STATEMENTS OF REDEEMABLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) - USD ($) shares in Thousands, $ in Thousands	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Series B1 Redeemable Convertible Preferred Stock	Total
Beginning balance at Dec. 31, 2017	$ 24	$ 712,165	$ (604,494)		$ 107,695
Beginning balance (in shares) at Dec. 31, 2017	24,453
Issuance of common stock under employee stock purchase plan		243			243
Issuance of common stock under employee stock purchase plan (in shares)	25
Issuance of common stock upon exercise of common stock options and warrants	$ 3	10,163			10,166
Issuance of common stock upon exercise of common stock options and warrants (in shares)	2,702
Issuance of common stock for services rendered		97			97
Issuance of common stock for services rendered (in shares)	8
Stock-based compensation expense		5,674			5,674
Net loss			(59,881)		(59,881)
Ending balance at Dec. 31, 2018	$ 27	728,342	(664,375)		63,994
Ending balance (in shares) at Dec. 31, 2018	27,188
Sale of common stock, net of issuance costs	$ 3	5,295			5,298
Sale of common stock, net of issuance costs (in shares)	2,068
Sale of redeemable convertible preferred stock				24
Deemed dividend related to December 2019 Private Placement (Note 7)		(28,043)			(28,043)
Issuance of commitment shares (in shares)	270
Issuance of common stock under employee stock purchase plan		121			121
Issuance of common stock under employee stock purchase plan (in shares)	61
Issuance of common stock upon exercise of common stock options and warrants		3			3
Issuance of common stock upon exercise of common stock options and warrants (in shares)	38
Issuance of common stock for services rendered		129			129
Issuance of common stock for services rendered (in shares)	47
Stock-based compensation expense		3,845			3,845
Net loss			(56,515)		(56,515)
Ending balance at Dec. 31, 2019	$ 30	709,692	(720,890)		$ (11,168)
Ending balance (in shares) at Dec. 31, 2019	29,672			24	29,672
Sale of common stock, net of issuance costs	$ 8	28,638			$ 28,646
Sale of common stock, net of issuance costs (in shares)	8,218
Issuance of common stock under equity incentive plan (vesting of restricted stock units) (in shares)	177
Issuance of common stock under employee stock purchase plan		113			113
Issuance of common stock under employee stock purchase plan (in shares)	76
Issuance of common stock upon exercise of warrants		15			15
Issuance of common stock upon exercise of warrants (in shares)	5
Issuance of common stock for services rendered		243			243
Issuance of common stock for services rendered (in shares)	143
Stock-based compensation expense		3,641			3,641
Net loss			(112,662)		(112,662)
Ending balance at Dec. 31, 2020	$ 38	$ 742,342	$ (833,552)		$ (91,172)
Ending balance (in shares) at Dec. 31, 2020	38,291			24	38,291