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, 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]
Unrealized loss on available-for-sale securities, net of tax	(106)
Net loss	(69,391)
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)
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)
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)		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)
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]
Exercise of stock options (in shares)	250,000
Unrealized loss on available-for-sale securities, net of tax	$ (121)
Net loss	(126,992)
Stockholders' equity ending balance (in shares) at Jun. 30, 2021		60,777,558
Stockholders' equity ending balance at Jun. 30, 2021	763,034	$ 61	1,767,811	(1,004,429)	(409)
Stockholders' equity beginning balance (in shares) at Mar. 31, 2021		60,638,494
Stockholders' equity beginning balance at Mar. 31, 2021	810,831	$ 60	1,752,096	(940,908)	(417)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	13,479		13,479
Exercise of stock options (in shares)		100,450
Exercise of stock options	4,055		4,055
Vesting of restricted stock units, net of shares withheld for taxes (in shares)		38,614
Vesting of restricted stock units, net of shares withheld for taxes	(1,818)	$ 1	(1,819)
Unrealized loss on available-for-sale securities, net of tax	8				8
Net loss	(63,521)			(63,521)
Stockholders' equity ending balance (in shares) at Jun. 30, 2021		60,777,558
Stockholders' equity ending balance at Jun. 30, 2021	$ 763,034	$ 61	$ 1,767,811	$ (1,004,429)	$ (409)

ACCELERON PHARMA INC 10-Q