Unaudited Consolidated Statements of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Underwritten Offering	Private Placement	Preferred Stock	Common Stock	Common Stock Underwritten Offering	Common Stock Private Placement	Additional paid-in capital	Additional paid-in capital Underwritten Offering	Additional paid-in capital Private Placement	Accumulated Deficit
Balance at beginning of period (in shares) at Dec. 31, 2018				0	41,397
Balance at beginning of period at Dec. 31, 2018	$ 123,959			$ 0	$ 41			$ 545,585			$ (421,667)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under employee stock compensation plan (in shares)					56
Issuance of common stock under employee stock compensation plans	41							41
Share-based compensation	2,470							2,470
Net loss	(12,501)										(12,501)
Balance at end of period (in shares) at Mar. 31, 2019				0	41,453
Balance at end of period at Mar. 31, 2019	113,969			$ 0	$ 41			548,096			(434,168)
Balance at beginning of period (in shares) at Dec. 31, 2018				0	41,397
Balance at beginning of period at Dec. 31, 2018	123,959			$ 0	$ 41			545,585			(421,667)
Increase (Decrease) in Stockholders' Equity
Net loss	(26,944)
Balance at end of period (in shares) at Jun. 30, 2019				0	41,477
Balance at end of period at Jun. 30, 2019	101,733			$ 0	$ 41			550,303			(448,611)
Balance at beginning of period (in shares) at Mar. 31, 2019				0	41,453
Balance at beginning of period at Mar. 31, 2019	113,969			$ 0	$ 41			548,096			(434,168)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under employee stock compensation plan (in shares)					24
Issuance of common stock under employee stock compensation plans	0							0
Share-based compensation	2,207							2,207
Net loss	(14,443)										(14,443)
Balance at end of period (in shares) at Jun. 30, 2019				0	41,477
Balance at end of period at Jun. 30, 2019	101,733			$ 0	$ 41			550,303			(448,611)
Balance at beginning of period (in shares) at Dec. 31, 2019				0	49,627
Balance at beginning of period at Dec. 31, 2019	117,203			$ 0	$ 50			597,679			(480,526)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under employee stock compensation plan (in shares)					105
Issuance of common stock under employee stock compensation plans	179							179
Share-based compensation	2,324							2,324
Net loss	(15,076)										(15,076)
Balance at end of period (in shares) at Mar. 31, 2020				0	49,732
Balance at end of period at Mar. 31, 2020	104,630			$ 0	$ 50			600,182			(495,602)
Balance at beginning of period (in shares) at Dec. 31, 2019				0	49,627
Balance at beginning of period at Dec. 31, 2019	117,203			$ 0	$ 50			597,679			(480,526)
Increase (Decrease) in Stockholders' Equity
Net loss	(33,665)
Balance at end of period (in shares) at Jun. 30, 2020				0	89,489
Balance at end of period at Jun. 30, 2020	238,041			$ 0	$ 89			752,143			(514,191)
Balance at beginning of period (in shares) at Mar. 31, 2020				0	49,732
Balance at beginning of period at Mar. 31, 2020	104,630			$ 0	$ 50			600,182			(495,602)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock, net of issuance costs (in shares)						28,503	8,649
Issuance of common stock, net of issuance costs		$ 116,883	$ 33,237			$ 28	$ 9		$ 116,855	$ 33,228
Issuance of common stock under the exercise of pre-funded warrants (in shares)					2,500
Issuance of common stock under the exercise of pre-funded warrants	0				$ 2			(2)
Issuance of common stock under employee stock compensation plan (in shares)					105
Issuance of common stock under employee stock compensation plans	7							7
Share-based compensation	1,873							1,873
Net loss	(18,589)										(18,589)
Balance at end of period (in shares) at Jun. 30, 2020				0	89,489
Balance at end of period at Jun. 30, 2020	$ 238,041			$ 0	$ 89			$ 752,143			$ (514,191)