9 million, respectively, 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	9 Months Ended
	Sep. 30, 2017
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 collaboration 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	9 Months Ended
	Sep. 30, 2017
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, 2017 and December 31, 2016 (in thousands): September 30, 2017 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 $ 224,053 $ — $ — $ 224,053 Corporate obligations — 56,256 — 56,256 U.S. Treasury securities — 13,967 — 13,967 Certificates of deposit — 10,997 — 10,997 Mortgage and other asset backed securities — 46,419 — 46,419 Restricted cash 1,132 — — 1,132 Total assets $ 225,185 $ 127,639 $ — $ 352,824 Liabilities: Warrants to purchase common stock $ — $ — $ 1,927 $ 1,927 Total liabilities $ — $ — $ 1,927 1,927 December 31, 2016 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 $ 19,818 $ — $ — $ 19,818 Corporate obligations — 88,492 — 88,492 U.S. Treasury securities — 33,968 — 33,968 Certificates of deposit — 23,373 — 23,373 Mortgage and other asset backed securities — 67,599 — 67,599 Restricted cash 946 — — 946 Total assets $ 20,764 $ 213,432 $ — $ 234,196 Liabilities: Warrants to purchase common stock $ — $ — $ 1,244 $ 1,244 Total liabilities $ — $ — $ 1,244 $ 1,244 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 nine months ended September 30, 2017 or the year ended December 31, 2016 . 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): Nine Months Ended September 30, 2017 2016 Beginning balance $ 1,244 $ 17,187 Change in fair value 683 (5,026 ) Ending balance $ 1,927 $ 12,161 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 September 30, 2017 , 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 nine months ended September 30, 2017 or the year ended December 31, 2016 .

Net Loss Per Share

Net Loss Per Share	9 Months Ended
	Sep. 30, 2017
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 Nine Months Ended 2017 2016 2017 2016 Outstanding stock options 3,459 3,193 3,459 3,193 Common stock warrants 64 397 64 397 Shares issuable under employee stock purchase plan 19 18 19 18 Outstanding restricted stock units 551 670 551 670 4,093 4,278 4,093 4,278

Comprehensive Loss

Comprehensive Loss	9 Months Ended
	Sep. 30, 2017
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 September 30, 2017 and December 31, 2016 .

Recent Accounting Pronouncement

Recent Accounting Pronouncements	9 Months Ended
	Sep. 30, 2017
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Recent Accounting Pronouncements	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. In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606) , which supersedes all existing revenue recognition requirements, including most industry-specific guidance. The new standard requires a company to recognize revenue when it transfers goods or services to customers in an amount that reflects the consideration that the company expects to receive for those goods or services. The new standard will be effective for the Company on January 1, 2018. Topic 606 allows for either a full retrospective application, in which the standard is applied to all periods presented, or a modified retrospective application, in which the standard is applied to the most current period presented in the financial statements. As of September 30, 2017 , revenue is generated exclusively from the Company's collaboration agreement with Celgene. The Company is currently evaluating the potential impact that Topic 606 may have on its financial position and results of operations as it relates to this single arrangement, and expects to elect the modified retrospective application as its transition method. In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842), Amendments to the FASB Accounting Standards Codification, which replaces the existing guidance for leases. ASU 2016-02 requires the identification of arrangements that should be accounted for as leases by lessees. In general, for lease arrangements exceeding a twelve month term, these arrangements must now be recognized as assets and liabilities on the balance sheet of the lessee. Under ASU 2016-02, a right-of-use asset and lease obligation will be recorded for all leases, whether operating or financing, while the income statement will reflect lease expense for operating leases and amortization/interest expense for financing leases. The balance sheet amount recorded for existing leases at the date of adoption of ASU 2016-02 must be calculated using the applicable incremental borrowing rate at the date of adoption. In addition, ASU 2016-02 requires the use of the modified retrospective method, which will require adjustment to all comparative periods presented in the consolidated financial statements. This guidance is effective for annual and interim periods beginning after December 15, 2018 and requires retrospective application. The Company is currently assessing the impact that adopting ASU 2016-02 will have on its consolidated financial statements and related disclosures. In November 2016, the FASB issued ASU 2016-18, Statement of Cash Flows - Restricted Cash (Topic 230) . This new standard requires companies to include amounts generally described as restricted cash and restricted cash equivalents in cash and cash equivalents when reconciling beginning-of-period and end-of-period total amounts shown on the statement of cash flows. This guidance is effective for annual and interim reporting periods beginning after December 15, 2017, and requires retrospective application. The Company is currently assessing the impact that adopting ASU 2016-18 will have on its consolidated financial statements and related disclosures. In March 2017, the FASB issued ASU 2017-08, Receivables - Nonrefundable Fees and Other Costs (Subtopic 310-20): Premium Amortization on Purchased Callable Debt Securities. This new standard shortens the amortization period for certain callable debt securities held at a premium. Specifically, the amendment requires the premium to be amortized to the earliest call date. The amendment does not require an accounting change for securities held at a discount; the discount continues to be amortized to maturity. The guidance is effective for annual and interim periods beginning after December 15, 2018, and early adoption is permitted. The amendment should be applied on a modified retrospective basis, with the cumulative-effect adjustment directly to retained earnings as of the beginning of the period of adoption. The Company is currently assessing the impact that adopting ASU 2017-08 will have on its consolidated financial statements and related disclosures.

Warrants

Warrants	9 Months Ended
	Sep. 30, 2017
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 September 30, 2017 December 31, 2016 Price Per Share Expiration September 30, 2017 December 31, 2016 Warrants to purchase common stock 61 61 $ 5.88 June 10, 2020 - July 9, 2020 Liability Liability Warrants to purchase common stock 3 4 7.40 December 31, 2017 Equity(1) (2) Equity(1) (2) All warrants 64 65 $ 5.94 (1) In March 2017, warrant holders exercised warrants to purchase 1,187 shares of Common Stock on a net basis, resulting in the issuance of 1,014 shares of Common Stock. (2) Warrants to purchase common stock were issued in connection with various debt financing transactions that were consummated in periods prior to our initial public offering. 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 loss for each reporting period thereafter. The fair value of the common stock warrants was recorded as a discount to the preferred stock issued, and the preferred stock was accreted to the redemption value. At the end of each reporting period, the Company re-measured the fair value of the outstanding warrants, using current assumptions, resulting in an increase in fair value of $0.4 million and $0.8 million for the three months ended September 30, 2017 and 2016 , respectively, and an increase in fair value of $0.7 million and a decrease of $5.0 million for the nine months ended September 30, 2017 and 2016 , respectively, which was recorded in other (expense) income in the accompanying consolidated statements of operations and comprehensive loss. 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. All remaining outstanding warrants were fully vested and exercisable as of September 30, 2017 and December 31, 2016 .

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2017
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 September 30, 2017 , and, to the best of its knowledge, no material legal proceedings are currently pending or threatened. 99 Erie Street Operating Lease On March 16, 2017, the Company entered into an approximately six and a half year lease for approximately 11,825 square feet of rentable office and lab space located at 99 Erie Street, Cambridge, Massachusetts. The lease commenced on May 1, 2017 and ends on September 30, 2023. Excluding operating costs and real estate taxes, rent for the first year is $0.7 million with annual rent escalations thereafter. The total operating lease obligation for the term of this agreement, excluding operating costs and real estate taxes, is $5.1 million . The Company has the option to extend the lease by an additional three years. In accordance with the lease, the Company entered into a cash-collateralized, irrevocable standby letter of credit in the amount of $0.2 million , naming the landlord as beneficiary. On July 27, 2017, the Company entered into a one -year sublease with Rubius Therapeutics, Inc. (the "sublessee") under which Rubius will sublease approximately 11,825 square feet of office and lab space at 99 Erie Street, Cambridge, Massachusetts (the "sublease"). The sublease commenced on August 1, 2017 and ends on July 31, 2018. The sublessee has the right to extend the sublease by an additional five months. The total operating lease obligation, excluding operating costs and real estate taxes, for the term of this agreement is approximately $0.7 million . In connection with this sublease, we recognized a loss of $0.1 million during the third quarter of 2017, which represents the difference between the Company's lease obligation to the landlord and the sublessee's lease obligation to the Company and other expenses associated with the sublease. 128/149 Sidney Street Operating Leases On July 18, 2017, the Company entered into a five -year lease extension for approximately 37,700 square feet of office, manufacturing and lab space located at 128 Sidney Street, Cambridge, Massachusetts, our principal headquarters and manufacturing facility. The lease commences on October 1, 2018 and ends on September 30, 2023. The total operating lease obligation for the term of this agreement, excluding operating costs and real estate taxes, is approximately $13.0 million . The Company has the option to extend the lease by an additional five years. On July 18, 2017, the Company entered into a five -year lease extension for approximately 37,116 square feet of office and lab space located at 149 Sidney Street, Cambridge, Massachusetts. The lease commences on October 1, 2018 and ends on September 30, 2023. The total operating lease obligation for the term of this agreement, excluding operating costs and real estate taxes, is approximately $12.8 million . The Company has the option to extend the lease by an additional five years. Other The Company is also party to various agreements, principally relating to licensed technology, that require future payments relating to milestones which had not been met at September 30, 2017 and December 31, 2016 , 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 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, 2017
Significant Agreements [Abstract]
Significant Agreements	Significant Agreements Celgene Overview On February 20, 2008, the Company entered into a collaboration, license, and option agreement with Celgene Corporation (Celgene) relating to sotatercept, which the Company and Celgene amended on August 2, 2011 (as amended, the Original Sotatercept Agreement), and then amended and restated in its entirety on September 18, 2017 (as amended and restated, the Restated Sotatercept Agreement). On August 2, 2011, the Company entered into a second collaboration, license and option agreement with Celgene for luspatercept (the Luspatercept Agreement). These agreements provide Celgene an exclusive license to luspatercept in all indications, an exclusive license to sotatercept outside of the pulmonary hypertension (PH) field, as well as exclusive rights to obtain a license to certain future compounds. Since December 31, 2016 , there have been no material changes to the key terms of the Luspatercept Agreement, and the material changes to the Original Sotatercept Agreement pursuant to the Restated Sotatercept Agreement are described below. For further information on the terms of the agreements as well as the historical accounting analysis, please see the notes to the consolidated financial statements included in the Company’s Form 10-K for the year ended December 31, 2016 . Restated Sotatercept Agreement Under the Restated Sotatercept Agreement, Celgene granted the Company worldwide rights to develop and commercialize sotatercept for the treatment, prevention, modulation and diagnosis of pulmonary hypertension in humans. In addition, Celgene agreed not to develop or commercialize in the field of pulmonary hypertension 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 of pulmonary hypertension. The Company has the right to license, transfer or sell its rights to develop and commercialize sotatercept in pulmonary hypertension, subject to Celgene’s right of first negotiation. The Company is responsible for 100% of the costs related to the Company’s development and commercialization of sotatercept in pulmonary hypertension. The Company is not required to make any upfront payments or milestone payments to Celgene in connection with the Company’s development and commercialization of sotatercept in pulmonary hypertension. If sotatercept is commercialized to treat pulmonary hypertension and the Company recognizes such revenue, then Celgene will be eligible to receive a royalty in the low 20% range on global net sales. In certain circumstances Celgene may recognize revenue related to the commercialization of sotatercept in pulmonary hypertension, and in this scenario, the Company will be eligible to receive a royalty from Celgene such that the economic position of the parties is equivalent to the scenario in which the Company recognizes such revenue. With respect to the development and commercialization of sotatercept outside of pulmonary hypertension or the development and commercialization of any other compound under the Restated Sotatercept Agreement, the terms of the Original Sotatercept Agreement remain unchanged, and Celgene will continue to have responsibility to fund and conduct all such development and commercialization activities, with the Company eligible to receive tiered royalty payments in the low-to-mid 20% range on global net sales. Pursuant to the Restated Sotatercept Agreement, Celgene will provide to the Company certain quantities of Celgene’s existing clinical supply of sotatercept at no cost to the Company. For clinical or commercial supply of sotatercept in excess of that which is agreed under the Restated Sotatercept Agreement, Celgene can elect to provide the Company with such clinical and commercial supply of sotatercept at a negotiated price or provide a tech transfer to the Company to enable the Company to manufacture on its own behalf. The conduct of the collaboration is managed by a Joint Development Committee and Joint Commercialization Committee. In the event of a deadlock of a committee, the Company shall determine the resolution of issues specifically related to the development or commercialization of sotatercept in pulmonary hypertension (other than pricing which shall be determined by consensus), and Celgene shall determine the resolution of all other issues. The Joint Commercialization Committee will oversee commercialization of sotatercept and sotatercept pricing will be determined by mutual agreement of the Company and Celgene in the Joint Commercialization Committee. The Restated Sotatercept Agreement is terminable by either party upon a breach that is uncured and continuing or by Celgene for convenience on a country by country or product by product basis, or in its entirety. Celgene may also terminate the Restated Sotatercept Agreement, in its entirety or on a product by product basis, for failure of a product to meet a development or clinical trial endpoint. Termination for cause by us or termination by Celgene for convenience or failure to meet an endpoint will have the effect of terminating the applicable license to Celgene and the rights granted to the Company with respect to the development of sotatercept in pulmonary hypertension shall become irrevocable. Termination for cause by either party shall result in reducing the remaining royalties due to the breaching party by a certain percentage. Upon termination by Celgene for convenience or for failure to meet an endpoint, Celgene and the Company will enter into a termination agreement pursuant to which, among other things, Celgene will continue to be eligible to receive a royalty in the low 20% range on global net sales of sotatercept in pulmonary hypertension. The Company and Celgene will continue to collaborate worldwide for the joint development and commercialization of sotatercept outside of the pulmonary hypertension field. The Company also previously granted Celgene an option to license three discovery stage compounds under the Original Sotatercept Agreement. The Company retained responsibility for research and development of sotatercept outside of the pulmonary hypertension field 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. Outside of pulmonary hypertension, Celgene will also 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. Through September 30, 2017 , the Company has received $44.1 million in research and development funding and milestone payments for sotatercept under the original and amended and restated agreements. The next likely clinical milestone payment would be $10.0 million and result from Celgene’s start of a Phase 3 study. 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 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 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 any subsequent Phase 2 and Phase 3 clinical studies. The Company 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. Through September 30, 2017 , the Company has received $96.6 million in research and development funding and milestone payments for luspatercept. The next likely milestone payment would be $25.0 million and result from U.S. Food and Drug Administration or European Medical Association acceptance of a Biologics Licensing Application or equivalent for luspatercept in either myelodysplastic syndromes or beta-thalassemia. 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. 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 a result of the changes to the economic terms under the Restated Sotatercept Agreement, the Company concluded the modification did not represent a material modification under ASU 2009-13. Further, the Company concluded there are no new deliverables which arise from the agreement. There have been no material changes to the accounting under the Luspatercept Agreement, or the Original Sotatercept Agreement pursuant to the Restated Sotatercept Agreement. For further information on the historical accounting analysis, please see the notes to the consolidated financial statements included in the Company’s Form 10-K for the year ended December 31, 2016 . During the three months ended September 30, 2017 and 2016 , the Company recognized $0.1 million and $0.1 million , respectively, and during the nine months ended September 30, 2017 and 2016 , $0.4 million and $0.4 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. In November 2013, the Company agreed to conduct additional activities for the benefit of the luspatercept program including certain clinical and non-clinical services. These activities are reimbursed under the same terms and rates of the existing Agreements and are accounted for as the services are delivered. Pursuant to the terms of the agreements, Celgene and the Company shared development costs incurred by either party, with Celgene responsible for substantially more than half of the costs for sotatercept and luspatercept until December 31, 2012. Beginning January 1, 2013, Celgene is responsible for 100% of the costs for luspatercept and for sotatercept outside of the pulmonary hypertension field. Payments from Celgene with respect to research and development costs incurred by the Company are recorded as cost-sharing revenue. The Company recorded net cost-sharing revenue of $2.9 million and $2.9 million during the three months ended September 30, 2017 and 2016 , respectively, and $9.4 million and $9.0 million during the nine months ended September 30, 2017 and 2016 , 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 thousand , 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, 2017 and 2016 , the Company expensed zero and zero , respectively, and during the nine months ended September 30, 2017 and 2016 , respectively, the Company expensed $0.1 million and $1.0 million of milestones and fees defined under the agreement. 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 September 30, 2017 and 2016 , the Company expensed $0.9 million and $0.4 million , respectively, and during the nine months ended September 30, 2017 and 2016 , the Company expensed $1.2 million and $0.9 million , respectively, of milestones and fees, which is recorded as research and development expense.

Stockholders' Equity

Stockholders' Equity	9 Months Ended
	Sep. 30, 2017
Equity [Abstract]
Stockholders' Equity	Stockholders' Equity On September 25, 2017, the Company completed its underwritten public offering of 5,405,406 shares of common stock at a public offering price of $37.00 per share. The aggregate net proceeds received by the Company, after underwriting discounts and other estimated offering expenses, were approximately $187.6 million . On October 4, 2017, in connection with the public offering of common stock on September 20, 2017, the underwriters of the Company's public offering fully exercised their over-allotment option to purchase an additional 810,810 shares of the Company's common stock at the public offering price of $37.00 per share, less underwriting discounts and commissions, resulting in additional net proceeds of approximately $28.2 million .

Stock-Based Compensation

Stock-Based Compensation	9 Months Ended
	Sep. 30, 2017
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) totaling $5.7 million , $4.6 million , $21.9 million , and $13.4 million during the three months ended September 30, 2017 and 2016 and the nine months ended September 30, 2017 and 2016 , 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 2017 2016 2017 2016 Research and development $ 3,173 $ 1,942 $ 10,268 $ 5,617 General and administrative 2,533 2,646 11,609 7,770 $ 5,706 $ 4,588 $ 21,877 $ 13,387 On January 1, 2017, the Company adopted ASU 2016-09, which identifies areas for simplification involving several aspects of accounting for share-based payments, including income tax consequences, classification of awards as either equity or liabilities, an option to make a policy election to recognize gross share based compensation expense with actual forfeitures recognized as they occur as well as certain classification changes on the statement of cash flows. In connection with the adoption of this standard, the Company changed its accounting policy to record actual forfeitures as they occur, rather than estimating forfeitures by applying a forfeiture rate. The provisions of the standard related to the timing of when excess tax benefits are recognized, minimum statutory withholding requirements and forfeitures were adopted using a modified retrospective transition method. Accordingly, a cumulative adjustment of $0.1 million was booked to retained earnings for the impact of the forfeitures. The Company also recorded $21.5 million f or the excess tax benefit related to equity awards, which was offset by a $21.5 million increase to the valuation allowance. The provisions of the standard related to the recognition of the excess tax benefits in the income statement and classification in the statement of cash flows were adopted prospectively and prior periods were not retrospectively adjusted. In December 2016, the Company entered into a consulting agreement with its former Chief Executive Officer. In accordance with the 2003 Plan and 2013 Plan, any vested shares remain exercisable and any outstanding and unvested options and restricted stock units will continue to vest in accordance with their terms so long as he continues to provides services as a non-employee consultant. During the three and nine months ended September 30, 2017 , the Company recognized $1.0 million and $3.1 million of stock-based compensation expense within research and development expense associated with these awards. In April 2017, the Company amended the employment agreement with its former Chief Operating Officer as a result of his diagnosis of amyotrophic lateral sclerosis (ALS). The amended agreement modified the vesting conditions of his stock options and restricted stock units in the event his termination of employment as a result of death or disability and extended the post termination exercise period of the options. These modifications resulted in zero and $3.6 million of stock-based compensation, recognized within general and administrative expense during the three and nine months ended September 30, 2017 , respectively. Stock Options 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 2017 2016 2017 2016 Expected volatility 65.1 % 65.4 % 65.8 % 64.5 % Expected term (in years) 6.0 6.0 6.0 5.9 Risk-free interest rate 1.9 % 1.2 % 2.1 % 1.4 % Expected dividend yield — % — % — % — % The following table summarizes the stock option activity under the Company’s 2003 Plan and 2013 Plan during the nine months ended September 30, 2017 (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, 2016 3,316 $ 25.96 7.03 Granted 669 $ 30.51 Exercised (442 ) $ 7.32 Canceled or forfeited (84 ) $ 34.16 Outstanding at September 30, 2017 3,459 $ 29.02 6.99 $ 32,169 Exercisable at September 30, 2017 1,907 $ 25.88 5.55 $ 24,324 (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, 2017 . During the nine months ended September 30, 2017 , the Company granted stock options to purchase an aggregate of 669,184 shares of its common stock, with a weighted-average grant date fair value of options granted of $18.44 per share. During the nine months ended September 30, 2017 , current and former employees of the Company exercised a total of 441,842 options, resulting in total proceeds of $3.2 million . The aggregate intrinsic value of options exercised during the nine months ended September 30, 2017 was $9.5 million . As of September 30, 2017 , there was $25.4 million of unrecognized compensation expense related to unvested stock options that is expected to be recognized over a weighted-average period of 2.57 years. Restricted Stock Units The following table summarizes the restricted stock unit (RSU) activity under the 2013 Plan during the nine months ended September 30, 2017 (in thousands, except per share amounts): Number Weighted- Unvested balance at December 31, 2016 732 $ 31.55 Granted 129 $ 29.53 Vested (289 ) $ 31.00 Forfeited (21 ) $ 32.27 Unvested balance at September 30, 2017 551 $ 31.34 During the nine months ended September 30, 2017 , the Company issued 100,692 RSUs to employees which are subject to time-based vesting. As of September 30, 2017 , 214,120 of these restricted stock units remained unvested and outstanding, and there was approximately $4.7 million of related unrecognized compensation cost, which the Company expects to recognize over a remaining weighted-average period of 1.63 years . During the nine months ended September 30, 2017 , the Company issued 28,333 RSUs to employees which are subject to performance-based vesting conditions, and vesting accelerates upon the occurrence of certain milestone events. In September 2019, any of these unvested RSUs will vest. As a result, when 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. As of September 30, 2017 , 337,122 of these performance-based RSUs remained outstanding, and there was approximately $5.6 million of related unrecognized compensation cost, which the Company expects to recognize over a remaining weighted-average period of 1.66 years . Employee Stock Purchase Plan The Company recorded stock-based compensation expense related to the ESPP Plan during the three months ended September 30, 2017 and 2016 of $0.1 million and $0.1 million , respectively, and during the nine months ended September 30, 2017 and 2016 of $0.2 million and $0.2 million , respectively.

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2017
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes For the three months and nine months ended September 30, 2017 , the Company recognized an income tax benefit of $41 thousand and $29 thousand and tax expense in other comprehensive loss of $59 thousand and $59 thousand related to the unrealized gain on available-for-sale securities. At September 30, 2017 , the Company recorded an accrued tax provision of $17 thousand related to this tax benefit included within accrued expenses and other current liabilities in the condensed consolidated balance sheet, which is expected to be generated from continuing operations. For the three and nine months ended September 30, 2016 , the Company recognized an income tax benefit of $20 thousand and $20 thousand and tax expense in other comprehensive loss of $53 thousand and $53 thousand related to the unrealized gain on available-for-sale securities. At September 30, 2016 , the Company recorded an accrued tax provision of $17 thousand related to this tax benefit included within accrued expenses and other current liabilities in the condensed consolidated balance sheet, which is expected to be generated from continuing operations. 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, 2017 and December 31, 2016 . The Company files income tax returns in the United States, and various state jurisdictions. The federal and state income tax returns are generally subject to tax examinations for the tax years ended December 31, 2013 through December 31, 2016 . 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 or state taxing authorities to the extent utilized in a future period. 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, 2017 and December 31, 2016 , the Company did not have any significant uncertain tax positions.

Related Party Transactions

Related Party Transactions	9 Months Ended
	Sep. 30, 2017
Related Party Transactions [Abstract]
Related Party Transactions	Related Party Transactions Celgene Corporation In connection with the Company's September 2017 public offering, Celgene purchased 745,592 shares of common stock. In connection with this and prior transactions, Celgene owned 12.7% and 12.9% of the Company’s fully diluted equity as of September 30, 2017 and December 31, 2016 , respectively. Refer to Note 14 for additional information regarding this collaboration arrangement. During the three and nine months ended September 30, 2017 and 2016 , all revenue recognized by the Company was recognized under the Celgene collaboration arrangement and, as of September 30, 2017 , the Company had $3.8 million of deferred revenue related to the Celgene collaboration arrangement.

Basis of Presentation (Policies

Basis of Presentation (Policies)	9 Months Ended
	Sep. 30, 2017
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, 2016 except for the adoption of Accounting Standards Update (ASU) No. 2016-09, Compensation - Stock Compensation (Topic 718), Improvements to Employee Share-Based Payment Accounting, which did not have a material impact , 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, 2017 , the results of its operations for the three and nine months ended September 30, 2017 and 2016 , and its cash flows for the nine months ended September 30, 2017 and 2016 . The results for the three and nine months ended September 30, 2017 are not necessarily indicative of the results to be expected for the year ending December 31, 2017 , 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, 2016 , 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, 2016 . 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 for certain estimates or to identify matters that require additional disclosure. Subsequent events have been evaluated as required. The Company has evaluated all subsequent events and determined that there are no material recognized or unrecognized events requiring disclosure, other than those disclosed in this Report on Form 10-Q. 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, 2017 , 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, 2016 , have not changed, except for the adoption of Accounting Standards Update (ASU) No. 2016-09, Compensation - Stock Compensation (Topic 718), Improvements to Employee Share-Based Payment Accounting, which is discussed further in Note 16.
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, 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.
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. In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606) , which supersedes all existing revenue recognition requirements, including most industry-specific guidance. The new standard requires a company to recognize revenue when it transfers goods or services to customers in an amount that reflects the consideration that the company expects to receive for those goods or services. The new standard will be effective for the Company on January 1, 2018. Topic 606 allows for either a full retrospective application, in which the standard is applied to all periods presented, or a modified retrospective application, in which the standard is applied to the most current period presented in the financial statements. As of September 30, 2017 , revenue is generated exclusively from the Company's collaboration agreement with Celgene. The Company is currently evaluating the potential impact that Topic 606 may have on its financial position and results of operations as it relates to this single arrangement, and expects to elect the modified retrospective application as its transition method. In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842), Amendments to the FASB Accounting Standards Codification, which replaces the existing guidance for leases. ASU 2016-02 requires the identification of arrangements that should be accounted for as leases by lessees. In general, for lease arrangements exceeding a twelve month term, these arrangements must now be recognized as assets and liabilities on the balance sheet of the lessee. Under ASU 2016-02, a right-of-use asset and lease obligation will be recorded for all leases, whether operating or financing, while the income statement will reflect lease expense for operating leases and amortization/interest expense for financing leases. The balance sheet amount recorded for existing leases at the date of adoption of ASU 2016-02 must be calculated using the applicable incremental borrowing rate at the date of adoption. In addition, ASU 2016-02 requires the use of the modified retrospective method, which will require adjustment to all comparative periods presented in the consolidated financial statements. This guidance is effective for annual and interim periods beginning after December 15, 2018 and requires retrospective application. The Company is currently assessing the impact that adopting ASU 2016-02 will have on its consolidated financial statements and related disclosures. In November 2016, the FASB issued ASU 2016-18, Statement of Cash Flows - Restricted Cash (Topic 230) . This new standard requires companies to include amounts generally described as restricted cash and restricted cash equivalents in cash and cash equivalents when reconciling beginning-of-period and end-of-period total amounts shown on the statement of cash flows. This guidance is effective for annual and interim reporting periods beginning after December 15, 2017, and requires retrospective application. The Company is currently assessing the impact that adopting ASU 2016-18 will have on its consolidated financial statements and related disclosures. In March 2017, the FASB issued ASU 2017-08, Receivables - Nonrefundable Fees and Other Costs (Subtopic 310-20): Premium Amortization on Purchased Callable Debt Securities. This new standard shortens the amortization period for certain callable debt securities held at a premium. Specifically, the amendment requires the premium to be amortized to the earliest call date. The amendment does not require an accounting change for securities held at a discount; the discount continues to be amortized to maturity. The guidance is effective for annual and interim periods beginning after December 15, 2018, and early adoption is permitted. The amendment should be applied on a modified retrospective basis, with the cumulative-effect adjustment directly to retained earnings as of the beginning of the period of adoption. The Company is currently assessing the impact that adopting ASU 2017-08 will have on its consolidated financial statements and related disclosures.

Cash Equivalents and Short-te26

Cash Equivalents and Short-term and Long-term Investments (Tables)	9 Months Ended
	Sep. 30, 2017
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 September 30, 2017 and December 31, 2016 (in thousands): September 30, 2017 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Cash and cash equivalents due in 90 days or less $ 238,959 $ — $ — $ 238,959 Available-for-sale securities: Corporate obligations due in one year or less 44,789 1 (47 ) 44,743 Corporate obligations due in more than one year 11,560 — (47 ) 11,513 U.S. Treasury securities due in one year or less 10,004 — (31 ) 9,973 U.S. Treasury securities due in more than one year 4,021 — (27 ) 3,994 Certificates of deposit due in one year or less 9,092 — — 9,092 Certificates of deposit due in more than one year 1,904 — — 1,904 Mortgage and other asset backed securities due in one year or less 34,497 — (39 ) 34,458 Mortgage and other asset backed securities due in more than one year 12,026 — (65 ) 11,961 Total available-for-sale securities $ 127,893 $ 1 $ (256 ) $ 127,638 Total cash, cash equivalents and available-for-sale securities $ 366,852 $ 1 $ (256 ) $ 366,597 December 31, 2016 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Cash and cash equivalents due in 90 days or less $ 20,950 $ — $ — $ 20,950 Available-for-sale securities: Corporate obligations due in one year or less 45,839 1 (58 ) 45,782 Corporate obligations due in more than one year 42,895 — (185 ) 42,710 U.S. Treasury securities due in one year or less 22,490 — (10 ) 22,480 U.S. Treasury securities due in more than one year 11,541 — (53 ) 11,488 Certificates of deposit due in one year or less 13,562 — — 13,562 Certificates of deposit due in more than one year 9,811 — — 9,811 Mortgage and other asset backed securities due in one year or less 36,948 — (32 ) 36,916 Mortgage and other asset backed securities due in more than one year 30,771 — (88 ) 30,683 Total available-for-sale securities $ 213,857 $ 1 $ (426 ) $ 213,432 Total cash, cash equivalents and available-for-sale securities $ 234,807 $ 1 $ (426 ) $ 234,382

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2017
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, 2017 and December 31, 2016 (in thousands): September 30, 2017 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 $ 224,053 $ — $ — $ 224,053 Corporate obligations — 56,256 — 56,256 U.S. Treasury securities — 13,967 — 13,967 Certificates of deposit — 10,997 — 10,997 Mortgage and other asset backed securities — 46,419 — 46,419 Restricted cash 1,132 — — 1,132 Total assets $ 225,185 $ 127,639 $ — $ 352,824 Liabilities: Warrants to purchase common stock $ — $ — $ 1,927 $ 1,927 Total liabilities $ — $ — $ 1,927 1,927 December 31, 2016 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 $ 19,818 $ — $ — $ 19,818 Corporate obligations — 88,492 — 88,492 U.S. Treasury securities — 33,968 — 33,968 Certificates of deposit — 23,373 — 23,373 Mortgage and other asset backed securities — 67,599 — 67,599 Restricted cash 946 — — 946 Total assets $ 20,764 $ 213,432 $ — $ 234,196 Liabilities: Warrants to purchase common stock $ — $ — $ 1,244 $ 1,244 Total liabilities $ — $ — $ 1,244 $ 1,244
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): Nine Months Ended September 30, 2017 2016 Beginning balance $ 1,244 $ 17,187 Change in fair value 683 (5,026 ) Ending balance $ 1,927 $ 12,161

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	9 Months Ended
	Sep. 30, 2017
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 Nine Months Ended 2017 2016 2017 2016 Outstanding stock options 3,459 3,193 3,459 3,193 Common stock warrants 64 397 64 397 Shares issuable under employee stock purchase plan 19 18 19 18 Outstanding restricted stock units 551 670 551 670 4,093 4,278 4,093 4,278

Warrants (Tables)

Warrants (Tables)	9 Months Ended
	Sep. 30, 2017
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 September 30, 2017 December 31, 2016 Price Per Share Expiration September 30, 2017 December 31, 2016 Warrants to purchase common stock 61 61 $ 5.88 June 10, 2020 - July 9, 2020 Liability Liability Warrants to purchase common stock 3 4 7.40 December 31, 2017 Equity(1) (2) Equity(1) (2) All warrants 64 65 $ 5.94 (1) In March 2017, warrant holders exercised warrants to purchase 1,187 shares of Common Stock on a net basis, resulting in the issuance of 1,014 shares of Common Stock. (2) Warrants to purchase common stock were issued in connection with various debt financing transactions that were consummated in periods prior to our initial public offering.

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Schedule of compensation costs recognized	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 2017 2016 2017 2016 Research and development $ 3,173 $ 1,942 $ 10,268 $ 5,617 General and administrative 2,533 2,646 11,609 7,770 $ 5,706 $ 4,588 $ 21,877 $ 13,387
Schedule of weighted-average assumptions used for estimating fair value	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 2017 2016 2017 2016 Expected volatility 65.1 % 65.4 % 65.8 % 64.5 % Expected term (in years) 6.0 6.0 6.0 5.9 Risk-free interest rate 1.9 % 1.2 % 2.1 % 1.4 % Expected dividend yield — % — % — % — %
Summary of stock option activity	The following table summarizes the stock option activity under the Company’s 2003 Plan and 2013 Plan during the nine months ended September 30, 2017 (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, 2016 3,316 $ 25.96 7.03 Granted 669 $ 30.51 Exercised (442 ) $ 7.32 Canceled or forfeited (84 ) $ 34.16 Outstanding at September 30, 2017 3,459 $ 29.02 6.99 $ 32,169 Exercisable at September 30, 2017 1,907 $ 25.88 5.55 $ 24,324 (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, 2017 .
Schedule of nonvested restricted stock units activity	The following table summarizes the restricted stock unit (RSU) activity under the 2013 Plan during the nine months ended September 30, 2017 (in thousands, except per share amounts): Number Weighted- Unvested balance at December 31, 2016 732 $ 31.55 Granted 129 $ 29.53 Vested (289 ) $ 31.00 Forfeited (21 ) $ 32.27 Unvested balance at September 30, 2017 551 $ 31.34

Segment Information (Details)

Segment Information (Details)	9 Months Ended
	Sep. 30, 2017segment
Segment Information
Number of operating segments	1

Cash Equivalents and Short-te32

Cash Equivalents and Short-term and Long-term Investments - Narrative (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Cash and Cash Equivalents [Abstract]
Unrealized loss position, less than twelve months, fair value	$ 53,100	$ 172,200
Continuous unrealized loss position, twelve months or longer, fair value	55,800	5,500
Continuous unrealized loss position, fair value	$ 100	$ 2

Cash Equivalents and Short-te33

Cash Equivalents and Short-term and Long-term Investments Summary of Cash, Cash Equivalents and Available-for-sale Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents due in 90 days or less	$ 238,959	$ 20,950	$ 31,016	$ 27,783
Available-for-sale securities:
Amortized Cost	127,893	213,857
Gross Unrealized Gains	1	1
Gross Unrealized Losses	(256)	(426)
Estimated Fair Value	127,638	213,432
Total cash, cash equivalents and available-for-sale securities
Amortized Cost	366,852	234,807
Gross Unrealized Gains	1	1
Gross Unrealized Losses	(256)	(426)
Estimated Fair Value	366,597	234,382
Corporate obligations due in one year or less
Available-for-sale securities:
Amortized Cost	44,789	45,839
Gross Unrealized Gains	1	1
Gross Unrealized Losses	(47)	(58)
Estimated Fair Value	44,743	45,782
Corporate obligations due in more than one year
Available-for-sale securities:
Amortized Cost	11,560	42,895
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(47)	(185)
Estimated Fair Value	11,513	42,710
U.S. Treasury securities due in one year or less
Available-for-sale securities:
Amortized Cost	10,004	22,490
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(31)	(10)
Estimated Fair Value	9,973	22,480
U.S. Treasury securities due in more than one year
Available-for-sale securities:
Amortized Cost	4,021	11,541
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(27)	(53)
Estimated Fair Value	3,994	11,488
Certificates of deposit due in one year or less
Available-for-sale securities:
Amortized Cost	9,092	13,562
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	9,092	13,562
Certificates of deposit due in more than one year
Available-for-sale securities:
Amortized Cost	1,904	9,811
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	1,904	9,811
Mortgage and other asset backed securities due in one year or less
Available-for-sale securities:
Amortized Cost	34,497	36,948
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(39)	(32)
Estimated Fair Value	34,458	36,916
Mortgage and other asset backed securities due in more than one year
Available-for-sale securities:
Amortized Cost	12,026	30,771
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(65)	(88)
Estimated Fair Value	$ 11,961	$ 30,683

Restricted Cash (Details)

Restricted Cash (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Certificates of Deposits and Money Market Funds | Letter of Credit
Accounts, Notes, Loans and Financing Receivable [Line Items]
Letters of credit held in the form of a money market account as collateral for lease obligations and credit cards	$ 1.1	$ 0.9

Fair Value Measurements Financi

Fair Value Measurements Financial Instrument at Fair Value Schedule (Details) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Liabilities:
Fair value measurement transfers between levels	$ 0	$ 0
Recurring
Assets:
Total assets	352,824,000	234,196,000
Liabilities:
Total liabilities	1,927,000	1,244,000
Recurring | Common stock warrants
Liabilities:
Total liabilities	1,927,000	1,244,000
Recurring | Money market funds
Assets:
Total assets	224,053,000	19,818,000
Recurring | Corporate obligations
Assets:
Total assets	56,256,000	88,492,000
Recurring | U.S. Treasury securities
Assets:
Total assets	13,967,000	33,968,000
Recurring | Certificates of deposit
Assets:
Total assets	10,997,000	23,373,000
Recurring | Mortgage and other asset backed securities
Assets:
Total assets	46,419,000	67,599,000
Recurring | Restricted cash
Assets:
Total assets	1,132,000	946,000
Recurring | Quoted Prices in Active Markets for Identical Items (Level 1)
Assets:
Total assets	225,185,000	20,764,000
Liabilities:
Total liabilities	0	0
Recurring | Quoted Prices in Active Markets for Identical Items (Level 1) | Common stock warrants
Liabilities:
Total liabilities	0	0
Recurring | Quoted Prices in Active Markets for Identical Items (Level 1) | Money market funds
Assets:
Total assets	224,053,000	19,818,000
Recurring | Quoted Prices in Active Markets for Identical Items (Level 1) | Corporate obligations
Assets:
Total assets	0	0
Recurring | Quoted Prices in Active Markets for Identical Items (Level 1) | U.S. Treasury securities
Assets:
Total assets	0	0
Recurring | Quoted Prices in Active Markets for Identical Items (Level 1) | Certificates of deposit
Assets:
Total assets	0	0
Recurring | Quoted Prices in Active Markets for Identical Items (Level 1) | Mortgage and other asset backed securities
Assets:
Total assets	0	0
Recurring | Quoted Prices in Active Markets for Identical Items (Level 1) | Restricted cash
Assets:
Total assets	1,132,000	946,000
Recurring | Significant Other Observable Inputs (Level 2)
Assets:
Total assets	127,639,000	213,432,000
Liabilities:
Total liabilities	0	0
Recurring | Significant Other Observable Inputs (Level 2) | Common stock warrants
Liabilities:
Total liabilities	0	0
Recurring | Significant Other Observable Inputs (Level 2) | Money market funds
Assets:
Total assets	0	0
Recurring | Significant Other Observable Inputs (Level 2) | Corporate obligations
Assets:
Total assets	56,256,000	88,492,000
Recurring | Significant Other Observable Inputs (Level 2) | U.S. Treasury securities
Assets:
Total assets	13,967,000	33,968,000
Recurring | Significant Other Observable Inputs (Level 2) | Certificates of deposit
Assets:
Total assets	10,997,000	23,373,000
Recurring | Significant Other Observable Inputs (Level 2) | Mortgage and other asset backed securities
Assets:
Total assets	46,419,000	67,599,000
Recurring | Significant Other Observable Inputs (Level 2) | Restricted cash
Assets:
Total assets	0	0
Recurring | Significant Unobservable Inputs (Level 3)
Assets:
Total assets	0	0
Liabilities:
Total liabilities	1,927,000	1,244,000
Recurring | Significant Unobservable Inputs (Level 3) | Common stock warrants
Liabilities:
Total liabilities	1,927,000	1,244,000
Recurring | Significant Unobservable Inputs (Level 3) | Money market funds
Assets:
Total assets	0	0
Recurring | Significant Unobservable Inputs (Level 3) | Corporate obligations
Assets:
Total assets	0	0
Recurring | Significant Unobservable Inputs (Level 3) | U.S. Treasury securities
Assets:
Total assets	0	0
Recurring | Significant Unobservable Inputs (Level 3) | Certificates of deposit
Assets:
Total assets	0	0
Recurring | Significant Unobservable Inputs (Level 3) | Mortgage and other asset backed securities
Assets:
Total assets	0	0
Recurring | Significant Unobservable Inputs (Level 3) | Restricted cash
Assets:
Total assets	$ 0	$ 0

Fair Value Measurements Preferr

Fair Value Measurements Preferred and Common Stock Warrant Liability Rollforward (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
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			$ 1,244	$ 17,187
Change in fair value	$ 400	$ 800	683	(5,026)
Ending balance	$ 1,927	$ 12,161	$ 1,927	$ 12,161

Net Loss Per Share (Details)

Net Loss Per Share (Details) - shares shares in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Net Income (Loss) Per Share
Anti-dilutive common stock equivalents excluded from the calculation of diluted net loss per share (in shares)	4,093	4,278	4,093	4,278
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	3,193	3,459	3,193
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)	64	397	64	397
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)	19	18	19	18
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)	551	670	551	670

Warrants (Details)

Warrants (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended	2 Months Ended	3 Months Ended		9 Months Ended
		Mar. 31, 2017	Jul. 31, 2010	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Warrants
Number of shares issuable upon exercise of outstanding warrants (in shares)				64,000		64,000		65,000
Weighted-average exercise price per share (in dollars per share)				$ 5.94		$ 5.94
Class of warrant or right number of warrants exercised on net basis (in shares)		1,187
Stock issued during period shares warrants exercise on net basis (in shares)		1,014
Increase (decrease) in fair value				$ 400	$ 800	$ 683	$ (5,026)
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)				61,000		61,000		61,000
Weighted-average exercise price per share (in dollars per share)				$ 5.88		$ 5.88
Warrants to purchase Common stock expiring between June 10, 2020 to July 9, 2020 | Warrants issued in connection with financing transactions
Warrants
Number of shares issuable upon exercise of outstanding warrants (in shares)			871,580
Expiration period			10 years
Warrants to purchase common stock having expiration on December 31, 2017
Warrants
Number of shares issuable upon exercise of outstanding warrants (in shares)	[1],[2]			3,000		3,000		4,000
Weighted-average exercise price per share (in dollars per share)	[1],[2]			$ 7.40		$ 7.40
[1]	In March 2017, warrant holders exercised warrants to purchase 1,187 shares of Common Stock on a net basis, resulting in the issuance of 1,014 shares of Common Stock.
[2]	Warrants to purchase common stock were issued in connection with various debt financing transactions that were consummated in periods prior to our initial public offering.

Commitments and Contingencies (

Commitments and Contingencies (Details) $ in Millions	Jul. 27, 2017USD ($)ft²	Jul. 18, 2017USD ($)ft²	Mar. 16, 2017USD ($)ft²
99 Erie Street, Cambridge, Massachusetts
Operating Leased Assets [Line Items]
Operating lease, term of contract			6 years 6 months
Operating leases, area (in sqft) | ft²			11,825
Operating leases, future minimum payments, remainder of fiscal year			$ 0.7
Total operating lease obligation			$ 5.1
Operating lease, extension term			3 years
Cash-collateralized irrevocable standby letter of credit			$ 0.2
128 Sidney Street, Cambridge Massachusetts
Operating Leased Assets [Line Items]
Operating lease, term of contract		5 years
Operating leases, area (in sqft) | ft²		37,700
Total operating lease obligation		$ 13
Operating lease, extension term		5 years
149 Sidney Street, Cambridge Massachusetts
Operating Leased Assets [Line Items]
Operating lease, term of contract		5 years
Operating leases, area (in sqft) | ft²		37,116
Total operating lease obligation		$ 12.8
Operating lease, extension term		5 years
Rubius Therapeutics, Inc. | 99 Erie Street, Cambridge, Massachusetts
Operating Leased Assets [Line Items]
Operating leases, area (in sqft) | ft²	11,825
Lessor, operating lease, term of contract	1 year
Lessor, operating lease, renewal term	5 months
Operating lease, payments to be received	$ 0.7
Operating lease, lease loss	$ 0.1

Significant Agreements - Celgen

Significant Agreements - Celgene (Details) $ in Thousands	Feb. 20, 2008discovery_stage_compound	Sep. 30, 2017USD ($)	Sep. 30, 2016USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2016USD ($)
Significant Agreements
Cost-sharing, net		$ 2,879	$ 2,870	$ 9,370	$ 8,987
Collaboration, License, and Option Agreement | Investor
Significant Agreements
Deferred revenue recognized		100	100	400	400
Cost-sharing, net		$ 2,900	$ 2,900	$ 9,400	$ 9,000
Collaboration, License, and Option Agreement | Investor | Until December 31, 2012
Significant Agreements
Percent of total costs to be reimbursed		50.00%		50.00%
Collaboration, License, and Option Agreement | Investor | January 1, 2013 And Thereafter
Significant Agreements
Percent of total costs to be reimbursed		100.00%		100.00%
Collaboration, License, and Option Agreement | Sotarcept | Investor
Significant Agreements
Number of license options granted | discovery_stage_compound	3
Payments received				$ 44,100
Collaboration, License, and Option Agreement | Sotarcept | Investor | Clinical milestones
Significant Agreements
Milestone payment receivable on commencement of Phase 2b clinical trial		$ 10,000		10,000
Collaboration, License, and Option Agreement | Luspatercept | Investor
Significant Agreements
Payments received				96,600
Collaboration, License, and Option Agreement | Luspatercept | Investor | Clinical milestones
Significant Agreements
Milestone payment receivable on commencement of Phase 2b clinical trial		$ 25,000		$ 25,000
Collaboration, License, and Option Agreement | Sotarcept and Luspatercept | Investor
Significant Agreements
Percentage of development costs for which collaborator is responsible				100.00%
Collaboration, License, and Option Agreement | Sotarcept and Luspatercept | Investor | Weighted Average
Significant Agreements
Potential royalty rate				20.00%

Significant Agreements - Other

Significant Agreements - Other Agreements (Details) - USD ($)	1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	May 31, 2014	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2004
Significant Agreements
Milestones and fees		$ 21,059,000	$ 17,102,000	$ 64,387,000	$ 49,492,000
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 | 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 | 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	0	0	100,000	1,000,000
Non-collaborative Arrangement Transactions | License Agreement with Non Profit Institution
Significant Agreements
Milestones and fees		$ 900,000	$ 400,000	$ 1,200,000	$ 900,000

Stockholders' Equity (Details)

Stockholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands	Oct. 04, 2017	Sep. 25, 2017	Sep. 30, 2017	Sep. 30, 2016
Class of Stock [Line Items]
Shares issued (in dollars per share)		$ 37
Proceeds from issuance of common stock from public offering, net of issuance costs		$ 187,600	$ 187,986	$ 140,697
Common Stock
Class of Stock [Line Items]
Stock issued during period, new issues (in shares)		5,405,406
Subsequent Event
Class of Stock [Line Items]
Shares issued (in dollars per share)	$ 37
Proceeds from issuance of common stock from public offering, net of issuance costs	$ 28,200
Subsequent Event | Common Stock
Class of Stock [Line Items]
Stock issued during period, new issues (in shares)	810,810

Stock-Based Compensation - (Det

Stock-Based Compensation - (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award
Stock-based compensation expense	$ 5,706	$ 4,588	$ 21,877	$ 13,387
Share-based compensation, excess tax benefit, amount			21,500
2016 Consulting Agreement with Former CEO
Share-based Compensation Arrangement by Share-based Payment Award
Stock-based compensation expense	1,000		3,100
Employment Agreement with Former Chief Operating Officer
Share-based Compensation Arrangement by Share-based Payment Award
Incremental value of the modifications	0		3,600
Employee Stock Purchase Plan 2013
Share-based Compensation Arrangement by Share-based Payment Award
Stock-based compensation expense	100	$ 100	200	$ 200
Accounting Standards Update 2016-09
Share-based Compensation Arrangement by Share-based Payment Award
Valuation allowance increase			21,500
Accounting Standards Update 2016-09 | Retained Earnings
Share-based Compensation Arrangement by Share-based Payment Award
Cumulative effect of new accounting principle in period of adoption	$ 100		$ 100

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Stock-Based Compensation Costs Recognized (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost
Stock-based compensation expense	$ 5,706	$ 4,588	$ 21,877	$ 13,387
Research and development
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost
Stock-based compensation expense	3,173	1,942	10,268	5,617
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost
Stock-based compensation expense	$ 2,533	$ 2,646	$ 11,609	$ 7,770

Stock-Based Compensation - Sc45

Stock-Based Compensation - Schedule of Weighted-Average Assumptions Used for Estimating Fair Value (Details) - Stock Option	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award
Expected volatility	65.10%	65.40%	65.80%	64.50%
Expected term (in years)	6 years	6 years	6 years	5 years 10 months 24 days
Risk-free interest rate	1.90%	1.20%	2.10%	1.40%
Expected dividend yield	0.00%	0.00%	0.00%	0.00%

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands		9 Months Ended	12 Months Ended
		Sep. 30, 2017	Dec. 31, 2016
Number of Stock Options
Granted (in shares)		669,184
Exercised (in shares)		(441,842)
Aggregate Intrinsic Value
Weighted average grant date fair value (in usd per share)		$ 18.44
Total proceeds from options exercised		$ 3,200
Aggregate intrinsic value of options exercised		$ 9,500
Stock Option
Number of Stock Options
Outstanding at the beginning of period (in shares)		3,316,498
Granted (in shares)		669,184
Exercised (in shares)		(441,842)
Canceled or forfeited (in shares)		(84,330)
Outstanding at the end of the period (in shares)		3,459,010	3,316,498
Exercisable at the end of the period (in shares)		1,906,649
Weighted- Average Exercise Price Per Share
Outstanding at the beginning of period (in usd per share)		$ 25.96
Granted (in usd per share)		30.51
Exercised (in usd per share)		7.32
Canceled or forfeited (in usd per share)		34.16
Outstanding at the end of the period (in usd per share)		29.02	$ 25.96
Exercisable at the end of the period (in usd per share)		$ 25.88
Weighted- Average Contractual Life (in years)
Outstanding at the beginning of the period		6 years 11 months 27 days	7 years 11 days
Outstanding at the end of the period		6 years 11 months 27 days	7 years 11 days
Exercisable at the end of the period		5 years 6 months 18 days
Aggregate Intrinsic Value
Outstanding at the end of the period	[1]	$ 32,169
Exercisable at the end of the period	[1]	24,324
Unrecognized compensation expense		$ 25,400
Weighted-average period for recognition		2 years 6 months 26 days
[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, 2017.

Stock-Based Compensation - Sc47

Stock-Based Compensation - Schedule of Nonvested Restricted Stock Activity (Details) $ / shares in Units, $ in Millions	9 Months Ended
	Sep. 30, 2017USD ($)$ / sharesshares
Outstanding restricted stock units
Number of Grants
Unvested balance at beginning of the period (in shares)	732,000
Granted (in shares)	129,000
Vested (in shares)	(289,000)
Forfeited (in shares)	(21,000)
Unvested balance at end of the period (in shares)	551,000
Weighted- Average Grant Date Fair Value
Unvested balance at beginning of the period (in usd per share) | $ / shares	$ 31.55
Granted (in usd per share) | $ / shares	29.53
Vested (in usd per share) | $ / shares	31
Forfeited (in usd per share) | $ / shares	32.27
Unvested balance at end of the period (in usd per share) | $ / shares	$ 31.34
Outstanding restricted stock units | Certain Individuals
Number of Grants
Granted (in shares)	100,692
Unvested balance at end of the period (in shares)	214,120
Weighted- Average Grant Date Fair Value
Unrecognized compensation cost | $	$ 4.7
Weighted-average period for recognition	1 year 7 months 17 days
Performance-Based Restricted Stock Units | Certain Individuals
Number of Grants
Granted (in shares)	28,333
Unvested balance at end of the period (in shares)	337,122
Weighted- Average Grant Date Fair Value
Unrecognized compensation cost | $	$ 5.6
Weighted-average period for recognition	1 year 7 months 28 days

Income Taxes (Details)

Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Income tax benefit	$ 41	$ 20	$ 29	$ 20
Unrealized holding gain (loss) on securities, tax	59	$ 53	59	$ 53
Other accrued liabilities, current	$ 17		$ 17		$ 17

Related Party Transactions (Det

Related Party Transactions (Details) - USD ($) $ in Millions	Sep. 25, 2017	Sep. 30, 2017	Dec. 31, 2016
Celgene
Related Party Transactions
Ownership percentage of entity's fully diluted equity		12.70%	12.90%
Deferred revenue		$ 3.8
Common Stock
Related Party Transactions
Stock issued during period, new issues (in shares)	5,405,406
Common Stock | Celgene
Related Party Transactions
Stock issued during period, new issues (in shares)	745,592