Condensed Consolidated Statements of Stockholders’ Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Deficit
Beginning balance (in shares) at Dec. 31, 2020		148,615,351
Beginning balance at Dec. 31, 2020	$ 130,266	$ 1,476	$ 755,874	$ (627,084)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock from option exercises (in shares)		6,021
Issuance of common stock from option exercises	19		19
Stock-based compensation	8,494		8,494
Issuance of common stock from releases of restricted stock units (in shares)		1,209,870
Issuance of common stock from releases of restricted stock units	0	$ 12	(12)
Tax withholding paid on behalf of employees for stock-based awards	(1,473)		(1,473)
Issuance of common stock upon public offering, net (in shares)		11,856,500
Issuance of common stock upon public offering (net of offering cost of $3,991)	53,513	$ 119	53,394
Issuance of common stock from warrant exercises (in shares)		42,621
Issuance of common stock from warrant exercises	2		2
Reclassification of warrant liability to additional paid-in capital upon warrant exercises	327		327
Net loss	(26,743)			(26,743)
Ending balance (in shares) at Mar. 31, 2021		161,730,363
Ending balance at Mar. 31, 2021	164,405	$ 1,607	816,625	(653,827)
Beginning balance (in shares) at Dec. 31, 2021		179,206,456
Beginning balance at Dec. 31, 2021	$ 169,795	$ 1,782	905,145	(737,132)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock from option exercises (in shares)	19,292	19,292
Issuance of common stock from option exercises	$ 56		56
Stock-based compensation	5,032		5,032
Issuance of common stock from releases of restricted stock units (in shares)		1,216,278
Issuance of common stock from releases of restricted stock units	0	$ 12	(12)
Tax withholding paid on behalf of employees for stock-based awards	(1,604)		(1,604)
Net loss	(25,774)			(25,774)
Ending balance (in shares) at Mar. 31, 2022		180,442,026
Ending balance at Mar. 31, 2022	$ 147,505	$ 1,794	$ 908,617	$ (762,906)