Acceleron Pharma (XLRN) 10-Q 5 Nov 20 2020 Q3 Quarterly report Financial data

Cover Page

Cover Page - shares	9 Months Ended
	Sep. 30, 2020	Oct. 31, 2020
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Sep. 30, 2020
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)		60,252,012
Entity Central Index Key	0001280600
Amendment Flag	false
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q3
Current Fiscal Year End Date	--12-31

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 729,685	$ 237,677
Collaboration receivables (all amounts are with a related party)	22,561	8,547
Prepaid expenses and other current assets	8,854	10,000
Short-term investments	152,677	193,692
Total current assets	913,777	449,916
Property and equipment, net	7,621	6,812
Operating lease - right of use asset, net	19,660	23,908
Other assets	1,708	1,793
Long-term investments	4,999	22,477
Total assets	947,765	504,906
Current liabilities:
Accounts payable	8,861	2,295
Accrued expenses	21,917	24,895
Operating lease liability - right of use	6,524	6,183
Total current liabilities	37,302	33,373
Operating lease liability - right of use, net of current portion	15,327	20,201
Warrants to purchase common stock	0	1,856
Total liabilities	52,629	55,430
Commitments and contingencies (Note 13)
Stockholders’ equity:
Common stock, $0.001 par value: 175,000,000 shares authorized; 60,052,046 and 53,123,567 shares issued and outstanding at September 30, 2020 and December 31, 2019, respectively	60	53
Additional paid-in capital	1,715,343	1,160,807
Accumulated deficit	(820,042)	(711,407)
Accumulated other comprehensive (loss) income	(225)	23
Total stockholders’ equity	895,136	449,476
Total liabilities and stockholders’ equity	$ 947,765	$ 504,906

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 2020	Dec. 31, 2019
Statement of Financial Position [Abstract]
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)	60,052,046	53,123,567
Common stock, shares outstanding (in shares)	60,052,046	53,123,567

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Collaboration revenue:
Revenues	$ 22,561	$ 4,208	$ 66,656	$ 34,655
Costs and expenses:
Research and development	40,747	37,630	116,663	105,125
Selling, general and administrative	21,042	15,501	59,705	40,394
Total costs and expenses	61,789	53,131	176,368	145,519
Loss from operations	(39,228)	(48,923)	(109,712)	(110,864)
Other (expense), income net	(6)	3,520	1,108	9,523
Loss before income taxes	(39,234)	(45,403)	(108,604)	(101,341)
Income tax (provision) benefit	(11)	34	(31)	58
Net loss	(39,245)	(45,369)	(108,635)	(101,283)
Other comprehensive (loss) income:
Net unrealized holding (losses) gains on short-term and long-term investments during the period, net of tax	(142)	(29)	(248)	691
Comprehensive loss	$ (39,387)	$ (45,398)	$ (108,883)	$ (100,592)
Net loss per share- basic and diluted (in dollars per share)	$ (0.66)	$ (0.86)	$ (1.95)	$ (1.94)
Weighted-average number of common shares used in computing net loss per share- basic and diluted (in shares)	59,640	52,882	55,635	52,239
Milestone
Collaboration revenue:
Revenues	$ 0	$ 0	$ 25,000	$ 25,000
Cost-sharing, net
Collaboration revenue:
Revenues	3,298	4,208	9,800	9,655
Royalty
Collaboration revenue:
Revenues	$ 19,263	$ 0	$ 31,856	$ 0

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Deficit	Comprehensive Income (Loss)
Stockholders' equity beginning balance (in shares) at Dec. 31, 2018		46,260,747
Stockholders' equity beginning balance at Dec. 31, 2018	$ 292,037	$ 47	$ 879,099	$ (586,549)	$ (560)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	6,992		6,992
Issuance of common stock, net of expense (in shares)		6,151,163
Issuance of common stock, net of expense	248,130	$ 6	248,124
Exercise of stock options (in shares)		35,919
Exercise of stock options	766		766
Vesting of restricted stock units, net of shares withheld for taxes (in shares)		75,028
Vesting of restricted stock units, net of shares withheld for taxes	(393)		(393)
Issuance of common stock related to ESPP (in shares)		19,661
Issuance of common stock related to ESPP	788		788
Unrealized loss on available-for-sale securities, net of tax	268				268
Net loss	(38,053)			(38,053)
Stockholders' equity ending balance (in shares) at Mar. 31, 2019		52,542,518
Stockholders' equity ending balance at Mar. 31, 2019	510,535	$ 53	1,135,376	(624,602)	(292)
Stockholders' equity beginning balance (in shares) at Dec. 31, 2018		46,260,747
Stockholders' equity beginning balance at Dec. 31, 2018	292,037	$ 47	879,099	(586,549)	(560)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Unrealized loss on available-for-sale securities, net of tax	691
Net loss	(101,283)
Stockholders' equity ending balance (in shares) at Sep. 30, 2019		53,027,063
Stockholders' equity ending balance at Sep. 30, 2019	464,801	$ 54	1,152,449	(687,833)	131
Stockholders' equity beginning balance (in shares) at Mar. 31, 2019		52,542,518
Stockholders' equity beginning balance at Mar. 31, 2019	510,535	$ 53	1,135,376	(624,602)	(292)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	5,012		5,012
Exercise of stock options (in shares)		64,174
Exercise of stock options	1,760		1,760
Vesting of restricted stock units, net of shares withheld for taxes (in shares)		146,162
Vesting of restricted stock units, net of shares withheld for taxes	0
Unrealized loss on available-for-sale securities, net of tax	452				452
Net loss	(17,862)			(17,862)
Stockholders' equity ending balance (in shares) at Jun. 30, 2019		52,752,854
Stockholders' equity ending balance at Jun. 30, 2019	499,897	$ 53	1,142,148	(642,464)	160
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	5,306		5,306
Exercise of stock options (in shares)		156,394
Exercise of stock options	4,744	$ 1	4,743
Vesting of restricted stock units, net of shares withheld for taxes (in shares)		104,217
Vesting of restricted stock units, net of shares withheld for taxes	(252)		(252)
Issuance of common stock related to ESPP (in shares)		13,598
Issuance of common stock related to ESPP	504		504
Unrealized loss on available-for-sale securities, net of tax	(29)				(29)
Net loss	(45,369)			(45,369)
Stockholders' equity ending balance (in shares) at Sep. 30, 2019		53,027,063
Stockholders' equity ending balance at Sep. 30, 2019	464,801	$ 54	1,152,449	(687,833)	131
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, 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]
Exercise of stock options (in shares)	1,143,000
Unrealized loss on available-for-sale securities, net of tax	$ (248)
Net loss	(108,635)
Stockholders' equity ending balance (in shares) at Sep. 30, 2020		60,052,046
Stockholders' equity ending balance at Sep. 30, 2020	895,136	$ 60	1,715,343	(820,042)	(225)
Stockholders' equity beginning balance (in shares) at Mar. 31, 2020		53,519,920
Stockholders' equity beginning balance at Mar. 31, 2020	413,822	$ 53	1,176,359	(762,346)	(244)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	7,140		7,140
Exercise of stock options (in shares)		617,441
Exercise of stock options	19,610	$ 1	19,609
Vesting of restricted stock units, net of shares withheld for taxes (in shares)		29,351
Vesting of restricted stock units, net of shares withheld for taxes	(663)		(663)
Unrealized loss on available-for-sale securities, net of tax	161				161
Net loss	(18,451)			(18,451)
Stockholders' equity ending balance (in shares) at Jun. 30, 2020		54,166,712
Stockholders' equity ending balance at Jun. 30, 2020	421,619	$ 54	1,202,445	(780,797)	(83)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	7,986		7,986
Issuance of common stock, net of expense (in shares)		5,594,593
Issuance of common stock, net of expense	492,414	$ 6	492,408
Exercise of stock options (in shares)		229,763
Exercise of stock options	8,284		8,284
Vesting of restricted stock units, net of shares withheld for taxes (in shares)		10,879
Vesting of restricted stock units, net of shares withheld for taxes	(313)		(313)
Issuance of common stock related to ESPP (in shares)		13,026
Issuance of common stock related to ESPP	804		804
Net exercise of warrants to purchase common stock (in shares)		37,073
Net exercise of warrants to purchase common stock	3,729		3,729
Unrealized loss on available-for-sale securities, net of tax	(142)				(142)
Net loss	(39,245)			(39,245)
Stockholders' equity ending balance (in shares) at Sep. 30, 2020		60,052,046
Stockholders' equity ending balance at Sep. 30, 2020	$ 895,136	$ 60	$ 1,715,343	$ (820,042)	$ (225)

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity (Parenthetical) - USD ($) $ in Thousands	3 Months Ended
	Sep. 30, 2020	Mar. 31, 2019
Statement of Stockholders' Equity [Abstract]
Issuance costs	$ 505	$ 500
Common stock, par value (in dollars per share)	$ 0.001

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Operating Activities
Net loss	$ (108,635)	$ (101,283)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	2,841	2,932
Stock-based compensation	21,805	17,310
Other non-cash items	1,552	(75)
Changes in assets and liabilities:
Prepaid expenses and other assets	1,231	(3,731)
Collaboration receivables (all amounts are with a related party)	(14,014)	164
Non-cash lease expense	4,249	3,804
Accounts payable	6,315	1,435
Accrued expenses	(3,025)	5,132
Operating lease obligations	(4,533)	(3,214)
Other changes in operating assets and liabilities	17	(27)
Net cash used in operating activities	(92,197)	(77,553)
Investing Activities
Purchases of investments	(161,741)	(379,096)
Proceeds from sales and maturities of investments	220,240	228,258
Purchases of property and equipment	(3,303)	(2,166)
Net cash provided by (used in) investing activities	55,196	(153,004)
Financing Activities
Proceeds from issuance of common stock from public offering, net of issuance costs	492,414	248,130
Net proceeds from exercises and vesting of stock awards, and ESPP contributions	36,595	7,917
Net cash provided by financing activities	529,009	256,047
Net increase in cash, cash equivalents and restricted cash	492,008	25,490
Cash, cash equivalents and restricted cash at beginning of period	239,274	145,649
Cash, cash equivalents and restricted cash at end of period	731,282	171,139
Supplemental Disclosure of Non-Cash Investing and Financing Activities:
Purchase of property and equipment included in accounts payable and accrued expenses	281	512
Reclassification of warrant liability to additional paid-in capital	$ 3,729	$ 0

Nature of Business

Nature of Business	9 Months Ended
	Sep. 30, 2020
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, the risk of relying on third parties, risks of clinical trial failures, dependence on key personnel, protection of proprietary technology, and compliance with government regulations.

Basis of Presentation

Basis of Presentation	9 Months Ended
	Sep. 30, 2020
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 September 30, 2020, 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, 2019, 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, 2019. 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 September 30, 2020, the results of its operations for the three and nine months ended September 30, 2020 and 2019, and its cash flows for the nine months ended September 30, 2020 and 2019. 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 and nine months ended September 30, 2020 are not necessarily indicative of the results to be expected for the year ending December 31, 2020, 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, 2019, 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, 2019.

Use of Estimates

Use of Estimates	9 Months Ended
	Sep. 30, 2020
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: accrued and prepaid clinical

Segment Information

Segment Information	9 Months Ended
	Sep. 30, 2020
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 Equivalents and Short-term

Cash Equivalents and Short-term and Long-term Investments	9 Months Ended
	Sep. 30, 2020
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 value. The Company determines the appropriate classification of marketable securities at the time of purchase and reevaluates such designation at each balance sheet date. The Company has classified all of its marketable securities at September 30, 2020 as “available-for-sale” pursuant to ASC 320, Investments – Debt and Equity Securities. 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. 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 as of September 30, 2020 (in thousands): Less than 12 months Fair Value Unrealized Losses Corporate obligations 18,756 (1) U.S. government agency securities 101,674 (16) Agency bond 4,999 (1) Total available-for sale securities in an unrealized loss position $ 125,429 $ (18) There were no securities in an unrealized loss position for greater than 12 months as of September 30, 2020. 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 September 30, 2020. Prior to January 1, 2020, the Company reviewed marketable securities for other-than-temporary impairment whenever the fair value of a marketable security was less than the amortized cost and evidence indicated that a marketable security’s carrying amount was not recoverable within a reasonable period of time. Other-than-temporary impairments of investments were recognized in the consolidated statements of operations if the Company had experienced a credit loss, had the intent to sell the marketable security, or if it was more likely than not that the Company would be required to sell the marketable security before recovery of the amortized cost basis. Evidence considered in this assessment included 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 December 31, 2019 was $35.8 million. The aggregate fair value of securities held by the Company in an unrealized loss position for more than twelve months as of December 31, 2019 was zero. The aggregate unrealized loss for those securities in an unrealized loss position for more than twelve months was zero. The Company determined it did not hold any investments with any other-than-temporary impairment as of December 31, 2019. The following is a summary of cash, cash equivalents and available-for-sale securities as of September 30, 2020 and December 31, 2019 (in thousands): September 30, 2020 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Cash and cash equivalents due in 90 days or less $ 729,686 $ — $ (1) $ 729,685 Available-for-sale securities: Corporate obligations 39,162 57 (1) 39,218 U.S. Treasury securities 113,220 8 (16) 113,212 Certificates of deposit 245 2 — 247 Mortgage and other asset backed securities 5,000 — (1) 4,999 Total available-for-sale securities (1) $ 157,627 $ 67 $ (18) $ 157,676 Total cash, cash equivalents and available-for-sale securities $ 887,313 $ 67 $ (19) $ 887,361 December 31, 2019 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Cash and cash equivalents due in 90 days or less $ 237,677 $ — $ — $ 237,677 Available-for-sale securities: Corporate obligations 124,676 219 (15) 124,880 U.S. Treasury securities 78,230 98 (1) 78,327 Certificates of deposit 490 3 — 493 Mortgage and other asset backed securities 12,476 5 (12) 12,469 Total available-for-sale securities (1) $ 215,872 $ 325 $ (28) $ 216,169 Total cash, cash equivalents and available-for-sale securities $ 453,549 $ 325 $ (28) $ 453,846 (1) All available-for-sale securities mature within two and three years as of September 30, 2020 and December 31, 2019, respectively.

Restricted Cash

Restricted Cash	9 Months Ended
	Sep. 30, 2020
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): September 30, 2020 2019 Cash and cash equivalents $ 729,685 $ 169,542 Restricted cash 1,597 1,597 Total cash, cash equivalents and restricted cash shown in the statement of cash flows $ 731,282 $ 171,139

Concentrations of Credit Risk a

Concentrations of Credit Risk and Off-Balance Sheet Risk	9 Months Ended
	Sep. 30, 2020
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 September 30, 2020.

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2020
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 September 30, 2020 and December 31, 2019 (in thousands): September 30, 2020 Quoted Prices Significant Other Significant Total Assets: Money market funds $ 186,379 $ — $ — $ 186,379 Corporate obligations — 43,336 — 43,336 U.S. Treasury securities — 123,210 — 123,210 Certificates of deposit — 247 — 247 Mortgage and other asset backed securities — 4,999 — 4,999 Total assets $ 186,379 $ 171,792 $ — $ 358,171 December 31, 2019 Quoted Prices Significant other Significant Total Assets: Money market funds $ 193,867 $ — $ — $ 193,867 Corporate obligations — 138,369 — 138,369 U.S. Treasury securities — 83,819 — 83,819 Certificates of deposit — 493 — 493 Mortgage and other asset backed securities — 12,470 — 12,470 Total assets $ 193,867 $ 235,151 $ — $ 429,018 Liabilities: Warrants to purchase common stock $ — $ — $ 1,856 $ 1,856 Total liabilities $ — $ — $ 1,856 $ 1,856 The money market funds noted above are included in cash and cash equivalents in the accompanying condensed consolidated balance sheets. The Company recognizes transfers between levels of the fair value hierarchy as of the end of the reporting period. There were no transfers within the hierarchy during the nine months ended September 30, 2020 or the year ended December 31, 2019. Items measured at fair value on a recurring basis include short-term and long-term investments (Note 5), and warrants to purchase common stock (Note 12). During the periods presented, the Company has not changed the manner in which it values assets and liabilities that are measured at fair value using Level 3 inputs. The following table sets forth a summary of changes in the fair value of the Company’s common stock warrant liabilities, which represent a recurring measurement that is classified within Level 3 of the fair value hierarchy, wherein fair value is estimated using significant unobservable inputs (in thousands): Nine Months Ended September 30, 2020 2019 Beginning balance $ 1,856 $ 1,491 Change in fair value 1,873 (165) Settlement (3,729) — Ending balance $ — $ 1,326

Net Loss Per Share

Net Loss Per Share	9 Months Ended
	Sep. 30, 2020
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 September 30, Nine Months Ended September 30, 2020 2019 2020 2019 Outstanding stock options 3,546 3,920 3,546 3,920 Common stock warrants — 39 — 39 Shares issuable under employee stock purchase plan 19 23 19 23 Outstanding restricted stock units (1) 572 366 572 366 4,137 4,348 4,137 4,348 (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	9 Months Ended
	Sep. 30, 2020
Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract]
Comprehensive Loss	Comprehensive LossComprehensive 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) income, 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) income is presented separately on the consolidated balance sheets and consists entirely of unrealized holding gains and losses on investments as of September 30, 2020 and December 31, 2019.

Recent Accounting Pronouncement

Recent Accounting Pronouncements	9 Months Ended
	Sep. 30, 2020
Accounting Standards Update and Change in Accounting Principle [Abstract]
Recent Accounting Pronouncements	Recent Accounting Pronouncements Recently Adopted In June 2016, the FASB issued ASU 2016-13, Financial Instruments-Credit Losses . The new standard requires entities to measure all expected credit losses for financial assets held at the reporting date based on historical experience, current conditions and reasonable and supportable forecasts. For available-for-sale debt securities with unrealized losses, the standard requires allowances to be recorded instead of reducing the amortized cost of the investment. The standard limits the amount of credit losses to be recognized for available-for-sale debt securities to the amount by which the carrying value exceeds fair value and requires the reversal of previously recognized credit losses if fair value increases. On January 1, 2020 the Company adopted ASU 2016-13. For discussion regarding the impact of this accounting pronouncement and its amendments, refer to Note 5 within the notes to our consolidated financial statements appearing elsewhere in this Quarterly Report on Form 10-Q. In June 2018, the FASB issued ASU 2018-15, Intangible-Goodwill and Other Internal-Use Software (Subtopic 350-40): Customer's Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement that is a Service Contract. This amendment aligns the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software. On January 1, 2020, the Company adopted ASU 2018-15 on a prospective basis, with no material impact on its consolidated financial statements and related disclosures. In December 2019, the FASB issued ASU 2019-12, Simplifying the Accounting for Income Taxes. The ASU simplifies the accounting for income taxes by eliminating certain exceptions to the guidance in ASC 740, Income Taxes , related to the approach for allocating income tax expense or benefit for the year to continuing operations, discontinued operations, other comprehensive income, and other charges or credits recorded directly to shareholders’ equity; the methodology for calculating income taxes in an interim period; and the recognition of deferred tax liabilities for outside basis differences. On January 1,

Warrants

Warrants	9 Months Ended
	Sep. 30, 2020
Warrants and Rights Note Disclosure [Abstract]
Warrants	Warrants Below is a summary of the number of shares issuable upon exercise of outstanding warrants and the terms and accounting treatment for the outstanding warrants (in thousands, except per share data): Warrants as of Weighted- Balance Sheet September 30, 2020 December 31, 2019 Price Per Expiration September 30, 2020 December 31, 2019 Warrants to purchase common stock — 39 $ 5.88 July 9, 2020 N/A (1) Liability (1) All remaining outstanding warrants were automatically cashless exercised in full upon their expiration on July 9, 2020.

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2020
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies Legal Proceedings The Company, from time to time, may be party to litigation arising in the ordinary course of its business. The Company was not subject to any material legal proceedings during the three months ended September 30, 2020, 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 September 30, 2020 and December 31, 2019, or royalties on future sales of specified products. See Note 14 for discussion of these arrangements. The Company enters into standard indemnification agreements in the ordinary course of business. Pursuant to the agreements, the Company indemnifies, holds harmless, and agrees to reimburse the indemnified party for losses suffered or incurred by the indemnified party, generally the Company’s business partners or customers, in connection with any U.S. patent or any copyright or other intellectual property infringement claim by any third party with respect to the Company’s products. The term of these indemnification agreements is generally perpetual any time after execution of the agreement. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is unlimited. The Company has never incurred costs to defend lawsuits or settle claims related to these indemnification agreements.

Significant Agreements

Significant Agreements	9 Months Ended
	Sep. 30, 2020
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 (the Restated Sotatercept Agreement) and clarified certain responsibilities of the Company and BMS in a letter agreement to the Restated Sotatercept Agreement on March 10, 2020. 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, 2019, 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, 2019. 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 September 30, 2020, 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 September 30, 2020, the Company cannot determine if it is probable that a regulatory agency will approve the applications. In June 2020, the European Commission, which has the authority to approve medicines for the European Union, approved REBLOZYL based on the recommendation of the European Medicines Agency, or EMA, for the treatment of adult patients with transfusion-dependent anemia due to very low-, low- and intermediate-risk MDS with ring sideroblasts, who had an unsatisfactory response or are ineligible for erythropoietin-based therapy, and adult patients with transfusion-dependent anemia associated with beta-thalassemia. As a result, the $25.0 million milestone from the EMA approval of a BLA or equivalent for REBLOZYL was no longer constrained. As the Company did not have any remaining performance obligations under the agreement with BMS, the full $25.0 million was recognized as revenue during the three months ended June 30, 2020. Through September 30, 2020, under all BMS arrangements, the Company has received net cost-share payments, milestones, and royalties of $234.3 million and $44.9 million for REBLOZYL and sotatercept, respectively. The Company recorded net collaboration revenue of $22.6 million and $4.2 million during the three months ended September 30, 2020 and 2019, respectively, and $66.7 million and $34.7 million during the nine months ended September 30, 2020 and 2019, 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 also 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 under the licenses. During the three months ended September 30, 2020 and 2019, the Company expensed $1.0 million and zero, respectively, and during the nine months ended September 30, 2020 and 2019, the Company expensed $3.4 million and $1.6 million, respectively, of milestones, fees and royalties. Milestones and fees associated with development related activities are recorded as research and development expense. 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 September 30, 2020 and 2019, the Company expensed $0.1 million and $0.1 million, respectively, and during the nine months ended September 30, 2020 and 2019, the Company expensed $0.6 million and $2.1 million, respectively, of milestones and fees, which is recorded as research and development expense. 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 payments in the mid-single-digit to low double-digit range on net sales, as well as reimbursement for relevant research and development costs. During the three months ended September 30, 2020 and 2019, the Company expensed $0.6 million and

Stockholders' Equity

Stockholders' Equity	9 Months Ended
	Sep. 30, 2020
Equity [Abstract]
Stockholders' Equity	Stockholders' EquityOn July 6, 2020, the Company completed the sale in an underwritten public offering of 5,594,593 shares of common stock, including 729,729 shares of common stock sold pursuant to the underwriter's full exercise of their option to purchase additional shares, at a public offering price of $92.50 per share, resulting in net proceeds to the Company of $492.4 million.

Stock-Based Compensation

Stock-Based Compensation	9 Months Ended
	Sep. 30, 2020
Share-based Payment Arrangement [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 and nine months ended September 30, 2020 and 2019, respectively, as follows (in thousands): Three Months Ended September 30, Nine Months Ended September 30, 2020 2019 2020 2019 Research and development $ 3,794 $ 2,107 $ 10,194 $ 7,610 Selling, general and administrative 4,192 3,199 11,611 9,700 $ 7,986 $ 5,306 $ 21,805 $ 17,310 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 September 30, Nine Months Ended September 30, 2020 2019 2020 2019 Expected volatility 57.7 % 57.4 % 55.6 % 58.7 % Expected term (in years) 6.0 6.0 6.0 6.0 Risk-free interest rate 0.4 % 1.6 % 1.4 % 2.4 % Expected dividend yield — % — % — % — % The following table summarizes the stock option activity under the Company’s stock option plans during the nine months ended September 30, 2020 (in thousands, except per share amounts and years): Number Weighted- Weighted- Aggregate Outstanding at December 31, 2019 3,820 $ 36.26 6.81 Granted 1,041 $ 62.88 Exercised (1,143) $ 31.83 Canceled or forfeited (172) $ 42.71 Outstanding at September 30, 2020 3,546 $ 45.19 7.42 $ 238,802 Exercisable at September 30, 2020 1,843 $ 37.22 6.22 $ 138,783 (1) The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the estimated fair value of the common stock for the options that were in the money at September 30, 2020. The aggregate intrinsic value of options exercised during the nine months ended September 30, 2020 was $67.7 million. As of September 30, 2020, there was $44.1 million of unrecognized compensation expense related to unvested stock options that is expected to be recognized over a weighted-average period of 2.61 years. Restricted Stock Units The following table summarizes the restricted stock unit (RSU) activity under the 2013 Plan during the nine months ended September 30, 2020 (in thousands, except per share amounts): Number Weighted- Unvested balance at December 31, 2019 397 $ 39.20 Granted 275 67.45 Vested (137) 38.64 Forfeited (38) 42.80 Unvested balance at September 30, 2020 497 $ 54.68 As of September 30, 2020, there was approximately $20.7 million of related unrecognized compensation cost, which the Company expects to recognize over a remaining weighted-average period of 1.79 years. Performance-Based Restricted Stock Units On January 22, 2020, the Company granted performance-based restricted stock units (PSU) whereby vesting depends upon the occurrence of certain milestone events by December 31, 2022. As of September 30, 2020, 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 September 30, 2020, no related compensation cost had been recognized. The following table summarizes PSU activity under the 2013 Plan during the nine months ended September 30, 2020 (in thousands, except per share amounts): Number Weighted- Unvested balance at December 31, 2019 — $ — Granted (1) 78 52.99 Vested — — Forfeited (3) 52.99 Unvested balance at September 30, 2020 75 $ 52.99 (1) Pursuant to the terms of the awards granted on January 22, 2020, the actual number of awards earned could range between 0% and 200% of the number of awards granted. As of September 30, 2020, there was approximately $4.0 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	9 Months Ended
	Sep. 30, 2020
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	9 Months Ended
	Sep. 30, 2020
Related Party Transactions [Abstract]
Related Party Transactions	Related Party Transactions BMS BMS owned 10.9% and 12.0% of the Company’s fully diluted equity as of September 30, 2020 and December 31, 2019, respectively. Refer to Note 14 for additional information regarding this collaboration arrangement.

Subsequent Events

Subsequent Events	9 Months Ended
	Sep. 30, 2020
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 October 16, 2020, the Company entered into an amendment to the lease agreement for its office space at 125 Sidney Street to extend the lease term to May 31, 2026. Under the terms of this amendment, the base rental rate ranges from $65.0 thousand to $71.8 thousand per month, for a total commitment of $4.1 million during the extended lease term. The Company is currently evaluating the impact the amendment will have on its consolidated financial statements and related disclosures.

Basis of Presentation (Policies

Basis of Presentation (Policies)	9 Months Ended
	Sep. 30, 2020
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 September 30, 2020, 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, 2019, 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, 2019. 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 September 30, 2020, the results of its operations for the three and nine months ended September 30, 2020 and 2019, and its cash flows for the nine months ended September 30, 2020 and 2019. 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 and nine months ended September 30, 2020 are not necessarily indicative of the results to be expected for the year ending December 31, 2020, 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, 2019, 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, 2019.
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: accrued and prepaid clinical
Recent Accounting Pronouncements	In June 2016, the FASB issued ASU 2016-13, Financial Instruments-Credit Losses . The new standard requires entities to measure all expected credit losses for financial assets held at the reporting date based on historical experience, current conditions and reasonable and supportable forecasts. For available-for-sale debt securities with unrealized losses, the standard requires allowances to be recorded instead of reducing the amortized cost of the investment. The standard limits the amount of credit losses to be recognized for available-for-sale debt securities to the amount by which the carrying value exceeds fair value and requires the reversal of previously recognized credit losses if fair value increases. On January 1, 2020 the Company adopted ASU 2016-13. For discussion regarding the impact of this accounting pronouncement and its amendments, refer to Note 5 within the notes to our consolidated financial statements appearing elsewhere in this Quarterly Report on Form 10-Q. In June 2018, the FASB issued ASU 2018-15, Intangible-Goodwill and Other Internal-Use Software (Subtopic 350-40): Customer's Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement that is a Service Contract. This amendment aligns the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software. On January 1, 2020, the Company adopted ASU 2018-15 on a prospective basis, with no material impact on its consolidated financial statements and related disclosures. In December 2019, the FASB issued ASU 2019-12, Simplifying the Accounting for Income Taxes. The ASU simplifies the accounting for income taxes by eliminating certain exceptions to the guidance in ASC 740, Income Taxes , related to the approach for allocating income tax expense or benefit for the year to continuing operations, discontinued operations, other comprehensive income, and other charges or credits recorded directly to shareholders’ equity; the methodology for calculating income taxes in an interim period; and the recognition of deferred tax liabilities for outside basis differences. On January 1,

Cash Equivalents and Short-te_2

Cash Equivalents and Short-term and Long-term Investments (Tables)	9 Months Ended
	Sep. 30, 2020
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 as of September 30, 2020 (in thousands): Less than 12 months Fair Value Unrealized Losses Corporate obligations 18,756 (1) U.S. government agency securities 101,674 (16) Agency bond 4,999 (1) Total available-for sale securities in an unrealized loss position $ 125,429 $ (18)
Available-for-Sale Securities	The following is a summary of cash, cash equivalents and available-for-sale securities as of September 30, 2020 and December 31, 2019 (in thousands): September 30, 2020 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Cash and cash equivalents due in 90 days or less $ 729,686 $ — $ (1) $ 729,685 Available-for-sale securities: Corporate obligations 39,162 57 (1) 39,218 U.S. Treasury securities 113,220 8 (16) 113,212 Certificates of deposit 245 2 — 247 Mortgage and other asset backed securities 5,000 — (1) 4,999 Total available-for-sale securities (1) $ 157,627 $ 67 $ (18) $ 157,676 Total cash, cash equivalents and available-for-sale securities $ 887,313 $ 67 $ (19) $ 887,361 December 31, 2019 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Cash and cash equivalents due in 90 days or less $ 237,677 $ — $ — $ 237,677 Available-for-sale securities: Corporate obligations 124,676 219 (15) 124,880 U.S. Treasury securities 78,230 98 (1) 78,327 Certificates of deposit 490 3 — 493 Mortgage and other asset backed securities 12,476 5 (12) 12,469 Total available-for-sale securities (1) $ 215,872 $ 325 $ (28) $ 216,169 Total cash, cash equivalents and available-for-sale securities $ 453,549 $ 325 $ (28) $ 453,846 (1) All available-for-sale securities mature within two and three years as of September 30, 2020 and December 31, 2019, respectively.

Restricted Cash (Tables)

Restricted Cash (Tables)	9 Months Ended
	Sep. 30, 2020
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): September 30, 2020 2019 Cash and cash equivalents $ 729,685 $ 169,542 Restricted cash 1,597 1,597 Total cash, cash equivalents and restricted cash shown in the statement of cash flows $ 731,282 $ 171,139
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): September 30, 2020 2019 Cash and cash equivalents $ 729,685 $ 169,542 Restricted cash 1,597 1,597 Total cash, cash equivalents and restricted cash shown in the statement of cash flows $ 731,282 $ 171,139

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2020
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 September 30, 2020 and December 31, 2019 (in thousands): September 30, 2020 Quoted Prices Significant Other Significant Total Assets: Money market funds $ 186,379 $ — $ — $ 186,379 Corporate obligations — 43,336 — 43,336 U.S. Treasury securities — 123,210 — 123,210 Certificates of deposit — 247 — 247 Mortgage and other asset backed securities — 4,999 — 4,999 Total assets $ 186,379 $ 171,792 $ — $ 358,171 December 31, 2019 Quoted Prices Significant other Significant Total Assets: Money market funds $ 193,867 $ — $ — $ 193,867 Corporate obligations — 138,369 — 138,369 U.S. Treasury securities — 83,819 — 83,819 Certificates of deposit — 493 — 493 Mortgage and other asset backed securities — 12,470 — 12,470 Total assets $ 193,867 $ 235,151 $ — $ 429,018 Liabilities: Warrants to purchase common stock $ — $ — $ 1,856 $ 1,856 Total liabilities $ — $ — $ 1,856 $ 1,856
Summary of Changes in the Fair Value of the Preferred and Common Stock Warrant Liability	The following table sets forth a summary of changes in the fair value of the Company’s common stock warrant liabilities, which represent a recurring measurement that is classified within Level 3 of the fair value hierarchy, wherein fair value is estimated using significant unobservable inputs (in thousands): Nine Months Ended September 30, 2020 2019 Beginning balance $ 1,856 $ 1,491 Change in fair value 1,873 (165) Settlement (3,729) — Ending balance $ — $ 1,326

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	9 Months Ended
	Sep. 30, 2020
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 September 30, Nine Months Ended September 30, 2020 2019 2020 2019 Outstanding stock options 3,546 3,920 3,546 3,920 Common stock warrants — 39 — 39 Shares issuable under employee stock purchase plan 19 23 19 23 Outstanding restricted stock units (1) 572 366 572 366 4,137 4,348 4,137 4,348 (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)	9 Months Ended
	Sep. 30, 2020
Warrants and Rights Note Disclosure [Abstract]
Summary of the Number of Shares Issuable Upon Exercise of Outstanding Warrants and the Terms and Accounting Treatment for the Outstanding Warrants	Below is a summary of the number of shares issuable upon exercise of outstanding warrants and the terms and accounting treatment for the outstanding warrants (in thousands, except per share data): Warrants as of Weighted- Balance Sheet September 30, 2020 December 31, 2019 Price Per Expiration September 30, 2020 December 31, 2019 Warrants to purchase common stock — 39 $ 5.88 July 9, 2020 N/A (1) Liability

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2020
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 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 and nine months ended September 30, 2020 and 2019, respectively, as follows (in thousands): Three Months Ended September 30, Nine Months Ended September 30, 2020 2019 2020 2019 Research and development $ 3,794 $ 2,107 $ 10,194 $ 7,610 Selling, general and administrative 4,192 3,199 11,611 9,700 $ 7,986 $ 5,306 $ 21,805 $ 17,310
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 September 30, Nine Months Ended September 30, 2020 2019 2020 2019 Expected volatility 57.7 % 57.4 % 55.6 % 58.7 % Expected term (in years) 6.0 6.0 6.0 6.0 Risk-free interest rate 0.4 % 1.6 % 1.4 % 2.4 % 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 nine months ended September 30, 2020 (in thousands, except per share amounts and years): Number Weighted- Weighted- Aggregate Outstanding at December 31, 2019 3,820 $ 36.26 6.81 Granted 1,041 $ 62.88 Exercised (1,143) $ 31.83 Canceled or forfeited (172) $ 42.71 Outstanding at September 30, 2020 3,546 $ 45.19 7.42 $ 238,802 Exercisable at September 30, 2020 1,843 $ 37.22 6.22 $ 138,783 (1) The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the estimated fair value of the common stock for the options that were in the money at September 30, 2020.
Schedule of Nonvested Restricted Stock Units Activity	The following table summarizes the restricted stock unit (RSU) activity under the 2013 Plan during the nine months ended September 30, 2020 (in thousands, except per share amounts): Number Weighted- Unvested balance at December 31, 2019 397 $ 39.20 Granted 275 67.45 Vested (137) 38.64 Forfeited (38) 42.80 Unvested balance at September 30, 2020 497 $ 54.68
Share-Based Compensation, Performance Shares Award Nonvested Activity	The following table summarizes PSU activity under the 2013 Plan during the nine months ended September 30, 2020 (in thousands, except per share amounts): Number Weighted- Unvested balance at December 31, 2019 — $ — Granted (1) 78 52.99 Vested — — Forfeited (3) 52.99 Unvested balance at September 30, 2020 75 $ 52.99 (1) Pursuant to the terms of the awards granted on January 22, 2020, the actual number of awards earned could range between 0% and 200% of the number of awards granted.

Segment Information (Details)

Segment Information (Details)	9 Months Ended
	Sep. 30, 2020segment
Segment Information
Number of operating segments	1

Cash Equivalents and Short-te_3

Cash Equivalents and Short-term and Long-term Investments Summary of Available-for-sale Securities With Unrealized Losses (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Fair Value	$ 125,429
Unrealized Losses	(18)	$ (35,800)
Corporate obligations
Debt Securities, Available-for-sale [Line Items]
Fair Value	18,756
Unrealized Losses	(1)
U.S. government agency securities
Debt Securities, Available-for-sale [Line Items]
Fair Value	101,674
Unrealized Losses	(16)
Agency bond
Debt Securities, Available-for-sale [Line Items]
Fair Value	4,999
Unrealized Losses	$ (1)

Cash Equivalents and Short-te_4

Cash Equivalents and Short-term and Long-term Investments - Narrative (Details) - USD ($)	Sep. 30, 2020	Dec. 31, 2019
Cash and Cash Equivalents [Abstract]
Continuous unrealized loss position, twelve months or longer, fair value	$ 0	$ 0
Unrealized loss position, less than twelve months, fair value	$ 18,000	35,800,000
Continuous unrealized loss position, fair value		$ 0

Cash Equivalents and Short-te_5

Cash Equivalents and Short-term and Long-term Investments - Summary of Cash, Cash Equivalents and Available-for-sale Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
Cash and cash equivalents due in 90 days or less
Amortized Cost	$ 729,686	$ 237,677
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1)	0
Estimated Fair Value	729,685	237,677	$ 169,542
Available-for-sale securities:
Amortized Cost	157,627	215,872
Gross Unrealized Gains	67	325
Gross Unrealized Losses	(18)	(28)
Debt Securities, Available-for-sale	157,676	216,169
Total cash, cash equivalents and available-for-sale securities
Amortized Cost	887,313	453,549
Gross Unrealized Gains	67	325
Gross Unrealized Losses	(19)	(28)
Estimated Fair Value	887,361	453,846
Corporate obligations
Available-for-sale securities:
Amortized Cost	39,162	124,676
Gross Unrealized Gains	57	219
Gross Unrealized Losses	(1)	(15)
Debt Securities, Available-for-sale	39,218	124,880
U.S. Treasury securities
Available-for-sale securities:
Amortized Cost	113,220	78,230
Gross Unrealized Gains	8	98
Gross Unrealized Losses	(16)	(1)
Debt Securities, Available-for-sale	113,212	78,327
Certificates of deposit
Available-for-sale securities:
Amortized Cost	245	490
Gross Unrealized Gains	2	3
Gross Unrealized Losses	0	0
Debt Securities, Available-for-sale	247	493
Mortgage and other asset backed securities
Available-for-sale securities:
Amortized Cost	5,000	12,476
Gross Unrealized Gains	0	5
Gross Unrealized Losses	(1)	(12)
Debt Securities, Available-for-sale	$ 4,999	$ 12,469

Restricted Cash (Details)

Restricted Cash (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Cash and cash equivalents	$ 729,685	$ 237,677	$ 169,542
Restricted cash	1,597		1,597
Total cash, cash equivalents and restricted cash shown in the statement of cash flows	731,282	239,274	$ 171,139	$ 145,649
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)	Sep. 30, 2020USD ($)
Risks and Uncertainties [Abstract]
Allowance for credit losses	$ 0

Fair Value Measurements - Finan

Fair Value Measurements - Financial Instrument at Fair Value Schedule (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Assets:
Available-for-sale debt securities	$ 157,676	$ 216,169
Corporate obligations
Assets:
Available-for-sale debt securities	39,218	124,880
Certificates of deposit
Assets:
Available-for-sale debt securities	247	493
Mortgage and other asset backed securities
Assets:
Available-for-sale debt securities	4,999	12,469
Recurring
Assets:
Total assets	358,171	429,018
Liabilities:
Total liabilities		1,856
Recurring | Warrants to purchase common stock
Liabilities:
Total liabilities		1,856
Recurring | Money market funds
Assets:
Available-for-sale debt securities	186,379	193,867
Recurring | Corporate obligations
Assets:
Available-for-sale debt securities	43,336	138,369
Recurring | U.S. Treasury securities
Assets:
Available-for-sale debt securities	123,210	83,819
Recurring | Certificates of deposit
Assets:
Available-for-sale debt securities	247	493
Recurring | Mortgage and other asset backed securities
Assets:
Available-for-sale debt securities	4,999	12,470
Recurring | Quoted Prices in Active Markets for Identical Items (Level 1)
Assets:
Total assets	186,379	193,867
Liabilities:
Total liabilities		0
Recurring | Quoted Prices in Active Markets for Identical Items (Level 1) | Warrants to purchase common stock
Liabilities:
Total liabilities		0
Recurring | Quoted Prices in Active Markets for Identical Items (Level 1) | Money market funds
Assets:
Available-for-sale debt securities	186,379	193,867
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	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	0
Recurring | Significant Other Observable Inputs (Level 2)
Assets:
Total assets	171,792	235,151
Liabilities:
Total liabilities		0
Recurring | Significant Other Observable Inputs (Level 2) | Warrants to purchase common stock
Liabilities:
Total liabilities		0
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	43,336	138,369
Recurring | Significant Other Observable Inputs (Level 2) | U.S. Treasury securities
Assets:
Available-for-sale debt securities	123,210	83,819
Recurring | Significant Other Observable Inputs (Level 2) | Certificates of deposit
Assets:
Available-for-sale debt securities	247	493
Recurring | Significant Other Observable Inputs (Level 2) | Mortgage and other asset backed securities
Assets:
Available-for-sale debt securities	4,999	12,470
Recurring | Significant Unobservable Inputs (Level 3)
Assets:
Total assets	0	0
Liabilities:
Total liabilities		1,856
Recurring | Significant Unobservable Inputs (Level 3) | Warrants to purchase common stock
Liabilities:
Total liabilities		1,856
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	0
Recurring | Significant Unobservable Inputs (Level 3) | Mortgage and other asset backed securities
Assets:
Available-for-sale debt securities	$ 0	$ 0

Fair Value Measurements - Prefe

Fair Value Measurements - Preferred and Common Stock Warrant Liability Rollforward (Details) - USD ($)	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Summary of changes in the fair value of the preferred and common stock warrant liability classified within Level 3 of the fair value hierarchy
Beginning balance	$ 1,856,000	$ 1,491,000
Change in fair value	1,873,000	(165,000)
Settlement	(3,729,000)	0
Ending balance	0	$ 1,326,000
Warrants outstanding	$ 0

Net Loss Per Share (Details)

Net Loss Per Share (Details) - shares shares in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Net Income (Loss) Per Share
Anti-dilutive common stock equivalents excluded from the calculation of diluted net loss per share (in shares)	4,137	4,348	4,137	4,348
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)	3,546	3,920	3,546	3,920
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	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)	19	23	19	23
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)	572	366	572	366

Warrants (Details)

Warrants (Details) - Warrants to purchase Common stock expiring between June 10, 2020 to July 9, 2020 - $ / shares shares in Thousands	Sep. 30, 2020	Dec. 31, 2019
Warrants
Warrants (in shares)	0	39
Weighted-average exercise price per share (in dollars per share)	$ 5.88

Significant Agreements - Celgen

Significant Agreements - Celgene, Accounting Analysis (Details) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Significant Agreements
Revenue from contract with customer	$ 22,561		$ 4,208	$ 66,656	$ 34,655
REBLOZYL Agreement | Collaboration, License, and Option Agreement
Significant Agreements
Revenue from contract with customer		$ 25,000
Payments received				234,300
Sotarcept | Collaboration, License, and Option Agreement
Significant Agreements
Payments received				44,900
Regulatory Milestone | Luspatercept | Collaboration, License, and Option Agreement
Significant Agreements
Collaborative arrangement potential milestone payments on acceptance of regulatory application	$ 20,000			$ 20,000

Significant Agreements - Other

Significant Agreements - Other Agreements (Details) - USD ($)	1 Months Ended		3 Months Ended		9 Months Ended		12 Months Ended
	Dec. 31, 2019	May 31, 2014	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2004
Significant Agreements
Milestones and fees			$ 40,747,000	$ 37,630,000	$ 116,663,000	$ 105,125,000
License agreement with Antibody Technology Company
Significant Agreements
Milestones and fees			100,000	100,000	600,000	2,100,000
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			600,000	0	1,700,000	0
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			$ 1,000,000	$ 0	$ 3,400,000	$ 1,600,000
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%
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%

Stockholders' Equity (Details)

Stockholders' Equity (Details) $ / shares in Units, $ in Millions	Jul. 06, 2020USD ($)$ / sharesshares
Public Stock Offering
Class of Stock [Line Items]
Number of shares issued in transaction (in shares)	5,594,593
Price per share (in dollars per share) | $ / shares	$ 92.50
Net proceeds on transaction | $	$ 492.4
Over-Allotment Option
Class of Stock [Line Items]
Number of shares issued in transaction (in shares)	729,729

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Stock-Based Compensation Costs Recognized (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost
Stock-based compensation expense	$ 7,986	$ 5,306	$ 21,805	$ 17,310
Research and development
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost
Stock-based compensation expense	3,794	2,107	10,194	7,610
Selling, general and administrative
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost
Stock-based compensation expense	$ 4,192	$ 3,199	$ 11,611	$ 9,700

Stock-Based Compensation - Sc_2

Stock-Based Compensation - Schedule of Weighted-Average Assumptions Used for Estimating Fair Value (Details) - Stock Option	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award
Expected volatility	57.70%	57.40%	55.60%	58.70%
Expected term (in years)	6 years	6 years	6 years	6 years
Risk-free interest rate	0.40%	1.60%	1.40%	2.40%
Expected dividend yield	0.00%	0.00%	0.00%	0.00%

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Option Activity (Details) $ / shares in Units, shares in Thousands, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020USD ($)$ / sharesshares	Dec. 31, 2019$ / sharesshares
Number of Stock Options
Outstanding at the beginning of period (in shares) | shares	3,820
Granted (in shares) | shares	1,041
Exercised (in shares) | shares	(1,143)
Canceled or forfeited (in shares) | shares	(172)
Outstanding at the end of the period (in shares) | shares	3,546	3,820
Exercisable at the end of the period (in shares) | shares	1,843
Weighted- Average Exercise Price Per Share
Outstanding at the beginning of period (in dollars per share) | $ / shares	$ 36.26
Granted (in dollars per share) | $ / shares	62.88
Exercised (in dollars per share) | $ / shares	31.83
Canceled or forfeited (in dollars per share) | $ / shares	42.71
Outstanding at the end of the period (in dollars per share) | $ / shares	45.19	$ 36.26
Exercisable at the end of the period (in dollars per share) | $ / shares	$ 37.22
Weighted- Average Contractual Life (in years)
Outstanding at the beginning of the period	7 years 5 months 1 day	6 years 9 months 21 days
Outstanding at the end of the period	7 years 5 months 1 day	6 years 9 months 21 days
Exercisable at the end of the period	6 years 2 months 19 days
Aggregate Intrinsic Value
Outstanding at the end of the period | $	$ 238,802
Exercisable at the end of the period | $	138,783
Aggregate intrinsic value of options exercised | $	67,700
Unrecognized compensation expense | $	$ 44,100
Weighted-average period for recognition	2 years 7 months 9 days

Stock-Based Compensation - Sc_3

Stock-Based Compensation - Schedule of Nonvested Restricted Stock Activity (Details) $ / shares in Units, shares in Thousands, $ in Millions	9 Months Ended
	Sep. 30, 2020USD ($)$ / sharesshares
Weighted- Average Grant Date Fair Value
Weighted-average period for recognition	2 years 7 months 9 days
Restricted Stock Units
Number of Grants
Unvested balance at beginning of the period (in shares) | shares	397
Granted (in shares) | shares	275
Vested (in shares) | shares	(137)
Forfeited (in shares) | shares	(38)
Unvested balance at end of the period (in shares) | shares	497
Weighted- Average Grant Date Fair Value
Unvested balance at beginning of the period (in dollars per share) | $ / shares	$ 39.20
Granted (in dollars per share) | $ / shares	67.45
Vested (in dollars per share) | $ / shares	38.64
Forfeited (in dollars per share) | $ / shares	42.80
Unvested balance at end of the period (in dollars per share) | $ / shares	$ 54.68
Unrecognized compensation cost | $	$ 20.7
Weighted-average period for recognition	1 year 9 months 14 days
Performance-Based Restricted Stock Units
Number of Grants
Unvested balance at beginning of the period (in shares) | shares	0
Granted (in shares) | shares	78
Vested (in shares) | shares	0
Forfeited (in shares) | shares	(3)
Unvested balance at end of the period (in shares) | shares	75
Weighted- Average Grant Date Fair Value
Unvested balance at beginning of the period (in dollars per share) | $ / shares	$ 0
Granted (in dollars per share) | $ / shares	52.99
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	$ 52.99
Unrecognized compensation cost | $	$ 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)	Sep. 30, 2020	Dec. 31, 2019
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	10.90%	12.00%

Subsequent Events (Details)

Subsequent Events (Details) - Subsequent Event	Oct. 16, 2020USD ($)
Subsequent Event [Line Items]
Operating lease, liability	$ 4,100,000
Minimum
Subsequent Event [Line Items]
Operating Leases, Rent Expense, Net	65,000
Maximum
Subsequent Event [Line Items]
Operating Leases, Rent Expense, Net	$ 71,800