CONDENSED STATEMENTS OF REDEEMABLE PREFERRED STOCK AND STOCKHOLDER’S EQUITY (DEFICIT) - USD ($) shares in Thousands, $ in Thousands	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Accumulated Other Comprehensive Income	Series B1 redeemable convertible preferred stock (Note 7); Designated — 278 shares, Issued and outstanding — 24 and 0 shares at December 31, 2019 and December 31, 2018, respectively	Total
Beginning balance at Dec. 31, 2016	$ 18	$ 641,818	$ (538,470)	$ (17)		$ 103,349
Beginning balance (in shares) at Dec. 31, 2016	18,633
Cumulative effect from adoption of new accounting standard (Note 2)		40	(40)
Sale of common stock, net of issuance costs	$ 5	53,741				53,746
Sale of common stock, net of issuance costs (in shares)	4,792
Issuance of common stock under employee stock purchase plan		253				253
Issuance of common stock under employee stock purchase plan (in shares)	22
Issuance of common stock upon exercise of common stock options and warrants	$ 1	5,443				5,444
Issuance of common stock upon exercise of common stock options and warrants (in shares)	996
Issuance of common stock for services rendered		150				150
Issuance of common stock for services rendered (in shares)	10
Stock-based compensation expense		10,720				10,720
Unrealized gain on marketable securities				$ 17		17
Net loss			(65,984)			(65,984)
Ending balance at Dec. 31, 2017	$ 24	712,165	(604,494)			107,695
Ending 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					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					23,684
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 warrants		3				3
Issuance of common stock upon exercise of 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				23,684	29,672