Consolidated Statements of Stockholders’ Equity (Deficit) - USD ($) $ in Thousands	Total	Aspire Capital Fund, LLC	Demand Sales Agreement	Common Stock	Common Stock Private Placement	Common Stock Aspire Capital Fund, LLC	Common Stock Demand Sales Agreement	Additional Paid-in Capital	Additional Paid-in Capital Private Placement	Additional Paid-in Capital Aspire Capital Fund, LLC	Additional Paid-in Capital Demand Sales Agreement	Accumulated Deficit	Accumulated Other Comprehensive Loss
Beginning balance (in shares) at Dec. 31, 2018				33,159,253
Beginning balance at Dec. 31, 2018	$ (4,496)			$ 332				$ 980,012				$ (984,840)	$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuances of common stock for purchases under the ESPP (in shares)				91,013
Issuances of common stock for purchases under the ESPP	68							68
Recognition of stock-based compensation	2,658							2,658
Cancellation of restricted stock awards (in shares)				(8,473)
Cancellation of restricted stock awards	0
Net loss	(32,141)											(32,141)
Ending balance (in shares) at Dec. 31, 2019				33,241,793
Ending balance at Dec. 31, 2019	(33,911)			$ 332				982,738				(1,016,981)	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuances of common stock for purchases under the ESPP (in shares)				80,544
Issuances of common stock for purchases under the ESPP	58							58
Recognition of stock-based compensation	2,698							2,698
Issuance of stock (in shares)					14,000,000	7,990,516	6,298,648
Issuance of stock	15,965	$ 8,099	$ 7,914		$ 140	$ 81	$ 63		$ 15,825	$ 8,018	$ 7,851
Issuance of common stock under registration statement, net of issuance costs (in shares)				29,500,000
Issuance of common stock under registration statement, net of issuance costs	159,031			$ 295				158,736
Exercise of stock options (in shares)				390,960
Exercise of stock options	727			$ 4				723
Unrealized loss on marketable securities	(3)												(3)
Net loss	$ (29,908)											(29,908)
Ending balance (in shares) at Dec. 31, 2020	91,502,461			91,502,461
Ending balance at Dec. 31, 2020	$ 130,670			$ 915				1,176,647				(1,046,889)	(3)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuances of common stock for purchases under the ESPP (in shares)				43,860
Issuances of common stock for purchases under the ESPP	135							135
Recognition of stock-based compensation	$ 5,279							5,279
Exercise of stock options (in shares)	98,886			99,048
Exercise of stock options	$ 165			$ 1				164
Unrealized loss on marketable securities	(106)												(106)
Net loss	$ (45,436)											(45,436)
Ending balance (in shares) at Dec. 31, 2021	91,645,369			91,645,369
Ending balance at Dec. 31, 2021	$ 90,707			$ 916				$ 1,182,225				$ (1,092,325)	$ (109)