Condensed Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Unrealized Gain/(Loss)	Accumulated Deficit
Beginning balance (in shares) at Dec. 31, 2019		121,674,568
Beginning balance at Dec. 31, 2019	$ 394,757	$ 1	$ 1,188,810	$ 0	$ (794,054)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock, net of issuance costs (in shares)		7,973,967
Issuance of common stock, net of    issuance costs	56,575		56,575
Proceeds from sale of stock under employee stock purchase plan (in shares)		115,024
Proceeds from sale of stock under    employee stock purchase plan	451		451
Share-based compensation expense	4,916		4,916
Exercise of options (in shares)		64,126
Exercise of options	412		412
Restricted stock unit vesting (in shares)		423,755
Net loss	(60,747)				(60,747)
Ending balance (in shares) at Mar. 31, 2020		130,251,440
Ending balance at Mar. 31, 2020	396,364	$ 1	1,251,164	0	(854,801)
Beginning balance (in shares) at Dec. 31, 2019		121,674,568
Beginning balance at Dec. 31, 2019	394,757	$ 1	1,188,810	0	(794,054)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Unrealized gain (loss)	6
Net loss	(296,457)
Ending balance (in shares) at Sep. 30, 2020		143,328,652
Ending balance at Sep. 30, 2020	317,962	$ 1	1,408,466	6	(1,090,511)
Beginning balance (in shares) at Dec. 31, 2019		121,674,568
Beginning balance at Dec. 31, 2019	$ 394,757	$ 1	1,188,810	0	(794,054)
Ending balance (in shares) at Dec. 31, 2020	148,074,085	148,074,085
Ending balance at Dec. 31, 2020	$ 247,618	$ 1	1,425,115	13	(1,177,511)
Beginning balance (in shares) at Mar. 31, 2020		130,251,440
Beginning balance at Mar. 31, 2020	396,364	$ 1	1,251,164	0	(854,801)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock, net of issuance costs (in shares)		12,650,000
Issuance of common stock, net of    issuance costs	142,383		142,383
Share-based compensation expense	6,864		6,864
Exercise of options (in shares)		48,103
Exercise of options	409		409
Restricted stock unit vesting (in shares)		179,866
Unrealized gain (loss)	(9)			(9)
Net loss	(175,751)				(175,751)
Ending balance (in shares) at Jun. 30, 2020		143,129,409
Ending balance at Jun. 30, 2020	370,260	$ 1	1,400,820	(9)	(1,030,552)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Proceeds from sale of stock under employee stock purchase plan (in shares)		120,634
Proceeds from sale of stock under    employee stock purchase plan	649		649
Share-based compensation expense	6,592		6,592
Exercise of options (in shares)		54,404
Exercise of options	405		405
Restricted stock unit vesting (in shares)		24,205
Unrealized gain (loss)	15			15
Net loss	(59,959)				(59,959)
Ending balance (in shares) at Sep. 30, 2020		143,328,652
Ending balance at Sep. 30, 2020	$ 317,962	$ 1	1,408,466	6	(1,090,511)
Beginning balance (in shares) at Dec. 31, 2020	148,074,085	148,074,085
Beginning balance at Dec. 31, 2020	$ 247,618	$ 1	1,425,115	13	(1,177,511)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock, net of issuance costs (in shares)		9,228,017
Issuance of common stock, net of    issuance costs	29,498	$ 1	29,497
Proceeds from sale of stock under employee stock purchase plan (in shares)		154,276
Proceeds from sale of stock under    employee stock purchase plan	367		367
Share-based compensation expense	5,992		5,992
Restricted stock unit vesting (in shares)		1,063,711
Unrealized gain (loss)	(4)			(4)
Net loss	(69,580)				(69,580)
Ending balance (in shares) at Mar. 31, 2021		158,520,089
Ending balance at Mar. 31, 2021	$ 213,891	$ 2	1,460,971	9	(1,247,091)
Beginning balance (in shares) at Dec. 31, 2020	148,074,085	148,074,085
Beginning balance at Dec. 31, 2020	$ 247,618	$ 1	1,425,115	13	(1,177,511)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Unrealized gain (loss)	(7)
Net loss	$ (212,162)
Ending balance (in shares) at Sep. 30, 2021	174,551,989	174,551,989
Ending balance at Sep. 30, 2021	$ 137,150	$ 2	1,526,815	6	(1,389,673)
Beginning balance (in shares) at Mar. 31, 2021		158,520,089
Beginning balance at Mar. 31, 2021	213,891	$ 2	1,460,971	9	(1,247,091)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock, net of issuance costs (in shares)		10,446,160
Issuance of common stock, net of    issuance costs	37,266		37,266
Share-based compensation expense	6,515		6,515
Restricted stock unit vesting (in shares)		685,174
Unrealized gain (loss)	(3)			(3)
Net loss	(83,038)				(83,038)
Ending balance (in shares) at Jun. 30, 2021		169,651,423
Ending balance at Jun. 30, 2021	174,631	$ 2	1,504,752	6	(1,330,129)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock, net of issuance costs (in shares)		4,730,466
Issuance of common stock, net of    issuance costs	16,092		16,092
Proceeds from sale of stock under employee stock purchase plan (in shares)		152,917
Proceeds from sale of stock under    employee stock purchase plan	379		379
Share-based compensation expense	5,592		5,592
Restricted stock unit vesting (in shares)		17,183
Unrealized gain (loss)	0
Net loss	$ (59,544)				(59,544)
Ending balance (in shares) at Sep. 30, 2021	174,551,989	174,551,989
Ending balance at Sep. 30, 2021	$ 137,150	$ 2	$ 1,526,815	$ 6	$ (1,389,673)