Acceleron Pharma (XLRN) 10-Q 2 Aug 18 2018 Q2 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2018	Jul. 31, 2018
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	Jun. 30, 2018
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)		46,002,467
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q2

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 79,592	$ 100,150
Collaboration receivables (all amounts are with related party)	3,669	3,570
Prepaid expenses and other current assets	7,517	4,446
Short-term investments	208,338	177,077
Total current assets	299,116	285,243
Property and equipment, net	6,868	6,966
Restricted cash	1,132	1,132
Other assets	97	113
Long-term investments	44,328	95,723
Total assets	351,541	389,177
Current liabilities:
Accounts payable	1,269	1,086
Accrued expenses	13,113	14,936
Deferred revenue	0	541
Deferred rent	190	182
Total current liabilities	14,572	16,745
Deferred revenue, net of current portion	0	3,161
Deferred rent, net of current portion	2,171	1,818
Warrants to purchase common stock	1,689	2,236
Total liabilities	18,432	23,960
Commitments and contingencies (Note 14)
Stockholders’ equity:
Undesignated preferred stock, $0.001 par value: 25,000,000 shares authorized and no shares issued or outstanding	0	0
Common stock, $0.001 par value: 175,000,000 shares authorized; 45,845,051 and 45,261,175 shares issued and outstanding at June 30, 2018 and December 31, 2017, respectively	47	46
Additional paid-in capital	858,582	839,090
Accumulated deficit	(524,477)	(473,024)
Accumulated other comprehensive loss	(1,043)	(895)
Total stockholders’ equity	333,109	365,217
Total liabilities and stockholders’ equity	$ 351,541	$ 389,177

Condensed Consolidated Balance3

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Jun. 30, 2018	Dec. 31, 2017
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	45,845,051	45,261,175
Common stock, shares outstanding	45,845,051	45,261,175

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Collaboration revenue:
Revenues	$ 3,685,000	$ 3,057,000	$ 6,917,000	$ 6,762,000
Costs and expenses:
Research and development	25,933,000	21,598,000	49,363,000	43,327,000
General and administrative	7,658,000	11,370,000	15,099,000	19,203,000
Total costs and expenses	33,591,000	32,968,000	64,462,000	62,530,000
Loss from operations	(29,906,000)	(29,911,000)	(57,545,000)	(55,768,000)
Other expense, net	(369,000)	(228,000)	(250,000)	(272,000)
Interest income	1,348,000	476,000	2,660,000	977,000
Total other income, net	979,000	248,000	2,410,000	705,000
Loss before income taxes	(28,927,000)	(29,663,000)	(55,135,000)	(55,063,000)
Income tax provision	(11,000)	(6,000)	(21,000)	(12,000)
Net loss	$ (28,938,000)	$ (29,669,000)	$ (55,156,000)	$ (55,075,000)
Net loss per share- basic and diluted (in dollars per share)	$ (0.63)	$ (0.77)	$ (1.21)	$ (1.43)
Weighted-average number of common shares used in computing net loss per share- basic and diluted (in shares)	45,789	38,631	45,654	38,515
Other comprehensive loss:
Net loss	$ (28,938,000)	$ (29,669,000)	$ (55,156,000)	$ (55,075,000)
Net unrealized holding gains (losses) on short-term and long-term investments during the period	280,000	46,000	(147,000)	71,000
Comprehensive loss	(28,658,000)	(29,623,000)	(55,303,000)	(55,004,000)
License and Service
Collaboration revenue:
Revenues	0	135,000	0	271,000
Cost Sharing
Collaboration revenue:
Revenues	$ 3,685,000	$ 2,922,000	$ 6,917,000	$ 6,491,000

Condensed Consolidated Stateme5

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Operating Activities
Net loss	$ (55,156)	$ (55,075)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	1,866	1,224
Stock-based compensation	11,655	16,172
Change in fair value of warrants	250	272
Other non-cash items	559	145
Changes in assets and liabilities:
Prepaid expenses and other assets	(3,055)	682
Collaboration receivables (all amounts are with a related party)	(99)	353
Accounts payable	183	71
Accrued expenses	(2,140)	(2,185)
Deferred revenue	0	(272)
Deferred rent	355	(311)
Net cash used in operating activities	(45,582)	(38,924)
Investing Activities
Purchases of investments	(66,113)	(175)
Proceeds from sales and maturities of investments	85,547	50,221
Purchases of property and equipment	(1,373)	(2,928)
Net cash provided by investing activities	18,061	47,118
Financing Activities
Payments for withholding taxes on restricted stock units	(363)	(140)
Payments for capital lease expenditures	(78)	0
Proceeds from exercise of stock options and warrants to purchase common stock	6,741	1,398
Proceeds from issuances of common stock related to employee stock purchase plan	663	510
Net cash provided by financing activities	6,963	1,768
Net (decrease) increase in cash, cash equivalents and restricted cash	(20,558)	9,962
Cash, cash equivalents and restricted cash at beginning of period	101,282	21,896
Cash, cash equivalents and restricted cash at end of period	80,724	31,858
Supplemental Disclosure of Non-Cash Investing and Financing Activities:
Purchase of property and equipment included in accounts payable and accrued expenses	256	305
Reclassification of warrant liability to additional paid-in capital	797	0
Acquisition of capital lease	$ 139	$ 0

Nature of Business

Nature of Business	6 Months Ended
	Jun. 30, 2018
Nature of Business
Nature of Business	Nature of Business Acceleron Pharma Inc. (Acceleron or the Company) is a Cambridge, Massachusetts-based clinical stage biopharmaceutical company dedicated to the discovery, development, and commercialization of therapeutics to treat serious and rare diseases. The Company’s leadership in the understanding of TGF-beta biology and protein engineering generates innovative compounds that engage the body’s ability to regulate cellular growth and repair. The Company is subject to risks common to companies in the biotechnology industry, including, but not limited to, the risk that the Company never achieves profitability, the need for substantial additional financing, risk of relying on third parties, risks of clinical trial failures, dependence on key personnel, protection of proprietary technology and compliance with government regulations.

Basis of Presentation

Basis of Presentation	6 Months Ended
	Jun. 30, 2018
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, 2017 except for the adoption of Accounting Standards Update (ASU) No. 2016-18, Restricted Cash, which did not have a material impact, and Topic No. 606, Revenue from Contracts with Customers, as discussed below, 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 June 30, 2018 , the results of its operations for the three and six months ended June 30, 2018 and 2017 , and its cash flows for the six months ended June 30, 2018 and 2017 . The results for the three and six months ended June 30, 2018 are not necessarily indicative of the results to be expected for the year ending December 31, 2018 , 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, 2017 , 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, 2017 . 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 June 30, 2018 , 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, 2017 , have not changed, except for the adoption of ASU No. 2016-18, Restricted Cash, which is discussed further in Note 6, and Accounting Standards Codification Topic No. 606, Revenue from Contracts with Customers, which is discussed further below. Revenue Recognition Effective January 1, 2018, the Company adopted Accounting Standards Codification Topic 606, Revenue from Contracts with Customers, (ASC 606), using the modified retrospective transition method. Under this method, results for reporting periods beginning January 1, 2018 are presented under ASC 606, while prior period amounts are not adjusted and continue to be reported in accordance with ASC 605. The Company has primarily generated revenue through collaboration, license and research arrangements, which are within the scope of ASC 606, with collaboration partners for the development and commercialization of therapeutic candidates. The arrangements generally contain performance obligations, which may include (1) licenses, or options to obtain licenses, to the Company's technology, (2) research and development activities performed for the collaboration partners (3) participation on joint development committees (JDCs), and (4) the manufacturing of clinical or preclinical material. Payments pursuant to these arrangements typically include non-refundable, upfront payments, milestone payments upon achieving significant development events, research and development reimbursements, sales milestones, exercises of options, and royalties on future product sales. Amounts received prior to satisfying the revenue recognition criteria are recorded as deferred revenue in the Company's consolidated balance sheets. Amounts expected to be recognized as revenue within the 12 months following the balance sheet date are classified as deferred revenue, current portion. Amounts not expected to be recognized as revenue within the 12 months following the balance sheet date are classified as deferred revenue, net of current portion. Amounts recognized as revenue, but not yet received or invoiced are generally recognized as contract assets, including collaboration receivables. To determine revenue recognition for arrangements within the scope of ASC 606, the Company performs the following five steps: (i) identify the contract(s) with the customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the entity satisfies a performance obligation. The Company only applies the five-step model to contracts when it is probable that the entity will collect the consideration it is entitled to in exchange for the goods or services it transfers to the customer. At contract inception, once the contract is determined to be within the scope of ASC 606, the Company assesses the goods or services promised within each contract, determines those that are performance obligations, and assesses whether each promised good or service is distinct. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) the performance obligation is satisfied. Depending on the nature of the performance obligation these assessments require management to make significant judgments and estimates. Exclusive Licenses If the license to the Company’s intellectual property is determined to be distinct from the other promises or performance obligations identified in the arrangement, the Company recognizes revenue from non-refundable, upfront fees allocated to the license when the license is transferred and the customer is able to use and benefit from the license. In order to assess whether the license is distinct, the Company considers the capabilities of the collaboration partner and the availability of the necessary expertise in the general marketplace to determine whether the collaboration partner can benefit from the license for its intended purpose without the receipt of the remaining elements. For licenses determined not to be distinct the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. The measure of progress, and thereby periods over which revenue should be recognized, are subject to estimates by management and may change over the course of the research and development and licensing agreement. Research and Development Services The promises under the Company’s collaboration and license agreements generally include research and development services to be performed by the Company on behalf of the collaboration partner. As the provision of research and development services is a part of the Company’s central operations, when the Company is principally responsible for the performance of these services under the agreements, the Company recognizes revenue on a gross basis for research and development services in accordance with the ASC 606 framework described above. Customer Options The Company's agreements may contain options which provide the collaboration partner the right to obtain additional licenses. If an arrangement is determined to contain customer options, the goods and services underlying the customer options are not considered to be performance obligations at the inception of the arrangement, and the associated option fees are not included in the transaction price. The Company evaluates the customer options to determine if they represent material rights, which may include options to acquire additional goods or services for free or at a discount. If the customer options are determined to represent a material right, the material right is recognized as a separate performance obligation at the outset of the arrangement. The Company allocates the transaction price to material rights based on the relative standalone selling price, which is determined based on the identified discount and the probability that the customer will exercise the option. Amounts allocated to a material right are not recognized as revenue until, at the earliest, the option is exercised. Milestone Payments At the inception of each arrangement that includes milestone payments, the Company evaluates whether the milestones are considered probable of being achieved and estimates the amount to be included in the transaction price using the most likely amount method. The Company evaluates factors such as the scientific, clinical, regulatory, commercial, and other risks that must be overcome to achieve the respective milestone in making this assessment. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the control of the Company or the licensee, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. At the end of each subsequent reporting period, the Company reevaluates the probability of achievement of all milestones subject to constraint and, if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect revenues and earnings in the period of adjustment. If a milestone or other variable consideration relates specifically to the Company's efforts to satisfy a single performance obligation or to a specific outcome from satisfying the performance obligation, the Company generally allocates the milestone amount entirely to that performance obligation. Royalties For arrangements that include sales-based royalties, including milestone payments based on a level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company recognizes revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied). To date, the Company has not recognized any royalty revenue resulting from any of its licensing arrangements. For a complete discussion of accounting for collaboration revenues, see Note 15.

Use of Estimates

Use of Estimates	6 Months Ended
	Jun. 30, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements, and the reported amounts expensed during the reporting period. Management considers many factors in selecting appropriate financial accounting policies and controls, and in developing the estimates and assumptions that are used in the preparation of these consolidated financial statements. Management must apply significant judgment in this process. In addition, other factors may affect estimates, including: expected business and operational changes, sensitivity and volatility associated with the assumptions used in developing estimates, and whether historical trends are expected to be representative of future trends. The estimation process often may yield a range of potentially reasonable estimates of the ultimate future outcomes and management must select an amount that falls within that range of reasonable estimates. This process may result in actual results differing materially from those estimated amounts used in the preparation of the consolidated financial statements if these results differ from historical experience, or other assumptions do not turn out to be substantially accurate, even if such assumptions are reasonable when made. In preparing these consolidated financial statements, management used significant estimates in the following areas, among others: revenue recognition and the related evaluation of any constrained milestones, stock-based compensation expense, the determination of the fair value of stock-based awards, the fair value of liability-classified warrants, and accrued expenses.

Segment Information

Segment Information	6 Months Ended
	Jun. 30, 2018
Segment Reporting [Abstract]
Segment Information	Segment Information Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision maker, or decision making group, in making decisions on how to allocate resources and assess performance. The Company’s chief operating decision maker is the chief executive officer. The Company and the chief executive officer view the Company’s operations and manage its business as one operating segment, which is the discovery, development and commercialization of highly innovative therapeutics to treat serious and rare diseases.

Cash Equivalents and Short-term

Cash Equivalents and Short-term and Long-term Investments	6 Months Ended
	Jun. 30, 2018
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 June 30, 2018 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 six months ended June 30, 2018 and 2017 . 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 June 30, 2018 and December 31, 2017 was $199.2 million and $193.6 million , respectively. The aggregate fair value of securities held by the Company in an unrealized loss position for more than twelve months as of June 30, 2018 and December 31, 2017 was $47.2 million and $67.0 million , respectively. The aggregate unrealized loss for those securities in an unrealized loss position for more than twelve months is $0.1 million and $0.3 million , respectively. As a result, the Company determined it did not hold any investments with any other-than-temporary impairment as of June 30, 2018 and December 31, 2017 . The following is a summary of cash, cash equivalents and available-for-sale securities as of June 30, 2018 and December 31, 2017 , (in thousands): June 30, 2018 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Cash and cash equivalents due in 90 days or less $ 79,592 $ — $ — $ 79,592 Available-for-sale securities: Corporate obligations due in one year or less 101,474 — (423 ) 101,051 Corporate obligations due in more than one year 20,848 — (182 ) 20,666 U.S. Treasury securities due in one year or less 58,825 — (123 ) 58,702 U.S. Treasury securities due in more than one year 12,342 — (39 ) 12,303 Certificates of deposit due in one year or less 5,765 — — 5,765 Certificates of deposit due in more than one year 490 — — 490 Mortgage and other asset backed securities due in one year or less 42,986 — (167 ) 42,819 Mortgage and other asset backed securities due in more than one year 10,978 — (108 ) 10,870 Total available-for-sale securities $ 253,708 $ — $ (1,042 ) $ 252,666 Total cash, cash equivalents and available-for-sale securities $ 333,300 $ — $ (1,042 ) $ 332,258 December 31, 2017 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Cash and cash equivalents due in 90 days or less $ 100,150 $ — $ — $ 100,150 Available-for-sale securities: Corporate obligations due in one year or less 99,792 — (219 ) 99,573 Corporate obligations due in more than one year 57,537 — (261 ) 57,276 U.S. Treasury securities due in one year or less 27,987 — (93 ) 27,894 U.S. Treasury securities due in more than one year 9,968 — (48 ) 9,920 Certificates of deposit due in one year or less 10,529 — — 10,529 Certificates of deposit due in more than one year 1,715 — — 1,715 Mortgage and other asset backed securities due in one year or less 39,236 — (155 ) 39,081 Mortgage and other asset backed securities due in more than one year 26,931 — (119 ) 26,812 Total available-for-sale securities $ 273,695 $ — $ (895 ) $ 272,800 Total cash, cash equivalents and available-for-sale securities $ 373,845 $ — $ (895 ) $ 372,950

Restricted Cash

Restricted Cash	6 Months Ended
	Jun. 30, 2018
Receivables [Abstract]
Restricted Cash	Restricted Cash On January 1, 2018, the Company adopted 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. As a result of the adoption, there was no impact to cash flows from investing or financing activities for the six months ended June 30, 2018 and June 30, 2017 . The $1.1 million of restricted cash related to collateral for the Company's facility lease obligation and its credit cards, which was previously reported as an adjustment to net loss in cash flows used in operating activities for the six months ended June 30, 2017 , is no longer presented within the net change in cash, cash equivalents, and restricted cash, as it is considered part of cash, cash equivalents, and restricted cash. The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the condensed consolidated balance sheet that sum to the total of the same such amounts shown in the statement of cash flows (in thousands): June 30, 2018 2017 Cash and cash equivalents $ 79,592 $ 30,726 Restricted cash 1,132 1,132 Total cash, cash equivalents and restricted cash shown in the statement of cash flows $ 80,724 $ 31,858 As of June 30, 2018 and December 31, 2017 , the Company maintained letters of credit totaling $1.1 million and $1.1 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	6 Months Ended
	Jun. 30, 2018
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	6 Months Ended
	Jun. 30, 2018
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 June 30, 2018 and December 31, 2017 (in thousands): June 30, 2018 Quoted Prices in Active Markets for Identical Items (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Total Assets: Money market funds $ 72,229 $ — $ — $ 72,229 Corporate obligations — 121,717 — 121,717 U.S. Treasury securities — 71,004 — 71,004 Certificates of deposit — 6,256 — 6,256 Mortgage and other asset backed securities — 53,689 — 53,689 Restricted cash 1,132 — — 1,132 Total assets $ 73,361 $ 252,666 $ — $ 326,027 Liabilities: Warrants to purchase common stock $ — $ — $ 1,689 $ 1,689 Total liabilities $ — $ — $ 1,689 1,689 December 31, 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 $ 90,702 $ — $ — $ 90,702 Corporate obligations — 158,849 — 158,849 U.S. Treasury securities — 37,813 — 37,813 Certificates of deposit — 12,244 — 12,244 Mortgage and other asset backed securities — 67,888 — 67,888 Restricted cash 1,132 — — 1,132 Total assets $ 91,834 $ 276,794 $ — $ 368,628 Liabilities: Warrants to purchase common stock $ — $ — $ 2,236 $ 2,236 Total liabilities $ — $ — $ 2,236 $ 2,236 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 six months ended June 30, 2018 or the year ended December 31, 2017 . 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 13). 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): Six Months Ended June 30, 2018 2017 Beginning balance $ 2,236 $ 1,244 Change in fair value 250 272 Exercises (797 ) — Ending balance $ 1,689 $ 1,516 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 June 30, 2018 , 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 six months ended June 30, 2018 or the year ended December 31, 2017 .

Net Loss Per Share

Net Loss Per Share	6 Months Ended
	Jun. 30, 2018
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 Six Months Ended 2018 2017 2018 2017 Outstanding stock options 3,663 3,541 3,663 3,541 Common stock warrants 39 64 39 64 Shares issuable under employee stock purchase plan 12 13 12 13 Outstanding restricted stock units (1) 667 810 667 810 4,381 4,428 4,381 4,428 (1) This balance is comprised of both the restricted stock units and performance-based restricted stock units described in Note 16.

Comprehensive Loss

Comprehensive Loss	6 Months Ended
	Jun. 30, 2018
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 June 30, 2018 and December 31, 2017 .

Subsequent Events

Subsequent Events	6 Months Ended
	Jun. 30, 2018
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events The Company considers events or transactions that occur after the balance sheet date but prior to the issuance of the financial statements to provide additional evidence relative to certain estimates or to identify matters that require additional disclosure. The Company has evaluated all subsequent events and determined that there are no material recognized or unrecognized subsequent events requiring disclosure except as described below. On July 30, 2018, the Company entered into a five -year lease for approximately 27,000 square feet of office and lab space located at 167 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 $9.9 million . The Company also has the right to extend the term of the lease for an additional three years, subject to landlord approval and subject to landlord's right to elect to redevelop 167 Sidney Street at the end of the current lease term in lieu of the Company's extension.

Recent Accounting Pronouncement

Recent Accounting Pronouncements	6 Months Ended
	Jun. 30, 2018
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 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 June 2018, the FASB issued ASU 2018-07, Compensation - Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting. This new standard expands the scope of Topic 718 to include share-based payment transactions from nonemployees. The guidance is effective for annual and interim periods beginning after December 15, 2018. Early adoption is permitted, but no earlier than a company's adoption date of Topic 606, Revenue from Contracts with Customers. The Company is currently assessing the impact that adopting ASU 2018-07 will have on its consolidated financial statements and related disclosures.

Warrants

Warrants	6 Months Ended
	Jun. 30, 2018
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 June 30, 2018 December 31, 2017 Price Per Share Expiration June 30, 2018 December 31, 2017 Warrants to purchase common stock 39 61 $ 5.88 June 10, 2020 - July 9, 2020 Liability (1) Liability (1) All warrants 39 61 $ 5.88 (1) In January 2018, warrant holders exercised warrants to purchase 21,258 shares of Common Stock on a net basis, resulting in the issuance of 18,449 shares of Common Stock.

Commitments and Contingencies

Commitments and Contingencies	6 Months Ended
	Jun. 30, 2018
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 June 30, 2018 , and, to the best of its knowledge, no material legal proceedings are currently pending or threatened. Other The Company is also party to various agreements, principally relating to licensed technology, that require future payments relating to milestones not met at June 30, 2018 and December 31, 2017 , or royalties on future sales of specified products. No milestones or royalty payments under these agreements are expected to be payable in the immediate future. See Note 15 for discussion of these arrangements. The Company enters into standard indemnification agreements in the ordinary course of business. Pursuant to the agreements, the Company indemnifies, holds harmless, and agrees to reimburse the indemnified party for losses suffered or incurred by the indemnified party, generally the Company’s business partners or customers, in connection with any U.S. patent or any copyright or other intellectual property infringement claim by any third party with respect to the Company’s products. The term of these indemnification agreements is generally perpetual any time after execution of the agreement. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is unlimited. The Company has never incurred costs to defend lawsuits or settle claims related to these indemnification agreements.

Significant Agreements

Significant Agreements	6 Months Ended
	Jun. 30, 2018
Significant Agreements [Abstract]
Significant Agreements	Significant Agreements Celgene Overview On February 20, 2008, the Company entered into an agreement with Celgene relating to sotatercept (the Original Sotatercept Agreement), which was amended on August 2, 2011 (as amended, the Amended Sotatercept Agreement). The Company further amended and restated the Original Sotatercept Agreement in its entirety on September 18, 2017, (the Restated Sotatercept Agreement). On August 2, 2011, the Company entered into a second agreement with Celgene for luspatercept, (the Luspatercept Agreement). Since December 31, 2017 , there have been no material changes to the key terms of the above agreements. For further information on the terms of the agreements, please see the notes to the consolidated financial statements included in the Company’s Form 10-K for the year ended December 31, 2017 . Restated Sotatercept Agreement The Restated Sotatercept Agreement provides Celgene with an exclusive license to sotatercept outside of the field of pulmonary hypertension, referred to as the PH field, and provides the Company with the worldwide rights to develop and commercialize sotatercept in the PH field. In connection with the Restated Sotatercept Agreement, Celgene agreed not to develop or commercialize in the PH field any compound developed under the Restated Sotatercept Agreement or the Luspatercept Agreement, and the Company agreed not to develop or commercialize any compound developed under the Restated Sotatercept Agreement or the Luspatercept Agreement in any field outside the PH field. The Company has the right to license, transfer, or sell its rights to develop and commercialize sotatercept in the PH field, subject to Celgene's first right of negotiation. The Company retained responsibility for research and development of sotatercept 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 is responsible for manufacturing or overseeing the manufacture of Phase 3 and commercial supplies. Luspatercept Agreement Under the terms of the Luspatercept Agreement, the Company and Celgene collaborate worldwide for the joint development and commercialization of luspatercept. The Company also granted Celgene an option for future products for which the Company files an Investigational New Drug application for the treatment of anemia. The Company retained responsibility for research and development through the end of Phase 1 and the Company's initial luspatercept beta-thalassemia and luspatercept MDS Phase 2 clinical trials, as well as manufacturing the clinical supplies for these studies. Celgene will conduct subsequent Phase 2 and Phase 3 clinical studies and will be responsible for overseeing the manufacture of Phase 3 and commercial supplies by third party contract manufacturing organizations. Beginning in November 2013, the Company agreed to conduct certain extension studies for the benefit of the luspatercept program, which included certain clinical and non-clinical services. These studies were mutually agreed to by both parties and are directed by the JDC. The Company is reimbursed for these services under the same terms and rates of the existing agreements. Both Agreements Under both agreements, Celgene is responsible for paying 100% of worldwide development costs and 100% of any commercialization costs worldwide for sotatercept (outside of the pulmonary hypertension field) and luspatercept. The Company has the right to co-promote sotatercept (outside of the pulmonary hypertension field), luspatercept and future products in North America. 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 On January 1, 2018, the Company adopted ASC 606, Revenue from Contracts with Customers , using the modified retrospective transition method, and has elected to use the practical expedient related to contract modifications that is permitted under the rules of adoption. The practical expedient included in the transition guidance allows companies to determine and allocate the transaction price of a modified contract as of the beginning of the earliest period presented instead of requiring them to separately evaluate the effects of every modification of the contract and eliminates the requirement to retrospectively restate a contract that has been modified at the date of adoption as is generally required under ASC 606. This practical expedient was applied in the assessment of the Restated Sotatercept Agreement and Luspatercept Agreement as of the date of adoption. The Company identified the following material promises under the Restated Sotatercept Agreement and Luspatercept Agreement: (1) licenses to develop and commercialize sotatercept and luspatercept; (2) performance of research and development services; (3) participation in the JDCs; and (4) the performance of the manufacturing services. The Company determined that the licenses to sotatercept and luspatercept technology, the research and development activities, participation in the JDCs and the manufacturing services are each distinct performance obligations. The option rights to future products related to the treatment of anemia under the Luspatercept Agreement are not considered to represent a material right as this right is a protective provision akin to exclusivity and does not represent a customer option to receive the rights or services at a discount. In addition, the Company is under no obligation to discover, develop, or deliver any new compounds that modulate anemia. Therefore, the option right under the Luspatercept Agreement is not a performance obligation. Commercialization support for each of sotatercept and luspatercept is considered to be a participatory right and not a performance obligation. The Company concluded that services provided for the extension studies do not represent a contract modification or a performance obligation but rather a separate services arrangement, which is accounted for as a separate contract. Each study includes one promise, the completion of the study, which is distinct from the performance obligations in the Restated Sotatercept Agreement and Luspatercept Agreement that is satisfied over time, and the consideration for each study approximates the stand-alone selling price. Revenue is recognized as the services for each study are provided. Future potential milestone payments were excluded from the transaction price as they are still subject to completion of on-going clinical studies or other risks that are outside of the Company's control and therefore the risk of significant reversal has not been resolved. The next likely clinical milestone payment for luspatercept would be $25.0 million and result from U.S. Food and Drug Administration (FDA) or European Medicines Agency (EMA) acceptance of a Biologics Licensing Application or equivalent for luspatercept in either myelodysplastic syndromes or beta-thalassemia. The Company and Celgene plan to submit market applications for luspatercept in the United States and Europe in the first half of 2019. Following application submission, the FDA will determine the acceptance of the application for "filing" by 60 days from the submission date. Similarly, the EMA will validate the application within 10 days of submission. In line with the Company's accounting policy regarding revenue recognition as described in Note 2, the revenue associated with this milestone will be recognized once the applications are accepted by review by the Health Authority. The transaction price includes the following payments received under the Restated Sotatercept and Luspatercept Agreement through the adoption date of December 31, 2017 for a total of $192.3 million , as follows: • $25.0 million upfront fee in connection with the closing of the Luspatercept Agreement; • $45.0 million of nonrefundable, upfront license and option payments in connection with the closing of the Original and Amended Sotatercept Agreements; • $14.9 million received for sotatercept development and manufacturing activities; • $47.9 million received for luspatercept development and manufacturing activities; and • $59.5 million milestone payments pursuant to the agreements. The Company allocated the total transaction price to the identified performance obligations (both satisfied and unsatisfied) using the estimated standalone selling price of each performance obligation as of the adoption date of ASC 606. The Company’s estimate of the standalone selling price requires judgment, in particular in estimating the value of the license rights for luspatercept and sotatercept, which includes assumptions over the projected revenues and expenses, probability of technical and regulatory success and appropriate discount rates. As of the ASC 606 adoption date, the only remaining undelivered element is participation in the JDC for which there was a deferred revenue balance of $3.7 million . The transaction price allocated to participation in the JDC based on the established standalone selling price of all performance obligations was de minimis as the sotatercept and luspatercept licenses carried the most significant portion of the value included in the agreements, and the Company's remaining effort on the JDC is minimal. As a result of adopting ASC 606 on January 1, 2018, the Company has recorded a cumulative-effect reduction to opening accumulated deficit of $3.7 million as of January 1, 2018 and a corresponding decrease to deferred revenue, of which $0.5 million was recorded to current deferred revenue and $3.2 million was recorded to long-term deferred revenue. License and milestone revenue for the three and six months ended June 30, 2018 was zero , and zero , respectively, as compared to the $0.1 million and $0.3 million , respectively, that would have been recorded under ASC 605. Deferred revenue as of June 30, 2018 was zero under ASC 606, as compared to a balance of $3.4 million , which would have resulted under ASC 605. Through June 30, 2018 , under all Celgene arrangements the Company has received net cost-share payments and milestones of $106.0 million and $44.4 million for luspatercept and sotatercept, respectively. The Company recorded net cost-sharing revenue of $3.7 million and $2.9 million during the three months ended June 30, 2018 and 2017 , respectively, and $6.9 million and $6.5 million during the six months ended June 30, 2018 and 2017 , respectively. Other Agreements Other In 2004, the Company entered into a license agreement with a non-profit institution for an exclusive, sublicensable, worldwide, royalty-bearing license to certain patents developed by the institution (Primary Licensed Products). In addition, the Company was granted a non-exclusive, non-sub-licensable license for Secondary Licensed Products. As compensation for the licenses, the Company issued 62,500 shares of its common stock to the institution, the fair value of which was $25,000 , and was expensed during 2004, to research and development expense. The Company also agreed to pay specified development milestone payments totaling up to $2.0 million for sotatercept and $0.7 million for luspatercept. In addition, the Company is obligated to pay milestone fees based on the Company’s research and development progress, and U.S. sublicensing revenue ranging from 10% - 25% , as well as royalties ranging from 1.0% - 3.5% of net sales on any products under the licenses. During the three months ended June 30, 2018 and 2017 , the Company expensed $0.1 million and $0.1 million , respectively, and during the six months ended June 30, 2018 and 2017 , the Company expensed $0.1 million and $0.1 million , respectively, 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 June 30, 2018 and 2017 , the Company expensed zero and $0.1 million , respectively, and during the six months ended June 30, 2018 and 2017 , the Company expensed zero and $0.2 million , respectively, of milestones and fees, which is recorded as research and development expense.

Stock-Based Compensation

Stock-Based Compensation	6 Months Ended
	Jun. 30, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock-Based Compensation	Stock-Based Compensation The Company recognized stock-based compensation expense related to the 2003 Stock Option and Restricted Stock Plan (the 2003 Plan), the 2013 Equity Incentive Plan (the 2013 Plan), and the 2013 Employee Stock Purchase Plan (the 2013 ESPP) in the consolidated statements of operations and comprehensive loss during the three months ended June 30, 2018 and 2017 , respectively, as follows (in thousands): Three Months Ended Six Months Ended 2018 2017 2018 2017 Research and development $ 3,050 $ 3,433 $ 5,919 $ 7,095 General and administrative 2,910 6,155 5,736 9,077 $ 5,960 $ 9,588 $ 11,655 $ 16,172 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 provide services as a non-employee consultant. During the three months ended June 30, 2018 and 2017 , the Company recognized $1.1 million and $1.0 million , respectively, and during the six months ended June 30, 2018 and 2017 , the Company recognized $1.3 million and $2.2 million , respectively, of stock-based compensation expense within research and development expense related to the agreement. Stock Options The fair value of each option issued to employees was estimated on the date of grant using the Black-Scholes option pricing model with the following weighted-average assumptions: Three Months Ended Six Months Ended 2018 2017 2018 2017 Expected volatility 62.2 % 65.8 % 62.9 % 65.9 % Expected term (in years) 6.0 6.0 6.0 6.0 Risk-free interest rate 2.8 % 1.9 % 2.7 % 2.2 % Expected dividend yield — % — % — % — % The following table summarizes the stock option activity under the Company’s stock option plans during the six months ended June 30, 2018 (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, 2017 3,452 $ 29.14 6.72 Granted 717 $ 40.22 Exercised (435 ) $ 15.51 Canceled or forfeited (71 ) $ 36.40 Outstanding at June 30, 2018 3,663 $ 32.79 7.34 $ 57,667 Exercisable at June 30, 2018 2,016 $ 30.39 6.17 $ 36,595 (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 June 30, 2018 . During the six months ended June 30, 2018 , the Company granted stock options to purchase an aggregate of 717,135 shares of its common stock, with a weighted-average grant date fair value of options granted of $23.80 per share. During the six months ended June 30, 2018 , current and former employees of the Company exercised a total of 434,627 options, resulting in total proceeds of $6.7 million . The aggregate intrinsic value of options exercised during the six months ended June 30, 2018 was $11.8 million . As of June 30, 2018 , there was $30.7 million of unrecognized compensation expense related to unvested stock options that is expected to be recognized over a weighted-average period of 2.58 years. Restricted Stock Units The following table summarizes the restricted stock unit (RSU) activity under the 2013 Plan during the six months ended June 30, 2018 (in thousands, except per share amounts): Number Weighted- Unvested balance at December 31, 2017 268 $ 32.44 Granted 208 $ 37.33 Vested (70 ) $ 31.26 Forfeited (8 ) $ 35.61 Unvested balance at June 30, 2018 398 $ 35.14 As of June 30, 2018 , there was approximately $11.3 million of related unrecognized compensation cost, which the Company expects to recognize over a remaining weighted-average period of 2.04 years. Performance-Based Restricted Stock Units The Company has granted performance-based restricted stock units (PSU) whereby vesting accelerates upon the occurrence of certain milestone events. In September 2019, any of these PSUs that remain unvested will vest. As a result, when achievement of a milestone becomes probable, compensation cost is recognized from the grant date through the estimated date of achievement. If achievement is not considered probable the expense is recognized from the grant date through September 2019. The following table summarizes PSU activity under the 2013 Plan during the six months ended June 30, 2018 (in thousands, except per share amounts): Number Weighted- Unvested balance at December 31, 2017 337 $ 31.53 Granted — $ — Vested (50 ) $ 29.92 Forfeited (18 ) $ 31.09 Unvested balance at June 30, 2018 269 $ 31.86 As of June 30, 2018 , there was approximately $2.8 million of related unrecognized compensation cost, which the Company expects to recognize over a remaining weighted-average period of 1.02 years. Employee Stock Purchase Plan During the three months ended June 30, 2018 and 2017 , the Company recorded $0.1 million and $0.1 million , respectively, and during the six months ended June 30, 2018 and 2017 , the Company recorded $0.2 million and $0.1 million , respectively, of stock-based compensation expense related to the 2013 ESPP.

Income Taxes

Income Taxes	6 Months Ended
	Jun. 30, 2018
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes U.S. Tax Reform In March 2018, the FASB issued ASU-2018-05, Income Taxes (Topic 740), Amendments to SEC Paragraphs Pursuant to SEC Staff Accounting Bulletin No. 118. This new standard addresses the accounting implications of the Tax Cuts and Jobs Act (the Tax Act) signed into law in December 2017. The Tax Act, among other changes, permanently lowers the corporate federal tax rate to 21% from the existing maximum rate of 35%, effective for tax years beginning January 1, 2018. As a result of the reduction of the corporate federal income tax rate to 21%, US GAAP requires companies to revalue their deferred tax assets and deferred tax liabilities as of the date of enactment, with the resulting tax effects accounted for in the reporting period of the enactment. This revaluation resulted in a provision of $59.3 million to income tax expense in continuing operations and a corresponding reduction in the valuation allowance as of December 31, 2017 . As a result, there was no impact on the Company's consolidated statements of operations from the reduction in the tax rate. The other provisions of the Tax Act did not have a material impact on the consolidated financial statements. The Company is still in the process of analyzing the impact to the Company of the Tax Act. Where the Company has been able to make reasonable estimates of the effects for which its analysis is not yet complete, the Company has recorded provisional amounts. Where the Company has not yet been able to make reasonable estimates of the impact of certain elements, the Company has not recorded any amounts related to those elements and has continued accounting for them in accordance with ASC 740 on the basis of the tax laws in effect immediately prior to the enactment of the Tax Act. Deferred tax assets and valuation allowance For the three months ended June 30, 2018 and 2017 , the Company recognized income tax expense of $11 thousand and $6 thousand , respectively, and during the six months ended June 30, 2018 and 2017 , the Company recognized income tax expense of $21 thousand and $12 thousand , respectively, related to state income taxes on its interest income. 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 June 30, 2018 and December 31, 2017 . 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, 2014 through December 31, 2017 . 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 June 30, 2018 and December 31, 2017 , the Company did not have any significant uncertain tax positions.

Related Party Transactions

Related Party Transactions	6 Months Ended
	Jun. 30, 2018
Related Party Transactions [Abstract]
Related Party Transactions	Related Party Transactions Celgene Corporation Celgene owned 12.3% and 12.5% of the Company’s fully diluted equity as of June 30, 2018 and December 31, 2017 , respectively. Refer to Note 15 for additional information regarding this collaboration arrangement. During the six months ended June 30, 2018 and 2017 , all revenue recognized by the Company was recognized under the Celgene collaboration arrangement and, as of June 30, 2018 , the Company had no deferred revenue related to the Celgene collaboration arrangement.

Basis of Presentation (Policies

Basis of Presentation (Policies)	6 Months Ended
	Jun. 30, 2018
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, 2017 except for the adoption of Accounting Standards Update (ASU) No. 2016-18, Restricted Cash, which did not have a material impact, and Topic No. 606, Revenue from Contracts with Customers, as discussed below, 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 June 30, 2018 , the results of its operations for the three and six months ended June 30, 2018 and 2017 , and its cash flows for the six months ended June 30, 2018 and 2017 . The results for the three and six months ended June 30, 2018 are not necessarily indicative of the results to be expected for the year ending December 31, 2018 , 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, 2017 , 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, 2017 . 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 June 30, 2018 , 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, 2017 , have not changed, except for the adoption of ASU No. 2016-18, Restricted Cash, which is discussed further in Note 6, and Accounting Standards Codification Topic No. 606, Revenue from Contracts with Customers, which is discussed further below.
Revenue Recognition	Revenue Recognition Effective January 1, 2018, the Company adopted Accounting Standards Codification Topic 606, Revenue from Contracts with Customers, (ASC 606), using the modified retrospective transition method. Under this method, results for reporting periods beginning January 1, 2018 are presented under ASC 606, while prior period amounts are not adjusted and continue to be reported in accordance with ASC 605. The Company has primarily generated revenue through collaboration, license and research arrangements, which are within the scope of ASC 606, with collaboration partners for the development and commercialization of therapeutic candidates. The arrangements generally contain performance obligations, which may include (1) licenses, or options to obtain licenses, to the Company's technology, (2) research and development activities performed for the collaboration partners (3) participation on joint development committees (JDCs), and (4) the manufacturing of clinical or preclinical material. Payments pursuant to these arrangements typically include non-refundable, upfront payments, milestone payments upon achieving significant development events, research and development reimbursements, sales milestones, exercises of options, and royalties on future product sales. Amounts received prior to satisfying the revenue recognition criteria are recorded as deferred revenue in the Company's consolidated balance sheets. Amounts expected to be recognized as revenue within the 12 months following the balance sheet date are classified as deferred revenue, current portion. Amounts not expected to be recognized as revenue within the 12 months following the balance sheet date are classified as deferred revenue, net of current portion. Amounts recognized as revenue, but not yet received or invoiced are generally recognized as contract assets, including collaboration receivables. To determine revenue recognition for arrangements within the scope of ASC 606, the Company performs the following five steps: (i) identify the contract(s) with the customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the entity satisfies a performance obligation. The Company only applies the five-step model to contracts when it is probable that the entity will collect the consideration it is entitled to in exchange for the goods or services it transfers to the customer. At contract inception, once the contract is determined to be within the scope of ASC 606, the Company assesses the goods or services promised within each contract, determines those that are performance obligations, and assesses whether each promised good or service is distinct. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) the performance obligation is satisfied. Depending on the nature of the performance obligation these assessments require management to make significant judgments and estimates. Exclusive Licenses If the license to the Company’s intellectual property is determined to be distinct from the other promises or performance obligations identified in the arrangement, the Company recognizes revenue from non-refundable, upfront fees allocated to the license when the license is transferred and the customer is able to use and benefit from the license. In order to assess whether the license is distinct, the Company considers the capabilities of the collaboration partner and the availability of the necessary expertise in the general marketplace to determine whether the collaboration partner can benefit from the license for its intended purpose without the receipt of the remaining elements. For licenses determined not to be distinct the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. The measure of progress, and thereby periods over which revenue should be recognized, are subject to estimates by management and may change over the course of the research and development and licensing agreement. Research and Development Services The promises under the Company’s collaboration and license agreements generally include research and development services to be performed by the Company on behalf of the collaboration partner. As the provision of research and development services is a part of the Company’s central operations, when the Company is principally responsible for the performance of these services under the agreements, the Company recognizes revenue on a gross basis for research and development services in accordance with the ASC 606 framework described above. Customer Options The Company's agreements may contain options which provide the collaboration partner the right to obtain additional licenses. If an arrangement is determined to contain customer options, the goods and services underlying the customer options are not considered to be performance obligations at the inception of the arrangement, and the associated option fees are not included in the transaction price. The Company evaluates the customer options to determine if they represent material rights, which may include options to acquire additional goods or services for free or at a discount. If the customer options are determined to represent a material right, the material right is recognized as a separate performance obligation at the outset of the arrangement. The Company allocates the transaction price to material rights based on the relative standalone selling price, which is determined based on the identified discount and the probability that the customer will exercise the option. Amounts allocated to a material right are not recognized as revenue until, at the earliest, the option is exercised. Milestone Payments At the inception of each arrangement that includes milestone payments, the Company evaluates whether the milestones are considered probable of being achieved and estimates the amount to be included in the transaction price using the most likely amount method. The Company evaluates factors such as the scientific, clinical, regulatory, commercial, and other risks that must be overcome to achieve the respective milestone in making this assessment. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the control of the Company or the licensee, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. At the end of each subsequent reporting period, the Company reevaluates the probability of achievement of all milestones subject to constraint and, if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect revenues and earnings in the period of adjustment. If a milestone or other variable consideration relates specifically to the Company's efforts to satisfy a single performance obligation or to a specific outcome from satisfying the performance obligation, the Company generally allocates the milestone amount entirely to that performance obligation. Royalties For arrangements that include sales-based royalties, including milestone payments based on a level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company recognizes revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied). To date, the Company has not recognized any royalty revenue resulting from any of its licensing arrangements. For a complete discussion of accounting for collaboration revenues, see Note 15.
Use of Estimates	The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements, and the reported amounts expensed during the reporting period. Management considers many factors in selecting appropriate financial accounting policies and controls, and in developing the estimates and assumptions that are used in the preparation of these consolidated financial statements. Management must apply significant judgment in this process. In addition, other factors may affect estimates, including: expected business and operational changes, sensitivity and volatility associated with the assumptions used in developing estimates, and whether historical trends are expected to be representative of future trends. The estimation process often may yield a range of potentially reasonable estimates of the ultimate future outcomes and management must select an amount that falls within that range of reasonable estimates. This process may result in actual results differing materially from those estimated amounts used in the preparation of the consolidated financial statements if these results differ from historical experience, or other assumptions do not turn out to be substantially accurate, even if such assumptions are reasonable when made. In preparing these consolidated financial statements, management used significant estimates in the following areas, among others: revenue recognition and the related evaluation of any constrained milestones, stock-based compensation expense, the determination of the fair value of stock-based awards, the fair value of liability-classified warrants, and accrued expenses.
Recent Accounting Pronouncements	From time to time, new accounting pronouncements are issued by the Financial Accounting Standards Board (FASB) or other standard setting bodies and adopted by the Company as of the specified effective date. Unless otherwise discussed, the Company believes that the impact of recently issued standards that are not yet effective will not have a material impact on its financial position or results of operations upon adoption. 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 June 2018, the FASB issued ASU 2018-07, Compensation - Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting. This new standard expands the scope of Topic 718 to include share-based payment transactions from nonemployees. The guidance is effective for annual and interim periods beginning after December 15, 2018. Early adoption is permitted, but no earlier than a company's adoption date of Topic 606, Revenue from Contracts with Customers. The Company is currently assessing the impact that adopting ASU 2018-07 will have on its consolidated financial statements and related disclosures.

Cash Equivalents and Short-te25

Cash Equivalents and Short-term and Long-term Investments (Tables)	6 Months Ended
	Jun. 30, 2018
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 June 30, 2018 and December 31, 2017 , (in thousands): June 30, 2018 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Cash and cash equivalents due in 90 days or less $ 79,592 $ — $ — $ 79,592 Available-for-sale securities: Corporate obligations due in one year or less 101,474 — (423 ) 101,051 Corporate obligations due in more than one year 20,848 — (182 ) 20,666 U.S. Treasury securities due in one year or less 58,825 — (123 ) 58,702 U.S. Treasury securities due in more than one year 12,342 — (39 ) 12,303 Certificates of deposit due in one year or less 5,765 — — 5,765 Certificates of deposit due in more than one year 490 — — 490 Mortgage and other asset backed securities due in one year or less 42,986 — (167 ) 42,819 Mortgage and other asset backed securities due in more than one year 10,978 — (108 ) 10,870 Total available-for-sale securities $ 253,708 $ — $ (1,042 ) $ 252,666 Total cash, cash equivalents and available-for-sale securities $ 333,300 $ — $ (1,042 ) $ 332,258 December 31, 2017 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Cash and cash equivalents due in 90 days or less $ 100,150 $ — $ — $ 100,150 Available-for-sale securities: Corporate obligations due in one year or less 99,792 — (219 ) 99,573 Corporate obligations due in more than one year 57,537 — (261 ) 57,276 U.S. Treasury securities due in one year or less 27,987 — (93 ) 27,894 U.S. Treasury securities due in more than one year 9,968 — (48 ) 9,920 Certificates of deposit due in one year or less 10,529 — — 10,529 Certificates of deposit due in more than one year 1,715 — — 1,715 Mortgage and other asset backed securities due in one year or less 39,236 — (155 ) 39,081 Mortgage and other asset backed securities due in more than one year 26,931 — (119 ) 26,812 Total available-for-sale securities $ 273,695 $ — $ (895 ) $ 272,800 Total cash, cash equivalents and available-for-sale securities $ 373,845 $ — $ (895 ) $ 372,950

Restricted Cash (Tables)

Restricted Cash (Tables)	6 Months Ended
	Jun. 30, 2018
Receivables [Abstract]
Reconciliation of Cash, Cash Equivalents and Restricted Cash	The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the condensed consolidated balance sheet that sum to the total of the same such amounts shown in the statement of cash flows (in thousands): June 30, 2018 2017 Cash and cash equivalents $ 79,592 $ 30,726 Restricted cash 1,132 1,132 Total cash, cash equivalents and restricted cash shown in the statement of cash flows $ 80,724 $ 31,858

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	6 Months Ended
	Jun. 30, 2018
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 June 30, 2018 and December 31, 2017 (in thousands): June 30, 2018 Quoted Prices in Active Markets for Identical Items (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Total Assets: Money market funds $ 72,229 $ — $ — $ 72,229 Corporate obligations — 121,717 — 121,717 U.S. Treasury securities — 71,004 — 71,004 Certificates of deposit — 6,256 — 6,256 Mortgage and other asset backed securities — 53,689 — 53,689 Restricted cash 1,132 — — 1,132 Total assets $ 73,361 $ 252,666 $ — $ 326,027 Liabilities: Warrants to purchase common stock $ — $ — $ 1,689 $ 1,689 Total liabilities $ — $ — $ 1,689 1,689 December 31, 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 $ 90,702 $ — $ — $ 90,702 Corporate obligations — 158,849 — 158,849 U.S. Treasury securities — 37,813 — 37,813 Certificates of deposit — 12,244 — 12,244 Mortgage and other asset backed securities — 67,888 — 67,888 Restricted cash 1,132 — — 1,132 Total assets $ 91,834 $ 276,794 $ — $ 368,628 Liabilities: Warrants to purchase common stock $ — $ — $ 2,236 $ 2,236 Total liabilities $ — $ — $ 2,236 $ 2,236
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): Six Months Ended June 30, 2018 2017 Beginning balance $ 2,236 $ 1,244 Change in fair value 250 272 Exercises (797 ) — Ending balance $ 1,689 $ 1,516

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	6 Months Ended
	Jun. 30, 2018
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 Six Months Ended 2018 2017 2018 2017 Outstanding stock options 3,663 3,541 3,663 3,541 Common stock warrants 39 64 39 64 Shares issuable under employee stock purchase plan 12 13 12 13 Outstanding restricted stock units (1) 667 810 667 810 4,381 4,428 4,381 4,428 (1) This balance is comprised of both the restricted stock units and performance-based restricted stock units described in Note 16.

Warrants (Tables)

Warrants (Tables)	6 Months Ended
	Jun. 30, 2018
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 June 30, 2018 December 31, 2017 Price Per Share Expiration June 30, 2018 December 31, 2017 Warrants to purchase common stock 39 61 $ 5.88 June 10, 2020 - July 9, 2020 Liability (1) Liability (1) All warrants 39 61 $ 5.88 (1) In January 2018, warrant holders exercised warrants to purchase 21,258 shares of Common Stock on a net basis, resulting in the issuance of 18,449 shares of Common Stock.

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	6 Months Ended
	Jun. 30, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Schedule of compensation costs recognized	The Company recognized stock-based compensation expense related to the 2003 Stock Option and Restricted Stock Plan (the 2003 Plan), the 2013 Equity Incentive Plan (the 2013 Plan), and the 2013 Employee Stock Purchase Plan (the 2013 ESPP) in the consolidated statements of operations and comprehensive loss during the three months ended June 30, 2018 and 2017 , respectively, as follows (in thousands): Three Months Ended Six Months Ended 2018 2017 2018 2017 Research and development $ 3,050 $ 3,433 $ 5,919 $ 7,095 General and administrative 2,910 6,155 5,736 9,077 $ 5,960 $ 9,588 $ 11,655 $ 16,172
Schedule of weighted-average assumptions used for estimating fair value	The fair value of each option issued to employees was estimated on the date of grant using the Black-Scholes option pricing model with the following weighted-average assumptions: Three Months Ended Six Months Ended 2018 2017 2018 2017 Expected volatility 62.2 % 65.8 % 62.9 % 65.9 % Expected term (in years) 6.0 6.0 6.0 6.0 Risk-free interest rate 2.8 % 1.9 % 2.7 % 2.2 % Expected dividend yield — % — % — % — %
Summary of stock option activity	The following table summarizes the stock option activity under the Company’s stock option plans during the six months ended June 30, 2018 (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, 2017 3,452 $ 29.14 6.72 Granted 717 $ 40.22 Exercised (435 ) $ 15.51 Canceled or forfeited (71 ) $ 36.40 Outstanding at June 30, 2018 3,663 $ 32.79 7.34 $ 57,667 Exercisable at June 30, 2018 2,016 $ 30.39 6.17 $ 36,595 (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 June 30, 2018 .
Schedule of nonvested restricted stock units activity	The following table summarizes the restricted stock unit (RSU) activity under the 2013 Plan during the six months ended June 30, 2018 (in thousands, except per share amounts): Number Weighted- Unvested balance at December 31, 2017 268 $ 32.44 Granted 208 $ 37.33 Vested (70 ) $ 31.26 Forfeited (8 ) $ 35.61 Unvested balance at June 30, 2018 398 $ 35.14
Share-based Compensation, Performance Shares Award Nonvested Activity	The following table summarizes PSU activity under the 2013 Plan during the six months ended June 30, 2018 (in thousands, except per share amounts): Number Weighted- Unvested balance at December 31, 2017 337 $ 31.53 Granted — $ — Vested (50 ) $ 29.92 Forfeited (18 ) $ 31.09 Unvested balance at June 30, 2018 269 $ 31.86

Segment Information (Details)

Segment Information (Details)	6 Months Ended
	Jun. 30, 2018segment
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 Millions	Jun. 30, 2018	Dec. 31, 2017
Cash and Cash Equivalents [Abstract]
Unrealized loss position, less than twelve months, fair value	$ 199.2	$ 193.6
Continuous unrealized loss position, twelve months or longer, fair value	47.2	67
Continuous unrealized loss position, fair value	$ 0.1	$ 0.3

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	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Debt Securities, Available-for-sale [Line Items]
Cash and cash equivalents, amortized cost basis	$ 79,592	$ 100,150
Cash and cash equivalents due in 90 days or less	79,592	100,150	$ 30,726
Available-for-sale securities:
Amortized Cost	253,708	273,695
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1,042)	(895)
Estimated Fair Value	252,666	272,800
Cash, Cash Equivalents, And Available-For-Sale Securities [Abstract]
Amortized Cost	333,300	373,845
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1,042)	(895)
Estimated Fair Value	332,258	372,950
Corporate obligations due in one year or less
Available-for-sale securities:
Amortized Cost	101,474	99,792
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(423)	(219)
Estimated Fair Value	101,051	99,573
Corporate obligations due in more than one year
Available-for-sale securities:
Amortized Cost	20,848	57,537
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(182)	(261)
Estimated Fair Value	20,666	57,276
U.S. Treasury securities due in one year or less
Available-for-sale securities:
Amortized Cost	58,825	27,987
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(123)	(93)
Estimated Fair Value	58,702	27,894
U.S. Treasury securities due in more than one year
Available-for-sale securities:
Amortized Cost	12,342	9,968
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(39)	(48)
Estimated Fair Value	12,303	9,920
Certificates of deposit due in one year or less
Available-for-sale securities:
Amortized Cost	5,765	10,529
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	5,765	10,529
Certificates of deposit due in more than one year
Available-for-sale securities:
Amortized Cost	490	1,715
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	490	1,715
Mortgage and other asset backed securities due in one year or less
Available-for-sale securities:
Amortized Cost	42,986	39,236
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(167)	(155)
Estimated Fair Value	42,819	39,081
Mortgage and other asset backed securities due in more than one year
Available-for-sale securities:
Amortized Cost	10,978	26,931
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(108)	(119)
Estimated Fair Value	$ 10,870	$ 26,812

Restricted Cash (Details)

Restricted Cash (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Cash and cash equivalents	$ 79,592	$ 100,150	$ 30,726
Restricted cash	1,132	1,132	1,132
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	80,724	101,282	31,858	$ 21,896
Certificates of Deposits and Money Market Funds | Letter of Credit
Accounts, Notes, Loans and Financing Receivable [Line Items]
Restricted cash	$ 1,100	$ 1,100
Accounting Standards Update 2016-18 | Certificates of Deposits and Money Market Funds | Letter of Credit
Accounts, Notes, Loans and Financing Receivable [Line Items]
Restricted cash			$ 1,100

Fair Value Measurements Financi

Fair Value Measurements Financial Instrument at Fair Value Schedule (Details) - USD ($)	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Liabilities:
Fair value measurement transfers between levels	$ 0	$ 0
Recurring
Assets:
Total assets	326,027,000	368,628,000
Liabilities:
Total liabilities	1,689,000	2,236,000
Recurring | Common stock warrants
Liabilities:
Total liabilities	1,689,000	2,236,000
Recurring | Money market funds
Assets:
Total assets	72,229,000	90,702,000
Recurring | Corporate obligations
Assets:
Total assets	121,717,000	158,849,000
Recurring | U.S. Treasury securities
Assets:
Total assets	71,004,000	37,813,000
Recurring | Certificates of deposit
Assets:
Total assets	6,256,000	12,244,000
Recurring | Mortgage and other asset backed securities
Assets:
Total assets	53,689,000	67,888,000
Recurring | Restricted cash
Assets:
Total assets	1,132,000	1,132,000
Recurring | Quoted Prices in Active Markets for Identical Items (Level 1)
Assets:
Total assets	73,361,000	91,834,000
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	72,229,000	90,702,000
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	1,132,000
Recurring | Significant Other Observable Inputs (Level 2)
Assets:
Total assets	252,666,000	276,794,000
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	121,717,000	158,849,000
Recurring | Significant Other Observable Inputs (Level 2) | U.S. Treasury securities
Assets:
Total assets	71,004,000	37,813,000
Recurring | Significant Other Observable Inputs (Level 2) | Certificates of deposit
Assets:
Total assets	6,256,000	12,244,000
Recurring | Significant Other Observable Inputs (Level 2) | Mortgage and other asset backed securities
Assets:
Total assets	53,689,000	67,888,000
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	1,689,000	2,236,000
Recurring | Significant Unobservable Inputs (Level 3) | Common stock warrants
Liabilities:
Total liabilities	1,689,000	2,236,000
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	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
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	$ 2,236	$ 1,244
Change in fair value	250	272
Exercises	(797)	0
Ending balance	$ 1,689	$ 1,516

Net Loss Per Share (Details)

Net Loss Per Share (Details) - shares shares in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Net Income (Loss) Per Share
Anti-dilutive common stock equivalents excluded from the calculation of diluted net loss per share (in shares)	4,381	4,428	4,381	4,428
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)	3,663	3,541	3,663	3,541
Common stock warrants
Net Income (Loss) Per Share
Anti-dilutive common stock equivalents excluded from the calculation of diluted net loss per share (in shares)	39	64	39	64
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)	12	13	12	13
Outstanding restricted stock units (1)
Net Income (Loss) Per Share
Anti-dilutive common stock equivalents excluded from the calculation of diluted net loss per share (in shares)	667	810	667	810

Subsequent Events (Details)

Subsequent Events (Details) - 167 Sidney Street, Cambridge, Massachusetts - Subsequent Event ft² in Thousands, $ in Millions	Jul. 30, 2018USD ($)ft²
Subsequent Event [Line Items]
Operating lease, term of contract	5 years
Operating leases, area (in sqft) | ft²	27
Total operating lease obligation | $	$ 9.9

Warrants (Details)

Warrants (Details) - $ / shares		1 Months Ended
		Jan. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Warrants
Number of shares issuable upon exercise of outstanding warrants (in shares)			39,000	61,000
Weighted-average exercise price per share (in dollars per share)			$ 5.88
Class of warrant or right number of warrants exercised on net basis (in shares)		21,258
Stock issued during period shares warrants exercise on net basis (in shares)		18,449
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)	[1]		39,000	61,000
Weighted-average exercise price per share (in dollars per share)	[1]		$ 5.88
[1]	In January 2018, warrant holders exercised warrants to purchase 21,258 shares of Common Stock on a net basis, resulting in the issuance of 18,449 shares of Common Stock.

Significant Agreements - Celgen

Significant Agreements - Celgene (Details) - Collaboration, License, and Option Agreement - Sotarcept and Luspatercept - Investor	6 Months Ended
	Jun. 30, 2018
Significant Agreements
Percentage of development costs for which collaborator is responsible	100.00%
Weighted Average
Significant Agreements
Potential royalty rate	20.00%

Significant Agreements - Accoun

Significant Agreements - Accounting Analysis (Details) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Jan. 01, 2018
Significant Agreements
Revenue from contract with customer	$ 3,685,000	$ 3,057,000	$ 6,917,000	$ 6,762,000
Deferred revenue	0		0
Accumulated deficit	(524,477,000)		(524,477,000)		$ (473,024,000)
Deferred revenue, current	0		0		541,000
Deferred revenue, net of current portion	0		0		3,161,000
Luspatercept and Amended Sotatercept Agreement
Significant Agreements
Revenue from contract with customer					192,300,000
Deferred revenue						$ 3,700,000
Development and Manufacturing Activities | Luspatercept Agreement
Significant Agreements
Revenue from contract with customer					47,900,000
Development and Manufacturing Activities | Amended Sotatercept Agreement
Significant Agreements
Revenue from contract with customer					14,900,000
Milestone Achievements | Luspatercept and Amended Sotatercept Agreement
Significant Agreements
Revenue from contract with customer					59,500,000
License and Service
Significant Agreements
Revenue from contract with customer	0	135,000	0	271,000
Upfront Fee | Luspatercept Agreement
Significant Agreements
Revenue from contract with customer					25,000,000
Upfront License and Option Payments | Collaborative Arrangement | Original and Amended Sotatercept Agreement
Significant Agreements
Revenue from contract with customer					$ 45,000,000
Cost Sharing
Significant Agreements
Revenue from contract with customer	3,685,000	2,922,000	6,917,000	6,491,000
Investor | Luspatercept | Collaboration, License, and Option Agreement
Significant Agreements
Payments received			106,000,000
Investor | Sotarcept | Collaboration, License, and Option Agreement
Significant Agreements
Payments received			44,400,000
Investor | Cost Sharing | Collaboration, License, and Option Agreement
Significant Agreements
Revenue from contract with customer	3,700,000	$ 2,900,000	6,900,000	$ 6,500,000
Clinical Milestones | Investor | Luspatercept | Collaboration, License, and Option Agreement
Significant Agreements
Collaborative arrangement potential milestone payments on commencement of clinical trial	25,000,000		25,000,000
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Significant Agreements
Accumulated deficit						3,700,000
Deferred revenue, current						500,000
Deferred revenue, net of current portion						$ 3,200,000
Calculated under Revenue Guidance in Effect before Topic 606
Significant Agreements
Deferred revenue	3,400,000		3,400,000
Calculated under Revenue Guidance in Effect before Topic 606 | License and Service
Significant Agreements
Revenue from contract with customer	$ 100,000		$ 300,000

Significant Agreements - Other

Significant Agreements - Other Agreements (Details) - USD ($)	1 Months Ended	3 Months Ended		6 Months Ended		12 Months Ended
	May 31, 2014	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2004
Significant Agreements
Milestones and fees		$ 25,933,000	$ 21,598,000	$ 49,363,000	$ 43,327,000
License Agreement | License agreement with a non-profit institution
Significant Agreements
Milestones and fees		100,000	100,000	100,000	100,000
License Agreement | License agreement with a non-profit institution | Minimum
Significant Agreements
Milestone fees payable as percentage of research and U.S. development progress and sublicensing revenue						10.00%
Royalty payable as percentage of net sales						1.00%
License Agreement | License agreement with a non-profit institution | Maximum
Significant Agreements
Milestone fees payable as percentage of research and U.S. development progress and sublicensing revenue						25.00%
Royalty payable as percentage of net sales						3.50%
License Agreement | License agreement with a non-profit institution | Sotarcept | Development milestone | Maximum
Significant Agreements
Total potential milestone payments (up to)						$ 2,000,000
License Agreement | License agreement with a non-profit institution | Luspatercept | Development milestone | Maximum
Significant Agreements
Total potential milestone payments (up to)						$ 700,000
License Agreement | Common Stock | License agreement with a non-profit institution
Significant Agreements
Number of shares issued as compensation for licenses (in shares)						62,500
Fair value of shares issued as compensation for licenses						$ 25,000
Collaborative Arrangement | License agreement with antibody technology company
Significant Agreements
Milestones and fees	$ 300,000
Non-collaborative Arrangement Transactions | License Agreement with Non Profit Institution
Significant Agreements
Milestones and fees		$ 0	$ 100,000	$ 0	$ 200,000

Stock-Based Compensation - (Det

Stock-Based Compensation - (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award
Stock-based compensation expense	$ 5,960	$ 9,588	$ 11,655	$ 16,172
2016 Consulting Agreement with Former CEO
Share-based Compensation Arrangement by Share-based Payment Award
Stock-based compensation expense	1,100	1,000	1,300	2,200
Employee Stock Purchase Plan 2013
Share-based Compensation Arrangement by Share-based Payment Award
Stock-based compensation expense	$ 100	$ 100	$ 200	$ 100

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Stock-Based Compensation Costs Recognized (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost
Stock-based compensation expense	$ 5,960	$ 9,588	$ 11,655	$ 16,172
Research and development
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost
Stock-based compensation expense	3,050	3,433	5,919	7,095
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost
Stock-based compensation expense	$ 2,910	$ 6,155	$ 5,736	$ 9,077

Stock-Based Compensation - Sc45

Stock-Based Compensation - Schedule of Weighted-Average Assumptions Used for Estimating Fair Value (Details) - Stock Option	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award
Expected volatility	62.20%	65.80%	62.90%	65.90%
Expected term (in years)	6 years	6 years	6 years	6 years
Risk-free interest rate	2.80%	1.90%	2.70%	2.20%
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		6 Months Ended	12 Months Ended
		Jun. 30, 2018	Dec. 31, 2017
Number of Stock Options
Granted (in shares)		717,135
Exercised (in shares)		(434,627)
Aggregate Intrinsic Value
Weighted average grant date fair value (in usd per share)		$ 23.80
Total proceeds from options exercised		$ 6,700
Aggregate intrinsic value of options exercised		$ 11,800
Stock Option
Number of Stock Options
Outstanding at the beginning of period (in shares)		3,452,000
Granted (in shares)		717,000
Exercised (in shares)		(435,000)
Canceled or forfeited (in shares)		(71,000)
Outstanding at the end of the period (in shares)		3,663,000	3,452,000
Exercisable at the end of the period (in shares)		2,016,000
Weighted- Average Exercise Price Per Share
Outstanding at the beginning of period (in usd per share)		$ 29.14
Granted (in usd per share)		40.22
Exercised (in usd per share)		15.51
Canceled or forfeited (in usd per share)		36.40
Outstanding at the end of the period (in usd per share)		32.79	$ 29.14
Exercisable at the end of the period (in usd per share)		$ 30.39
Weighted- Average Contractual Life (in years)
Outstanding at the beginning of the period		7 years 4 months 2 days	6 years 8 months 19 days
Outstanding at the end of the period		7 years 4 months 2 days	6 years 8 months 19 days
Exercisable at the end of the period		6 years 2 months 1 day
Aggregate Intrinsic Value
Outstanding at the end of the period	[1]	$ 57,667
Exercisable at the end of the period	[1]	36,595
Unrecognized compensation expense		$ 30,700
Weighted-average period for recognition		2 years 6 months 29 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 June 30, 2018.

Stock-Based Compensation - Sc47

Stock-Based Compensation - Schedule of Nonvested Restricted Stock Activity (Details) $ / shares in Units, shares in Thousands, $ in Millions	6 Months Ended
	Jun. 30, 2018USD ($)$ / sharesshares
Outstanding restricted stock units (1)
Number of Grants
Unvested balance at beginning of the period (in shares) | shares	268
Granted (in shares) | shares	208
Vested (in shares) | shares	(70)
Forfeited (in shares) | shares	(8)
Unvested balance at end of the period (in shares) | shares	398
Weighted- Average Grant Date Fair Value
Unvested balance at beginning of the period (in usd per share) | $ / shares	$ 32.44
Granted (in usd per share) | $ / shares	37.33
Vested (in usd per share) | $ / shares	31.26
Forfeited (in usd per share) | $ / shares	35.61
Unvested balance at end of the period (in usd per share) | $ / shares	$ 35.14
Unrecognized compensation cost | $	$ 11.3
Weighted-average period for recognition	2 years 15 days
Performance-Based Restricted Stock Units
Number of Grants
Unvested balance at beginning of the period (in shares) | shares	337
Granted (in shares) | shares	0
Vested (in shares) | shares	(50)
Forfeited (in shares) | shares	(18)
Unvested balance at end of the period (in shares) | shares	269
Weighted- Average Grant Date Fair Value
Unvested balance at beginning of the period (in usd per share) | $ / shares	$ 31.53
Granted (in usd per share) | $ / shares	0
Vested (in usd per share) | $ / shares	29.92
Forfeited (in usd per share) | $ / shares	31.09
Unvested balance at end of the period (in usd per share) | $ / shares	$ 31.86
Unrecognized compensation cost | $	$ 2.8
Weighted-average period for recognition	1 year 7 days

Income Taxes (Details)

Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Tax Cuts And Jobs Act Of 2017, provisional income tax expense (benefit)					$ 59,300
Income tax expense	$ 11	$ 6	$ 21	$ 12

Related Party Transactions (Det

Related Party Transactions (Details) - Celgene - USD ($)	Jun. 30, 2018	Dec. 31, 2017
Related Party Transactions
Ownership percentage of entity's fully diluted equity	12.30%	12.50%
Deferred revenue	$ 0