Acceleron Pharma (XLRN) 10-Q 6 May 21 2021 Q1 Quarterly report Financial data

Cover

Cover - shares	3 Months Ended
	Mar. 31, 2021	Apr. 30, 2021
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Mar. 31, 2021
Document Transition Report	false
Entity File Number	001-36065
Entity Registrant Name	ACCELERON PHARMA INC
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	27-0072226
Entity Address, Address Line One	128 Sidney Street
Entity Address, City or Town	Cambridge
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02139
City Area Code	617
Local Phone Number	649-9200
Title of 12(b) Security	Common Stock, $0.001 per share
Trading Symbol	XLRN
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding (in shares)		60,699,406
Entity Central Index Key	0001280600
Amendment Flag	false
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q1
Current Fiscal Year End Date	--12-31

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 455,513	$ 670,952
Short-term investments	231,736	178,951
Collaboration receivables (all amounts are with a related party)	25,207	26,101
Prepaid expenses and other current assets	25,156	17,265
Total current assets	737,612	893,269
Property and equipment, net	7,856	7,768
Operating lease - right of use asset, net	20,415	21,988
Long-term investments	108,193	7,585
Other assets	1,717	1,727
Total assets	875,793	932,337
Current liabilities:
Accounts payable	4,463	8,472
Accrued expenses	38,074	44,681
Operating lease liability - right of use	7,282	7,010
Total current liabilities	49,819	60,163
Operating lease liability - right of use, net of current portion	15,143	17,067
Total liabilities	64,962	77,230
Commitments and Contingencies	0	0
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; 60,638,494 and 60,383,867 shares issued and outstanding at March 31, 2021 and December 31, 2020, respectively	60	60
Additional paid-in capital	1,752,096	1,732,772
Accumulated deficit	(940,908)	(877,437)
Accumulated other comprehensive loss	(417)	(288)
Total stockholders’ equity	810,831	855,107
Total liabilities and stockholders’ equity	$ 875,793	$ 932,337

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Mar. 31, 2021	Dec. 31, 2020
Statement of Financial Position [Abstract]
Preferred stock, par value (in usd per share)	$ 0.001	$ 0.001
Preferred stock, shares authorized (in shares)	25,000,000	25,000,000
Preferred stock, shares issued (in shares)	0	0
Preferred stock, shares outstanding (in shares)	0	0
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common stock, shares authorized (in shares)	175,000,000	175,000,000
Common stock, shares issued (in shares)	60,638,494	60,383,867
Common stock, shares outstanding (in shares)	60,638,494	60,383,867

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Collaboration revenue:
Revenues	$ 24,758	$ 4,344
Costs and expenses:
Research and development	57,299	37,663
Selling, general and administrative	31,062	18,253
Total costs and expenses	88,361	55,916
Loss from operations	(63,603)	(51,572)
Other, income net	137	648
Loss before income taxes	(63,466)	(50,924)
Income tax provision	(5)	(15)
Net loss	(63,471)	(50,939)
Other comprehensive loss:
Net unrealized holding losses on short-term and long-term investments during the period, net of tax	(129)	(267)
Comprehensive loss	$ (63,600)	$ (51,206)
Net loss per share- basic and diluted (in dollars per share)	$ (1.05)	$ (0.95)
Weighted-average number of common shares used in computing net loss per share- basic and diluted (in shares)	60,579	53,361
Cost-sharing, net
Collaboration revenue:
Revenues	$ 2,362	$ 2,824
Royalty
Collaboration revenue:
Revenues	$ 22,396	$ 1,520

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Deficit	Accumulated Other Comprehensive Income (Loss)
Stockholders' equity beginning balance (in shares) at Dec. 31, 2019		53,123,567
Stockholders' equity beginning balance at Dec. 31, 2019	$ 449,476	$ 53	$ 1,160,807	$ (711,407)	$ 23
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	6,679		6,679
Exercise of stock options (in shares)		295,757
Exercise of stock options	8,485		8,485
Vesting of restricted stock units, net of shares withheld for taxes (in shares)		77,949
Vesting of restricted stock units, net of shares withheld for taxes	(472)		(472)
Issuance of common stock related to ESPP (in shares)		22,647
Issuance of common stock related to ESPP	860		860
Unrealized loss on available-for-sale securities, net of tax	(267)				(267)
Net loss	(50,939)			(50,939)
Stockholders' equity ending balance (in shares) at Mar. 31, 2020		53,519,920
Stockholders' equity ending balance at Mar. 31, 2020	413,822	$ 53	1,176,359	(762,346)	(244)
Stockholders' equity beginning balance (in shares) at Dec. 31, 2020		60,383,867
Stockholders' equity beginning balance at Dec. 31, 2020	855,107	$ 60	1,732,772	(877,437)	(288)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	$ 15,626		15,626
Exercise of stock options (in shares)	150,000	149,724
Exercise of stock options	$ 5,865		5,865
Vesting of restricted stock units, net of shares withheld for taxes (in shares)		84,902
Vesting of restricted stock units, net of shares withheld for taxes	(3,888)		(3,888)
Issuance of common stock related to ESPP (in shares)		20,001
Issuance of common stock related to ESPP	1,721		1,721
Unrealized loss on available-for-sale securities, net of tax	(129)				(129)
Net loss	(63,471)			(63,471)
Stockholders' equity ending balance (in shares) at Mar. 31, 2021		60,638,494
Stockholders' equity ending balance at Mar. 31, 2021	$ 810,831	$ 60	$ 1,752,096	$ (940,908)	$ (417)

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity (Parenthetical) - $ / shares	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Statement of Stockholders' Equity [Abstract]
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001	$ 0.001	$ 0.001

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Operating Activities
Net loss	$ (63,471)	$ (50,939)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	1,026	996
Stock-based compensation	15,626	6,679
Other non-cash items	(3,218)	1,365
Changes in assets and liabilities:
Prepaid expenses and other assets	(7,882)	828
Collaboration receivables (all amounts are with a related party)	894	112
Non-cash lease expense	1,574	1,377
Accounts payable	(4,178)	1,634
Accrued expenses	(6,643)	(7,148)
Operating lease obligations	(1,652)	(1,466)
Other changes in operating assets and liabilities	0	17
Net cash used in operating activities	(67,924)	(46,545)
Investing Activities
Purchases of investments	(270,144)	(10,454)
Proceeds from sales and maturities of investments	119,851	88,361
Purchases of property and equipment	(920)	(426)
Net cash (used in) provided by investing activities	(151,213)	77,481
Financing Activities
Net proceeds from exercises and vesting of stock awards, and ESPP contributions	3,698	8,873
Net cash provided by financing activities	3,698	8,873
Net (decrease) increase in cash, cash equivalents and restricted cash	(215,439)	39,809
Cash, cash equivalents and restricted cash at beginning of period	672,549	239,274
Cash, cash equivalents and restricted cash at end of period	457,110	279,083
Supplemental Disclosure of Non-Cash Investing and Financing Activities:
Purchase of property and equipment included in accounts payable and accrued expenses	$ 205	$ 425

Nature of Business

Nature of Business	3 Months Ended
	Mar. 31, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of Business	Nature of Business Acceleron Pharma Inc. (Acceleron or the Company) is a Cambridge, Massachusetts-based 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 or successfully commercializes its products, the need for substantial additional financing, risks of relying on third parties, risks of clinical trial failures, dependence on key personnel, protection of proprietary technology, and compliance with government regulations.

Basis of Presentation

Basis of Presentation	3 Months Ended
	Mar. 31, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	Basis of Presentation The accompanying interim condensed consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (GAAP). Any reference in these notes to applicable guidance is meant to refer to the authoritative United States generally accepted accounting principles as found in the Accounting Standards Codification (ASC) and Accounting Standards Update (ASU) of the Financial Accounting Standards Board (FASB). The accompanying interim condensed consolidated financial statements are unaudited and reflect the application of certain significant accounting policies as described below and elsewhere in these notes to the financial statements. As of March 31, 2021, 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, 2020, have not changed, and the unaudited interim financial statements have been prepared on the same basis as the audited annual financial statements as of and for the year ended December 31, 2020. In the opinion of management, the accompanying interim condensed consolidated financial statements reflect all adjustments, consisting of normal recurring adjustments, necessary for the fair presentation of the Company’s financial position as of March 31, 2021, the results of its operations for the three months ended March 31, 2021 and 2020, and its cash flows for the three months ended March 31, 2021 and 2020. The accompanying interim condensed consolidated financial statements include the results of operations of the Company and its wholly-owned subsidiaries. All intercompany accounts and transactions have been eliminated in consolidation. The results for the three months ended March 31, 2021 are not necessarily indicative of the results to be expected for the year ending December 31, 2021, 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, 2020, 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, 2020.

Use of Estimates

Use of Estimates	3 Months Ended
	Mar. 31, 2021
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 condensed 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 condensed 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 condensed consolidated financial statements, management used estimates in the following areas,

Segment Information

Segment Information	3 Months Ended
	Mar. 31, 2021
Segment Reporting [Abstract]
Segment Information	Segment InformationOperating 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, Cash Equivalents and Shor

Cash, Cash Equivalents and Short-term and Long-term Investments	3 Months Ended
	Mar. 31, 2021
Cash and Cash Equivalents [Abstract]
Cash, Cash Equivalents and Short-term and Long-term Investments	Cash, 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 value. The Company determines the appropriate classification of marketable securities at the time of purchase and reevaluates such designation at each balance sheet date. The Company has classified all of its marketable securities at March 31, 2021 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 years ended the three months ended March 31, 2021, and 2020. 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 interest and dividends on securities classified as available-for-sale in interest income in the accompanying condensed consolidated statements of operations and comprehensive loss. Accrued interest receivable relating to the Company's available-for-sale securities is presented within prepaid expenses and other current assets in the accompanying condensed consolidated balance sheets, and amounted to $1.2 million and $0.3 million at March 31, 2021 and December 31, 2020, respectively. In June 2016, the FASB issued Accounting Standards Update 2016-13, Financial Instruments-Credit Losses . The new standard requires an estimate of expected credit losses only when the fair value of an available-for-sale debt security is below its amortized cost basis, and credit losses are limited to the amount by which the security’s amortized cost basis exceeds its fair value. Credit-related impairment is recognized as an allowance for credit losses on the balance sheet with a corresponding adjustment to earnings. The standard additionally requires an investor to determine whether a decline in the fair value below the amortized cost basis of an available-for-sale debt security is due to credit-related factors or noncredit-related factors. Any impairment that is not credit related is recognized in other comprehensive income, net of applicable taxes. The Company adopted ASU 2016-13 effective January 1, 2020, with no material impact on its consolidated financial statements and related disclosures. The following is a summary of available-for-sale securities with unrealized losses for less than 12 months as of March 31, 2021 and December 31, 2020 (in thousands): March 31, 2021 December 31, 2020 Fair Value Unrealized Losses Fair Value Unrealized Losses Corporate obligations $ 229,629 $ (144) $ 54,724 $ (13) U.S. Treasury securities 19,554 (8) 72,289 (7) Mortgage and other asset backed securities — — 4,998 (2) Total available-for sale securities in an unrealized loss position $ 249,183 $ (152) $ 132,012 $ (22) At March 31, 2021, our security portfolio consisted of 99 securities related to investments in debt securities available-for-sale, of which 79 securities were in an unrealized loss position. There were no securities in an unrealized loss position for greater than 12 months as of March 31, 2021. The unrealized losses on the Company's available-for-sale securities were caused by central bank and market interest rate decreases on securities purchased at a premium. The contractual terms of these investments do not permit the issuer to settle the securities at a price less than the amortized cost bases of the investments. The Company does not intend to sell the investments and it is not more likely than not that the Company will be required to sell the investments before recovery of their amortized cost bases. The Company did not record an allowance for credit losses as of March 31, 2021. The following is a summary of cash, cash equivalents and available-for-sale securities as of March 31, 2021 and December 31, 2020 (in thousands): March 31, 2021 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Cash and cash equivalents due in 90 days or less $ 455,514 $ — $ (1) $ 455,513 Available-for-sale securities: Corporate obligations 265,321 4 (144) 265,181 U.S. Treasury securities 74,750 6 (8) 74,748 Total available-for-sale securities (1) $ 340,071 $ 10 $ (152) $ 339,929 Total cash, cash equivalents and available-for-sale securities $ 795,585 $ 10 $ (153) $ 795,442 December 31, 2020 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Cash and cash equivalents due in 90 days or less $ 670,952 $ — $ — $ 670,952 Available-for-sale securities: Corporate obligations 45,989 5 (13) 45,981 U.S. Treasury securities 135,315 3 (7) 135,311 Certificates of deposit 245 1 — 246 Mortgage and other asset backed securities 5,000 — (2) 4,998 Total available-for-sale securities (1) $ 186,549 $ 9 $ (22) $ 186,536 Total cash, cash equivalents and available-for-sale securities $ 857,501 $ 9 $ (22) $ 857,488 (1) All available-for-sale securities mature within two and three years as of March 31, 2021 and December 31, 2020, respectively.

Restricted Cash

Restricted Cash	3 Months Ended
	Mar. 31, 2021
Receivables [Abstract]
Restricted Cash	Restricted Cash The following table provides a reconciliation of cash, cash equivalents and restricted cash reported within the condensed consolidated balance sheet that sum to the total of the same such amounts shown in the statements of cash flows (in thousands): March 31, 2021 2020 Cash and cash equivalents $ 455,513 $ 277,486 Restricted cash 1,597 1,597 Total cash, cash equivalents and restricted cash shown in the statement of cash flows $ 457,110 $ 279,083 As of March 31, 2021 and December 31, 2020, the Company maintained letters of credit totaling $1.6 million held in the form of certificates of deposit and money market funds as collateral for the Company's facility lease obligation and its credit cards. Restricted cash is included within other assets in our condensed consolidated balance sheet.

Concentrations of Credit Risk a

Concentrations of Credit Risk and Off-Balance Sheet Risk	3 Months Ended
	Mar. 31, 2021
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 collaboration partner. 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 allowance for credit losses has been recorded for the Company's collaboration receivables as of March 31, 2021 and December 31, 2020.

Fair Value Measurements

Fair Value Measurements	3 Months Ended
	Mar. 31, 2021
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 that is significant to each financial instrument as of March 31, 2021 and December 31, 2020 (in thousands): March 31, 2021 Quoted Prices Significant Other Significant Total Assets: Money market funds $ 396,088 $ — $ — $ 396,088 Corporate obligations — 276,382 — 276,382 U.S. Treasury securities — 74,748 — 74,748 Total assets $ 396,088 $ 351,130 $ — $ 747,218 December 31, 2020 Quoted Prices Significant Other Significant Total Assets: Money market funds $ 266,562 $ — $ — $ 266,562 Corporate obligations — 60,982 — 60,982 U.S. Treasury securities — 215,310 — 215,310 Certificates of deposit — 246 — 246 Mortgage and other asset backed securities — 4,998 — 4,998 Total assets $ 266,562 $ 281,536 $ — $ 548,098 The money market funds noted above are included in cash and cash equivalents in the accompanying condensed consolidated balance sheets. The Company recognizes transfers between levels of the fair value hierarchy as of the end of the reporting period. There were no transfers within the hierarchy during the three months ended March 31, 2021 or the year ended December 31, 2020. Items measured at fair value on a recurring basis include short-term and long-term investments (Note 5).

Net Loss Per Share

Net Loss Per Share	3 Months Ended
	Mar. 31, 2021
Earnings Per Share [Abstract]
Net Loss Per Share	Net Loss Per ShareThe 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 March 31, 2021 2020 Outstanding stock options 3,766 4,232 Common stock warrants — 39 Shares issuable under employee stock purchase plan 9 13 Outstanding restricted stock units (1) 707 557 4,482 4,841 (1) This balance is comprised of both the restricted stock units and performance-based restricted stock units described in Note 14.

Comprehensive Income (Loss)

Comprehensive Income (Loss)	3 Months Ended
	Mar. 31, 2021
Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract]
Comprehensive Income (Loss)	Comprehensive Income (Loss)Comprehensive income (loss) is defined as the change in equity of a business enterprise during a period from transactions, other events, and circumstances from non-owner sources. Comprehensive income (loss) consists of net loss and other comprehensive income (loss), which includes certain changes in equity that are excluded from net loss. Comprehensive income (loss) has been disclosed in the accompanying condensed consolidated statements of operations and comprehensive loss. Accumulated other comprehensive loss is presented separately on the condensed consolidated balance sheets and consists entirely of unrealized holdings gains or losses on investments as of March 31, 2021 and December 31, 2020.

Recent Accounting Pronouncement

Recent Accounting Pronouncements	3 Months Ended
	Mar. 31, 2021
Accounting Standards Update and Change in Accounting Principle [Abstract]
Recent Accounting Pronouncements	Recent Accounting Pronouncements Not Yet Adopted From time to time, new accounting pronouncements are issued by the FASB or other standard setting bodies and adopted by the Company as of the specified effective date. Unless otherwise discussed, the Company believes that the impact of recently issued standards that are not yet effective will not have a material impact on its financial position or results of operations upon adoption.

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 2021
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies Operating Leases The Company leases its facilities under non-cancelable operating leases that expire at various dates from September 2023 through May 2026. All of the Company's leases contain escalating rent clauses, which require higher rent payments in future years. See Note 2 and Note 14 in our Annual Report on Form 10-K for the year ended December 31, 2020 for additional information regarding the Company's operating leases. Legal Proceedings The Company, from time to time, may be party to litigation arising in the ordinary course of its business. The Company was not subject to any material legal proceedings during the three months ended March 31, 2021, and, to the best of its knowledge, no material legal proceedings are currently pending or threatened. Other The Company is also party to various agreements, principally relating to licensed technology, that require future payments relating to milestones not met at March 31, 2021 and December 31, 2020, or royalties on future sales of specified products. See Note 13 for discussion of these arrangements. The Company enters into standard indemnification agreements in the ordinary course of business. Pursuant to the agreements, the Company indemnifies, holds harmless, and agrees to reimburse the indemnified party for losses suffered or incurred by the indemnified party, generally the Company’s business partners or customers, in connection with any U.S. patent or any copyright or other intellectual property infringement claim by any third party with respect to the Company’s products. The term of these indemnification agreements is generally perpetual any time after execution of the agreement. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is unlimited. The Company has never incurred costs to defend lawsuits or settle claims related to these indemnification agreements.

Significant Agreements

Significant Agreements	3 Months Ended
	Mar. 31, 2021
Significant Agreements [Abstract]
Significant Agreements	Significant Agreements BMS (Bristol Myers Squibb Company) Overview On February 20, 2008, the Company entered into an agreement with Celgene, which was acquired by BMS in November 2019 and is now referred to herein as BMS, 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, and clarified certain responsibilities of the Company and BMS in a letter agreement to the Restated Sotatercept Agreement on March 10, 2020 (collectively, the Restated Sotatercept Agreement). On August 2, 2011, the Company entered into a second agreement with BMS for REBLOZYL® (luspatercept-aamt) (the REBLOZYL Agreement, formerly the Luspatercept Agreement). Since December 31, 2020, 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, 2020. Accounting Analysis Upon adoption of ASC 606, all of the Company’s performance obligations pursuant to its arrangements with BMS were completed and all remaining potential milestone payments were fully constrained as they related to future regulatory events that were outside of the Company’s control, and therefore the risk of significant reversal in the amount of cumulative revenue had not been resolved. As of March 31, 2021, the last remaining regulatory milestone payment for REBLOZYL would be $20.0 million and would result from approval by a regulatory authority in Asia (as defined in the REBLOZYL Agreement) of a Biologics License Application (BLA) or equivalent for luspatercept-aamt in either myelodysplastic syndromes or beta-thalassemia. In accordance with the Company's accounting policy regarding revenue recognition as described in Note 2 to its Annual Report on Form 10-K, the revenue associated with this milestone will be recognized once it is probable that the application is approved by the regulatory authority. Milestone payments that are not within the control of the Company or the licensee are not considered probable of being achieved until those approvals are received. The approval of the application is not within the control of the Company or the licensee, and therefore, as of March 31, 2021, the Company cannot determine if it is probable that a regulatory agency will approve the applications. Through March 31, 2021, under all BMS arrangements, the Company has received net cost-share payments, milestones, and royalties of $282.2 million and $45.0 million for REBLOZYL and sotatercept, respectively. Other Agreements 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. The Company agreed to pay specified development milestone payments totaling up to $2.0 million for sotatercept and $0.7 million for REBLOZYL. 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 developed under the licenses. During the three months ended March 31, 2021 and 2020, the Company expensed zero and $0.2 million, respectively, of milestones and fees. Milestones and fees associated with development related activities are recorded as research and development expense. During the three months ended March 31, 2021 and 2020, the Company expensed $1.2 million and $0.1 million, respectively, of royalties. Costs related to royalties on sales of commercial products are recorded as selling, general and administrative expense. In May 2014, the Company executed a collaboration agreement with a research technology company, and such collaboration agreement was amended and restated in March 2019. The Company paid an upfront research fee of $0.3 million upon execution of the original agreement. The Company also received an option to obtain a commercial license to the molecules developed during the collaboration. During the three months ended March 31, 2021 and 2020, the Company did not reach any milestones defined under the agreement and, therefore, no amounts have been paid or expensed. In December 2019, the Company executed a license and collaboration agreement with Fulcrum Therapeutics to identify small molecules designed to modulate specific pathways associated with a targeted indication within the pulmonary disease space. The Company paid an upfront research fee of $10.0 million upon execution of this agreement, which was expensed to research and development. The Company also agreed to pay specified research, development and commercial milestone payments of up to $295.0 million for a first product commercialized and up to a maximum of $143.5 million in additional milestone payments for all subsequent products commercialized. Fulcrum will additionally receive tiered royalty

Stock-Based Compensation

Stock-Based Compensation	3 Months Ended
	Mar. 31, 2021
Share-based Payment Arrangement [Abstract]
Stock-Based Compensation	Stock-Based Compensation During the quarter ended March 31, 2021, the Company updated the equity award agreements with respect to the 2013 Equity Incentive Plan (the 2013 Plan) to enhance the vesting terms for retirement eligible employees and directors for awards granted in 2021 and future awards. Where awards are granted with non-substantive vesting periods (for instance, where all or a portion of the award continues to vest upon retirement eligibility), the Company recognizes expense based on the period from the grant date to the date the employee becomes retirement eligible. The Company recognized stock-based compensation expense related to the 2003 Stock Option and Restricted Stock Plan (the 2003 Plan), the 2013 Plan, and the 2013 Employee Stock Purchase Plan (the 2013 ESPP) in the condensed consolidated statements of operations and comprehensive loss during the three months ended March 31, 2021 and 2020, respectively, as follows (in thousands): Three Months Ended March 31, 2021 2020 Research and development $ 8,158 $ 3,141 Selling, general and administrative 7,468 3,538 $ 15,626 $ 6,679 Stock Options The following table summarizes the stock option activity under the Company’s stock option plans during the three months ended March 31, 2021 (in thousands, except per share amounts and years): Number Weighted- Weighted- Aggregate Outstanding at December 31, 2020 3,378 $ 49.78 7.35 $ 264,046 Granted 573 $ 116.85 Exercised (150) $ 39.17 Canceled or forfeited (35) $ 70.11 Outstanding at March 31, 2021 3,766 $ 60.22 7.63 $ 284,022 Exercisable at March 31, 2021 1,781 $ 40.26 6.29 $ 169,844 (1) The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the estimated fair value of the common stock for the options that were in the money at March 31, 2021. The aggregate intrinsic value of options exercised during the three months ended March 31, 2021 was $13.1 million. As of March 31, 2021, there was $71.6 million of unrecognized compensation expense related to unvested stock options that is expected to be recognized over a weighted-average period of 2.78 years. Restricted Stock Units The following table summarizes the restricted stock unit (RSU) activity under the 2013 Plan during the three months ended March 31, 2021 (in thousands, except per share amounts): Number Weighted- Unvested balance at December 31, 2020 494 $ 59.16 Granted 180 116.30 Vested (116) 48.69 Forfeited (7) 75.17 Unvested balance at March 31, 2021 551 $ 79.81 As of March 31, 2021, there was approximately $35.8 million of related unrecognized compensation cost, which the Company expects to recognize over a remaining weighted-average period of 1.91 years. Performance-Based Restricted Stock Units On January 22, 2020 and January 29, 2021, the Company granted performance-based restricted stock units (PSU) whereby vesting depends upon the occurrence of certain milestone events by December 31, 2022 and December 31, 2023, respectively. As of March 31, 2021, none of the PSU milestones had been achieved. When achievement of a milestone becomes probable, compensation cost will be recognized from the grant date over the requisite service period. As of March 31, 2021, no related compensation cost has been recognized. The following table summarizes PSU activity under the 2013 Plan during the three months ended March 31, 2021 (in thousands, except per share amounts): Number Weighted- Unvested balance at December 31, 2020 75 $ 52.99 Granted (1) 83 115.53 Vested — — Forfeited (2) 52.99 Unvested balance at March 31, 2021 156 $ 86.01 (1) Pursuant to the terms of the awards granted on January 22, 2020 and January 29, 2021, the actual number of awards earned could range between 0% and 200% of the number of awards granted. As of March 31, 2021, there was approximately $13.4 million of related unrecognized compensation cost. Depending on the actual number of awards earned, the actual expense recognized could range between 0% and 200% of this amount.

Income Taxes

Income Taxes	3 Months Ended
	Mar. 31, 2021
Income Tax Disclosure [Abstract]
Income Taxes	Income TaxesDeferred tax assets and deferred tax liabilities are recognized based on temporary differences between the financial reporting and tax basis of assets and liabilities using statutory rates. A valuation allowance is recorded against deferred tax assets if it is more likely than not that some or all of the deferred tax assets will not be realized. Due to the uncertainty surrounding the realization of the favorable tax attributes in future tax returns, the Company has recorded a full valuation allowance against the Company’s otherwise recognizable net deferred tax assets.

Related Party Transactions

Related Party Transactions	3 Months Ended
	Mar. 31, 2021
Related Party Transactions [Abstract]
Related Party Transactions	Related Party Transactions BMS BMS owned 10.7% and 10.8% of the Company’s fully diluted equity as of March 31, 2021 and December 31, 2020, respectively. Refer to Note 13 for additional information regarding this collaboration arrangement.

Subsequent Events

Subsequent Events	3 Months Ended
	Mar. 31, 2021
Subsequent Events [Abstract]
Subsequent Events	Subsequent EventsThe 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.

Basis of Presentation (Policies

Basis of Presentation (Policies)	3 Months Ended
	Mar. 31, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	The accompanying interim condensed consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (GAAP). Any reference in these notes to applicable guidance is meant to refer to the authoritative United States generally accepted accounting principles as found in the Accounting Standards Codification (ASC) and Accounting Standards Update (ASU) of the Financial Accounting Standards Board (FASB). The accompanying interim condensed consolidated financial statements are unaudited and reflect the application of certain significant accounting policies as described below and elsewhere in these notes to the financial statements. As of March 31, 2021, 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, 2020, have not changed, and the unaudited interim financial statements have been prepared on the same basis as the audited annual financial statements as of and for the year ended December 31, 2020. In the opinion of management, the accompanying interim condensed consolidated financial statements reflect all adjustments, consisting of normal recurring adjustments, necessary for the fair presentation of the Company’s financial position as of March 31, 2021, the results of its operations for the three months ended March 31, 2021 and 2020, and its cash flows for the three months ended March 31, 2021 and 2020. The accompanying interim condensed consolidated financial statements include the results of operations of the Company and its wholly-owned subsidiaries. All intercompany accounts and transactions have been eliminated in consolidation. The results for the three months ended March 31, 2021 are not necessarily indicative of the results to be expected for the year ending December 31, 2021, 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, 2020, 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, 2020.
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 condensed 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 condensed 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 condensed consolidated financial statements, management used estimates in the following areas,
Recent Accounting Pronouncements	Not Yet Adopted From time to time, new accounting pronouncements are issued by the FASB or other standard setting bodies and adopted by the Company as of the specified effective date. Unless otherwise discussed, the Company believes that the impact of recently issued standards that are not yet effective will not have a material impact on its financial position or results of operations upon adoption.

Cash, Cash Equivalents and Sh_2

Cash, Cash Equivalents and Short-term and Long-term Investments (Tables)	3 Months Ended
	Mar. 31, 2021
Cash and Cash Equivalents [Abstract]
Available-for-Sale Securities with Unrealized Losses	The following is a summary of available-for-sale securities with unrealized losses for less than 12 months as of March 31, 2021 and December 31, 2020 (in thousands): March 31, 2021 December 31, 2020 Fair Value Unrealized Losses Fair Value Unrealized Losses Corporate obligations $ 229,629 $ (144) $ 54,724 $ (13) U.S. Treasury securities 19,554 (8) 72,289 (7) Mortgage and other asset backed securities — — 4,998 (2) Total available-for sale securities in an unrealized loss position $ 249,183 $ (152) $ 132,012 $ (22)
Available-for-Sale Securities	The following is a summary of cash, cash equivalents and available-for-sale securities as of March 31, 2021 and December 31, 2020 (in thousands): March 31, 2021 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Cash and cash equivalents due in 90 days or less $ 455,514 $ — $ (1) $ 455,513 Available-for-sale securities: Corporate obligations 265,321 4 (144) 265,181 U.S. Treasury securities 74,750 6 (8) 74,748 Total available-for-sale securities (1) $ 340,071 $ 10 $ (152) $ 339,929 Total cash, cash equivalents and available-for-sale securities $ 795,585 $ 10 $ (153) $ 795,442 December 31, 2020 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Cash and cash equivalents due in 90 days or less $ 670,952 $ — $ — $ 670,952 Available-for-sale securities: Corporate obligations 45,989 5 (13) 45,981 U.S. Treasury securities 135,315 3 (7) 135,311 Certificates of deposit 245 1 — 246 Mortgage and other asset backed securities 5,000 — (2) 4,998 Total available-for-sale securities (1) $ 186,549 $ 9 $ (22) $ 186,536 Total cash, cash equivalents and available-for-sale securities $ 857,501 $ 9 $ (22) $ 857,488 (1) All available-for-sale securities mature within two and three years as of March 31, 2021 and December 31, 2020, respectively.

Restricted Cash (Tables)

Restricted Cash (Tables)	3 Months Ended
	Mar. 31, 2021
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 statements of cash flows (in thousands): March 31, 2021 2020 Cash and cash equivalents $ 455,513 $ 277,486 Restricted cash 1,597 1,597 Total cash, cash equivalents and restricted cash shown in the statement of cash flows $ 457,110 $ 279,083
Restrictions on Cash and Cash Equivalents	The following table provides a reconciliation of cash, cash equivalents and restricted cash reported within the condensed consolidated balance sheet that sum to the total of the same such amounts shown in the statements of cash flows (in thousands): March 31, 2021 2020 Cash and cash equivalents $ 455,513 $ 277,486 Restricted cash 1,597 1,597 Total cash, cash equivalents and restricted cash shown in the statement of cash flows $ 457,110 $ 279,083

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	3 Months Ended
	Mar. 31, 2021
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 that is significant to each financial instrument as of March 31, 2021 and December 31, 2020 (in thousands): March 31, 2021 Quoted Prices Significant Other Significant Total Assets: Money market funds $ 396,088 $ — $ — $ 396,088 Corporate obligations — 276,382 — 276,382 U.S. Treasury securities — 74,748 — 74,748 Total assets $ 396,088 $ 351,130 $ — $ 747,218 December 31, 2020 Quoted Prices Significant Other Significant Total Assets: Money market funds $ 266,562 $ — $ — $ 266,562 Corporate obligations — 60,982 — 60,982 U.S. Treasury securities — 215,310 — 215,310 Certificates of deposit — 246 — 246 Mortgage and other asset backed securities — 4,998 — 4,998 Total assets $ 266,562 $ 281,536 $ — $ 548,098

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	3 Months Ended
	Mar. 31, 2021
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 March 31, 2021 2020 Outstanding stock options 3,766 4,232 Common stock warrants — 39 Shares issuable under employee stock purchase plan 9 13 Outstanding restricted stock units (1) 707 557 4,482 4,841 (1) This balance is comprised of both the restricted stock units and performance-based restricted stock units described in Note 14.

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	3 Months Ended
	Mar. 31, 2021
Share-based Payment Arrangement [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 Plan, and the 2013 Employee Stock Purchase Plan (the 2013 ESPP) in the condensed consolidated statements of operations and comprehensive loss during the three months ended March 31, 2021 and 2020, respectively, as follows (in thousands): Three Months Ended March 31, 2021 2020 Research and development $ 8,158 $ 3,141 Selling, general and administrative 7,468 3,538 $ 15,626 $ 6,679
Summary of Stock Option Activity	The following table summarizes the stock option activity under the Company’s stock option plans during the three months ended March 31, 2021 (in thousands, except per share amounts and years): Number Weighted- Weighted- Aggregate Outstanding at December 31, 2020 3,378 $ 49.78 7.35 $ 264,046 Granted 573 $ 116.85 Exercised (150) $ 39.17 Canceled or forfeited (35) $ 70.11 Outstanding at March 31, 2021 3,766 $ 60.22 7.63 $ 284,022 Exercisable at March 31, 2021 1,781 $ 40.26 6.29 $ 169,844 (1) The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the estimated fair value of the common stock for the options that were in the money at March 31, 2021.
Schedule of Nonvested Restricted Stock Units Activity	The following table summarizes the restricted stock unit (RSU) activity under the 2013 Plan during the three months ended March 31, 2021 (in thousands, except per share amounts): Number Weighted- Unvested balance at December 31, 2020 494 $ 59.16 Granted 180 116.30 Vested (116) 48.69 Forfeited (7) 75.17 Unvested balance at March 31, 2021 551 $ 79.81
Share-Based Compensation, Performance Shares Award Nonvested Activity	The following table summarizes PSU activity under the 2013 Plan during the three months ended March 31, 2021 (in thousands, except per share amounts): Number Weighted- Unvested balance at December 31, 2020 75 $ 52.99 Granted (1) 83 115.53 Vested — — Forfeited (2) 52.99 Unvested balance at March 31, 2021 156 $ 86.01 (1) Pursuant to the terms of the awards granted on January 22, 2020 and January 29, 2021, the actual number of awards earned could range between 0% and 200% of the number of awards granted.

Segment Information (Details)

Segment Information (Details)	3 Months Ended
	Mar. 31, 2021segment
Segment Information
Number of operating segments	1

Cash, Cash Equivalents and Sh_3

Cash, Cash Equivalents and Short-term and Long-term Investments - Narrative (Details) $ in Millions	Mar. 31, 2021USD ($)debt_security	Dec. 31, 2020USD ($)
Cash and Cash Equivalents [Abstract]
Interest Receivable | $	$ 1.2	$ 0.3
Number of positions	99
Unrealized loss position, number of positions	79
Continuous unrealized loss position, 12 months or longer, number of positions	0

Cash, Cash Equivalents and Sh_4

Cash, Cash Equivalents and Short-term and Long-term Investments - Summary of Available-for-sale Securities With Unrealized Losses (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Fair Value	$ 249,183	$ 132,012
Unrealized Losses	(152)	(22)
Corporate obligations
Debt Securities, Available-for-sale [Line Items]
Fair Value	229,629	54,724
Unrealized Losses	(144)	(13)
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Fair Value	19,554	72,289
Unrealized Losses	(8)	(7)
Mortgage and other asset backed securities
Debt Securities, Available-for-sale [Line Items]
Fair Value	0	4,998
Unrealized Losses	$ 0	$ (2)

Cash, Cash Equivalents and Sh_5

Cash, Cash Equivalents and Short-term and Long-term Investments - Summary of Cash, Cash Equivalents and Available-for-sale Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Cash and cash equivalents due in 90 days or less
Amortized Cost	$ 455,514	$ 670,952
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1)	0
Estimated Fair Value	455,513	670,952	$ 277,486
Available-for-sale securities:
Amortized Cost	340,071	186,549
Gross Unrealized Gains	10	9
Gross Unrealized Losses	(152)	(22)
Estimated Fair Value	339,929	186,536
Total cash, cash equivalents and available-for-sale securities
Amortized Cost	795,585	857,501
Gross Unrealized Gains	10	9
Gross Unrealized Losses	(153)	(22)
Estimated Fair Value	795,442	857,488
Corporate obligations
Available-for-sale securities:
Amortized Cost	265,321	45,989
Gross Unrealized Gains	4	5
Gross Unrealized Losses	(144)	(13)
Estimated Fair Value	265,181	45,981
U.S. Treasury securities
Available-for-sale securities:
Amortized Cost	74,750	135,315
Gross Unrealized Gains	6	3
Gross Unrealized Losses	(8)	(7)
Estimated Fair Value	$ 74,748	135,311
Certificates of deposit
Available-for-sale securities:
Amortized Cost		245
Gross Unrealized Gains		1
Gross Unrealized Losses		0
Estimated Fair Value		246
Mortgage and other asset backed securities
Available-for-sale securities:
Amortized Cost		5,000
Gross Unrealized Gains		0
Gross Unrealized Losses		(2)
Estimated Fair Value		$ 4,998

Restricted Cash (Details)

Restricted Cash (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Cash and cash equivalents	$ 455,513	$ 670,952	$ 277,486
Restricted cash	1,597		1,597
Total cash, cash equivalents and restricted cash shown in the statement of cash flows	457,110	672,549	$ 279,083	$ 239,274
Certificates of Deposits and Money Market Funds | Letter of Credit
Accounts, Notes, Loans and Financing Receivable [Line Items]
Restricted cash	$ 1,600	$ 1,600

Concentrations of Credit Risk_2

Concentrations of Credit Risk and Off-Balance Sheet Risk (Details) - USD ($)	Mar. 31, 2021	Dec. 31, 2020
Risks and Uncertainties [Abstract]
Allowance for credit losses	$ 0	$ 0

Fair Value Measurements - Finan

Fair Value Measurements - Financial Instrument at Fair Value Schedule (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Assets:
Available-for-sale debt securities	$ 339,929	$ 186,536
Corporate obligations
Assets:
Available-for-sale debt securities	265,181	45,981
Certificates of deposit
Assets:
Available-for-sale debt securities		246
Mortgage and other asset backed securities
Assets:
Available-for-sale debt securities		4,998
Recurring
Assets:
Total assets	747,218	548,098
Recurring | Money market funds
Assets:
Available-for-sale debt securities	396,088	266,562
Recurring | Corporate obligations
Assets:
Available-for-sale debt securities	276,382	60,982
Recurring | U.S. Treasury securities
Assets:
Available-for-sale debt securities	74,748	215,310
Recurring | Certificates of deposit
Assets:
Available-for-sale debt securities		246
Recurring | Mortgage and other asset backed securities
Assets:
Available-for-sale debt securities		4,998
Recurring | Quoted Prices in Active Markets for Identical Items (Level 1)
Assets:
Total assets	396,088	266,562
Recurring | Quoted Prices in Active Markets for Identical Items (Level 1) | Money market funds
Assets:
Available-for-sale debt securities	396,088	266,562
Recurring | Quoted Prices in Active Markets for Identical Items (Level 1) | Corporate obligations
Assets:
Available-for-sale debt securities	0	0
Recurring | Quoted Prices in Active Markets for Identical Items (Level 1) | U.S. Treasury securities
Assets:
Available-for-sale debt securities	0	0
Recurring | Quoted Prices in Active Markets for Identical Items (Level 1) | Certificates of deposit
Assets:
Available-for-sale debt securities		0
Recurring | Quoted Prices in Active Markets for Identical Items (Level 1) | Mortgage and other asset backed securities
Assets:
Available-for-sale debt securities		0
Recurring | Significant Other Observable Inputs (Level 2)
Assets:
Total assets	351,130	281,536
Recurring | Significant Other Observable Inputs (Level 2) | Money market funds
Assets:
Available-for-sale debt securities	0	0
Recurring | Significant Other Observable Inputs (Level 2) | Corporate obligations
Assets:
Available-for-sale debt securities	276,382	60,982
Recurring | Significant Other Observable Inputs (Level 2) | U.S. Treasury securities
Assets:
Available-for-sale debt securities	74,748	215,310
Recurring | Significant Other Observable Inputs (Level 2) | Certificates of deposit
Assets:
Available-for-sale debt securities		246
Recurring | Significant Other Observable Inputs (Level 2) | Mortgage and other asset backed securities
Assets:
Available-for-sale debt securities		4,998
Recurring | Significant Unobservable Inputs (Level 3)
Assets:
Total assets	0	0
Recurring | Significant Unobservable Inputs (Level 3) | Money market funds
Assets:
Available-for-sale debt securities	0	0
Recurring | Significant Unobservable Inputs (Level 3) | Corporate obligations
Assets:
Available-for-sale debt securities	0	0
Recurring | Significant Unobservable Inputs (Level 3) | U.S. Treasury securities
Assets:
Available-for-sale debt securities	$ 0	0
Recurring | Significant Unobservable Inputs (Level 3) | Certificates of deposit
Assets:
Available-for-sale debt securities		0
Recurring | Significant Unobservable Inputs (Level 3) | Mortgage and other asset backed securities
Assets:
Available-for-sale debt securities		$ 0

Net Loss Per Share (Details)

Net Loss Per Share (Details) - shares shares in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Net Income (Loss) Per Share
Anti-dilutive common stock equivalents excluded from the calculation of diluted net loss per share (in shares)	4,482	4,841
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)	3,766	4,232
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)	0	39
Shares issuable under employee stock purchase plan
Net Income (Loss) Per Share
Anti-dilutive common stock equivalents excluded from the calculation of diluted net loss per share (in shares)	9	13
Outstanding restricted stock units
Net Income (Loss) Per Share
Anti-dilutive common stock equivalents excluded from the calculation of diluted net loss per share (in shares)	707	557

Significant Agreements - Celgen

Significant Agreements - Celgene, Accounting Analysis (Details) - Collaboration, License, and Option Agreement $ in Millions	3 Months Ended
	Mar. 31, 2021USD ($)
REBLOZYL Agreement
Significant Agreements
Payments received	$ 282.2
Sotarcept
Significant Agreements
Payments received	45
Regulatory Milestone | Luspatercept
Significant Agreements
Collaborative arrangement potential milestone payments on acceptance of regulatory application	$ 20

Significant Agreements - Other

Significant Agreements - Other Agreements (Details) - USD ($)	1 Months Ended		3 Months Ended		12 Months Ended
	Dec. 31, 2019	May 31, 2014	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2004
Significant Agreements
Milestones and fees			$ 57,299,000	$ 37,663,000
Selling, general and administrative			31,062,000	18,253,000
License agreement with Antibody Technology Company
Significant Agreements
Milestones and fees			0	0
License agreement with Antibody Technology Company | Additional Research Fees
Significant Agreements
Collaborative arrangement amount paid and expensed		$ 300,000
License Agreement With Fulcrum Therapeutics
Significant Agreements
Milestones and fees			500,000	300,000
License Agreement With Fulcrum Therapeutics | Additional Research Fees
Significant Agreements
Collaborative arrangement amount paid and expensed	$ 10,000,000
License Agreement With Fulcrum Therapeutics | Additional Research Fees | Maximum
Significant Agreements
Total potential milestone payments (up to)	143,500,000
License Agreement With Fulcrum Therapeutics | Research, Development and Commercial Milestone Payments | Maximum
Significant Agreements
Total potential milestone payments (up to)	$ 295,000,000
Non-collaborative Arrangement Transactions | Sotarcept | Development Milestone | Maximum
Significant Agreements
Total potential milestone payments (up to)					$ 2,000,000
Non-collaborative Arrangement Transactions | REBLOZYL Agreement | Development Milestone | Maximum
Significant Agreements
Total potential milestone payments (up to)					$ 700,000
Non-collaborative Arrangement Transactions | License Agreement with Non Profit Institution
Significant Agreements
Milestones and fees			0	200,000
Selling, general and administrative			$ 1,200,000	$ 100,000
Non-collaborative Arrangement Transactions | License Agreement with 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%
Non-collaborative Arrangement Transactions | License Agreement with 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%

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Stock-Based Compensation Costs Recognized (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost
Stock-based compensation expense	$ 15,626	$ 6,679
Research and development
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost
Stock-based compensation expense	8,158	3,141
Selling, general and administrative
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost
Stock-based compensation expense	$ 7,468	$ 3,538

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Number of Shares (in thousands)
Outstanding at the beginning of period (in shares)	3,378
Granted (in shares)	573
Exercised (in shares)	(150)
Canceled or forfeited (in shares)	(35)
Outstanding at the end of the period (in shares)	3,766	3,378
Exercisable at the end of the period (in shares)	1,781
Weighted- Average Exercise Price Per Share
Outstanding at the beginning of period (in dollars per share)	$ 49.78
Granted (in dollars per share)	116.85
Exercised (in dollars per share)	39.17
Canceled or forfeited (in dollars per share)	70.11
Outstanding at the end of the period (in dollars per share)	60.22	$ 49.78
Exercisable at the end of the period (in dollars per share)	$ 40.26
Weighted- Average Remaining Contractual Life  (in years)
Outstanding at the beginning of the period	7 years 7 months 17 days	7 years 4 months 6 days
Outstanding at the end of the period	7 years 7 months 17 days	7 years 4 months 6 days
Exercisable at the end of the period	6 years 3 months 14 days
Aggregate Intrinsic Value
Outstanding at the end of the period	$ 284,022	$ 264,046
Exercisable at the end of the period	169,844
Aggregate intrinsic value of options exercised	13,100
Unrecognized compensation expense	$ 71,600
Weighted-average period for recognition	2 years 9 months 10 days

Stock-Based Compensation - Sc_2

Stock-Based Compensation - Schedule of Nonvested Restricted Stock Activity (Details) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended
	Mar. 31, 2021USD ($)$ / sharesshares
Weighted- Average Grant Date Fair Value
Weighted-average period for recognition	2 years 9 months 10 days
Restricted Stock Units
Number of Grants
Unvested balance at beginning of the period (in shares) | shares	494
Granted (in shares) | shares	180
Vested (in shares) | shares	(116)
Forfeited (in shares) | shares	(7)
Unvested balance at end of the period (in shares) | shares	551
Weighted- Average Grant Date Fair Value
Unvested balance at beginning of the period (in dollars per share) | $ / shares	$ 59.16
Granted (in dollars per share) | $ / shares	116.30
Vested (in dollars per share) | $ / shares	48.69
Forfeited (in dollars per share) | $ / shares	75.17
Unvested balance at end of the period (in dollars per share) | $ / shares	$ 79.81
Unrecognized compensation cost | $	$ 35.8
Weighted-average period for recognition	1 year 10 months 28 days
Performance-Based Restricted Stock Units
Number of Grants
Unvested balance at beginning of the period (in shares) | shares	75
Granted (in shares) | shares	83
Vested (in shares) | shares	0
Forfeited (in shares) | shares	(2)
Unvested balance at end of the period (in shares) | shares	156
Weighted- Average Grant Date Fair Value
Unvested balance at beginning of the period (in dollars per share) | $ / shares	$ 52.99
Granted (in dollars per share) | $ / shares	115.53
Vested (in dollars per share) | $ / shares	0
Forfeited (in dollars per share) | $ / shares	52.99
Unvested balance at end of the period (in dollars per share) | $ / shares	$ 86.01
Unrecognized compensation cost | $	$ 13.4
Minimum
Weighted- Average Grant Date Fair Value
Actual awards earned percentage of expected expense recognized	0.00%
Maximum
Weighted- Average Grant Date Fair Value
Actual awards earned percentage of expected expense recognized	200.00%

Related Party Transactions (Det

Related Party Transactions (Details)	Mar. 31, 2021	Dec. 31, 2020
Bristol-Myers Squibb Company (BMS, formerly Celgene) | Bristol-Myers Squibb Company (BMS, formerly Celgene)
Related Party Transactions
Ownership percentage of entity's fully diluted equity	10.70%	10.80%