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, 2018		116,887,518
Beginning balance at Dec. 31, 2018	$ 635,928	$ 1	$ 1,150,583	$ (261)	$ (514,395)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Proceeds from sale of stock under employee stock purchase plan (in shares)		39,977
Proceeds from sale of stock under    employee stock purchase plan	188		188
Exercise of options (in shares)		62,204
Exercise of options	365		365
Share-based compensation expense	2,094		2,094
Restricted stock unit vesting (in shares)		132,563
Restricted stock unit vesting	0
Unrealized gain (loss)	225			225
Net loss	(72,421)				(72,421)
Ending balance (in shares) at Mar. 31, 2019		117,122,262
Ending balance at Mar. 31, 2019	566,379	$ 1	1,153,230	(36)	(586,816)
Beginning balance (in shares) at Dec. 31, 2018		116,887,518
Beginning balance at Dec. 31, 2018	635,928	$ 1	1,150,583	(261)	(514,395)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Unrealized gain (loss)	267
Net loss	(185,176)
Ending balance (in shares) at Sep. 30, 2019		118,863,735
Ending balance at Sep. 30, 2019	467,562	$ 1	1,167,126	6	(699,571)
Beginning balance (in shares) at Dec. 31, 2018		116,887,518
Beginning balance at Dec. 31, 2018	$ 635,928	$ 1	1,150,583	(261)	(514,395)
Ending balance (in shares) at Dec. 31, 2019	121,674,568	121,674,568
Ending balance at Dec. 31, 2019	$ 394,757	$ 1	1,188,810	0	(794,054)
Beginning balance (in shares) at Mar. 31, 2019		117,122,262
Beginning balance at Mar. 31, 2019	566,379	$ 1	1,153,230	(36)	(586,816)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock, net of issuance costs (in shares)		1,384,520
Issuance of common stock, net of    issuance costs	9,035		9,035
Exercise of options (in shares)		300,592
Exercise of options	195		195
Retired shares (in shares)		(55,324)
Retired shares	(426)		(426)
Share-based compensation expense	2,284		2,284
Restricted stock unit vesting (in shares)		35,251
Restricted stock unit vesting	0
Unrealized gain (loss)	59			59
Net loss	(58,170)				(58,170)
Ending balance (in shares) at Jun. 30, 2019		118,787,301
Ending balance at Jun. 30, 2019	519,356	$ 1	1,164,318	23	(644,986)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Proceeds from sale of stock under employee stock purchase plan (in shares)		47,553
Proceeds from sale of stock under    employee stock purchase plan	195		195
Share-based compensation expense	2,613		2,613
Restricted stock unit vesting (in shares)		28,881
Restricted stock unit vesting	0
Unrealized gain (loss)	(17)			(17)
Net loss	(54,585)				(54,585)
Ending balance (in shares) at Sep. 30, 2019		118,863,735
Ending balance at Sep. 30, 2019	$ 467,562	$ 1	1,167,126	6	(699,571)
Beginning balance (in shares) at Dec. 31, 2019	121,674,568	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
Exercise of options (in shares)		64,126
Exercise of options	412		412
Share-based compensation expense	4,916		4,916
Restricted stock unit vesting (in shares)		423,755
Restricted stock unit vesting	0
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	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	143,328,652
Ending balance at Sep. 30, 2020	$ 317,962	$ 1	1,408,466	6	(1,090,511)
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
Exercise of options (in shares)		48,103
Exercise of options	409		409
Share-based compensation expense	6,864		6,864
Restricted stock unit vesting (in shares)		179,866
Restricted stock unit vesting	0
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
Exercise of options (in shares)		54,404
Exercise of options	405		405
Share-based compensation expense	6,592		6,592
Restricted stock unit vesting (in shares)		24,205
Restricted stock unit vesting	0
Unrealized gain (loss)	15			15
Net loss	$ (59,959)				(59,959)
Ending balance (in shares) at Sep. 30, 2020	143,328,652	143,328,652
Ending balance at Sep. 30, 2020	$ 317,962	$ 1	$ 1,408,466	$ 6	$ (1,090,511)