Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Deficit
Beginning balance (in shares) at Dec. 31, 2018		96,332,023
Beginning balance at Dec. 31, 2018	$ 167,309	$ 952	$ 565,334	$ (398,977)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock from option exercises (in shares)		2,219,251
Issuance of common stock from option exercises	9,641	$ 23	9,618
Issuance of common stock from releases of restricted stock units (in shares)		393,722
Issuance of common stock from releases of restricted stock units	0	$ 4	(4)
Tax withholding paid on behalf of employees for stock-based awards	(2,410)		(2,410)
Stock-based compensation	19,444		19,444
Issuance of common stock upon public offering, net of offering costs (in shares)		47,782,500
Issuance of common stock upon public offering, net of offering costs	138,413	$ 478	137,935
Issuance of common stock from warrant exercises (in shares)		464,199
Issuance of common stock from warrant exercises	0	$ 5	(5)
Reclassification of warrant liability to additional paid-in capital upon warrant exercises	3,976		3,976
Net loss	(120,199)			(120,199)
Ending balance (in shares) at Dec. 31, 2019		147,191,695
Ending balance at Dec. 31, 2019	216,174	$ 1,462	733,888	(519,176)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock from option exercises (in shares)		20,579
Issuance of common stock from option exercises	15		15
Issuance of common stock from releases of restricted stock units (in shares)		1,214,786
Issuance of common stock from releases of restricted stock units	0	$ 12	(12)
Tax withholding paid on behalf of employees for stock-based awards	(1,376)		(1,376)
Stock-based compensation	22,805		22,805
Issuance of common stock from employee stock purchase plan (in shares)		188,291
Issuance of common stock from employee stock purchase plan	365	$ 2	363
Write-down of offering costs related to previous issuance of common stock upon public offering	191		191
Net loss	(107,908)			(107,908)
Ending balance (in shares) at Dec. 31, 2020		148,615,351
Ending 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)	156,199	156,199
Issuance of common stock from option exercises	$ 775	$ 3	772
Issuance of common stock from releases of restricted stock units (in shares)		4,015,380
Issuance of common stock from releases of restricted stock units	0	$ 40	(40)
Tax withholding paid on behalf of employees for stock-based awards	(4,600)		(4,600)
Stock-based compensation	23,871		23,871
Issuance of common stock upon public offering, net of offering costs (in shares)		26,231,500
Issuance of common stock upon public offering, net of offering costs	128,626	$ 262	128,364
Issuance of common stock from employee stock purchase plan (in shares)		142,974
Issuance of common stock from employee stock purchase plan	550	$ 1	549
Fair value of warrants upon exercise	352		352
Issuance of common stock from warrant exercises (in shares)		44,287
Issuance of common stock from warrant exercise (cash exercise) (in shares)		765
Issuance of common stock from warrant exercise (cash exercise)	3		3
Net loss	(110,048)			(110,048)
Ending balance (in shares) at Dec. 31, 2021		179,206,456
Ending balance at Dec. 31, 2021	$ 169,795	$ 1,782	$ 905,145	$ (737,132)