Condensed Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Unrealized Gain/(Loss)	Accumulated Deficit
Balance at Dec. 31, 2018	$ 635,928	$ 1	$ 1,150,583	$ (261)	$ (514,395)
Balance, shares at Dec. 31, 2018		116,887,518
Proceeds from sale of stock under employee stock purchase plan	188		188
Proceeds from sale of stock under employee stock purchase plan, shares		39,977
Exercise of options	365		365
Exercise of options, shares		62,204
Share-based compensation expense	2,094		2,094
Restricted stock unit vesting, shares		132,563
Unrealized gain	225			225
Net loss	(72,421)				(72,421)
Balance at Mar. 31, 2019	566,379	$ 1	1,153,230	(36)	(586,816)
Balance, shares at Mar. 31, 2019		117,122,262
Balance at Dec. 31, 2018	635,928	$ 1	1,150,583	$ (261)	(514,395)
Balance, shares at Dec. 31, 2018		116,887,518
Balance at Dec. 31, 2019	$ 394,757	$ 1	1,188,810		(794,054)
Balance, shares at Dec. 31, 2019	121,674,568	121,674,568
Issuance of common stock, net of issuance costs	$ 56,575		56,575
Issuance of common stock, net of issuance costs, shares		7,973,967
Proceeds from sale of stock under employee stock purchase plan	451		451
Proceeds from sale of stock under employee stock purchase plan, shares		115,024
Exercise of options	412		412
Exercise of options, shares		64,126
Share-based compensation expense	4,916		4,916
Restricted stock unit vesting, shares		423,755
Net loss	(60,747)				(60,747)
Balance at Mar. 31, 2020	$ 396,364	$ 1	$ 1,251,164		$ (854,801)
Balance, shares at Mar. 31, 2020	130,251,440	130,251,440