Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Loss [Member]	Accumulated Deficit [Member]
Beginning balance at Dec. 31, 2018	$ 79,737	$ 31	$ 494,947	$ (23)	$ (415,218)
Beginning balance, shares at Dec. 31, 2018		30,685,412
Issuance of common stock upon exercise of stock options	5		5
Issuance of common stock upon exercise of stock options, shares		2,912
Issuance of common stock under employee stock purchase plan	242		242
Issuance of common stock under employee stock purchase plan, shares		125,887
Stock-based compensation expense	4,890		4,890
Net loss	(44,675)				(44,675)
Unrealized (loss) gain on available-for-sale securities	34			34
Ending balance at Dec. 31, 2019	40,233	$ 31	500,084	11	(459,893)
Ending balance, shares at Dec. 31, 2019		30,814,211
Issuance of common stock and pre-funded warrants, net of issuance costs	64,187	$ 17	64,170
Issuance of common stock and prefunded warrants, net of issuance costs (unaudited), shares		17,275,000
Issuance of common stock upon exercise of stock options	206		206
Issuance of common stock upon exercise of stock options, shares		92,035
Issuance of common stock under employee stock purchase plan	270		270
Issuance of common stock under employee stock purchase plan, shares		137,724
Stock-based compensation expense	6,111		6,111
Net loss	(44,731)				(44,731)
Unrealized (loss) gain on available-for-sale securities	(10)			(10)
Ending balance at Dec. 31, 2020	66,266	$ 48	570,841	1	(504,624)
Ending balance, shares at Dec. 31, 2020		48,318,970
Issuance of common stock and pre-funded warrants, net of issuance costs	32,207	$ 9	32,198
Issuance of common stock and prefunded warrants, net of issuance costs (unaudited), shares		8,298,890
Issuance of common stock upon exercise of stock options	$ 6		6
Issuance of common stock upon exercise of stock options, shares	3,000	2,836
Issuance of common stock under employee stock purchase plan	$ 213		213
Issuance of common stock under employee stock purchase plan, shares		111,778
Stock-based compensation expense	7,397		7,397
Net loss	(51,181)				(51,181)
Unrealized (loss) gain on available-for-sale securities	(1)			(1)
Ending balance at Dec. 31, 2021	$ 54,907	$ 57	$ 610,655	$ 0	$ (555,805)
Ending balance, shares at Dec. 31, 2021		56,732,474