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	Accumulated Other Comprehensive Income (Loss)	AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-sale, Parent
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)		$ 0
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)	$ 0
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)		0
Increase (Decrease) in Stockholders' Equity
Net loss	(41,381)
Balance at end of period (in shares) at Sep. 30, 2019				0	41,602
Balance at end of period at Sep. 30, 2019	89,462			$ 0	$ 42			552,468			(463,048)	0	0
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)	0
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)	0	0
Increase (Decrease) in Stockholders' Equity
Issuance of common stock, net of issuance costs (in shares)					75
Issuance of common stock, net of issuance costs	0
Issuance of common stock under employee stock compensation plan (in shares)					50
Issuance of common stock under employee stock compensation plans	40				$ 1			39
Share-based compensation	2,126							2,126
Net loss	(14,437)										(14,437)
Balance at end of period (in shares) at Sep. 30, 2019				0	41,602
Balance at end of period at Sep. 30, 2019	89,462			$ 0	$ 42			552,468			(463,048)	0	0
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)		0
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)	0
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)		0
Increase (Decrease) in Stockholders' Equity
Net loss	(59,115)
Balance at end of period (in shares) at Sep. 30, 2020				0	89,554
Balance at end of period at Sep. 30, 2020	214,586			$ 0	$ 90			754,136			(539,641)	1	1
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)	0
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)	0	0
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under employee stock compensation plan (in shares)					65
Issuance of common stock under employee stock compensation plans	147				$ 1			146
Share-based compensation	1,847							1,847
Unrealized gain on available for sale securities, net of tax	1												1
Net loss	(25,450)										(25,450)
Balance at end of period (in shares) at Sep. 30, 2020				0	89,554
Balance at end of period at Sep. 30, 2020	$ 214,586			$ 0	$ 90			$ 754,136			$ (539,641)	$ 1	$ 1