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, 2019		147,191,695
Beginning 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)		2,870
Issuance of common stock from option exercises	2		2
Stock-based compensation	5,501		5,501
Issuance of common stock from releases of restricted stock units (in shares)		202,420
Issuance of common stock from releases of restricted stock units	0	$ 2	(2)
Tax withholding paid on behalf of employees for stock-based awards	(131)		(131)
Net loss	(27,532)			(27,532)
Ending balance (in shares) at Mar. 31, 2020		147,396,985
Ending balance at Mar. 31, 2020	194,014	$ 1,464	739,258	(546,708)
Beginning balance (in shares) at Dec. 31, 2019		147,191,695
Beginning balance at Dec. 31, 2019	216,174	$ 1,462	733,888	(519,176)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(81,809)
Ending balance (in shares) at Sep. 30, 2020		147,954,426
Ending balance at Sep. 30, 2020	151,036	$ 1,469	750,552	(600,985)
Beginning balance (in shares) at Dec. 31, 2019		147,191,695
Beginning balance at Dec. 31, 2019	216,174	$ 1,462	733,888	(519,176)
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)
Beginning balance (in shares) at Mar. 31, 2020		147,396,985
Beginning balance at Mar. 31, 2020	194,014	$ 1,464	739,258	(546,708)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock from option exercises (in shares)		17,709
Issuance of common stock from option exercises	13		13
Stock-based compensation	5,802		5,802
Issuance of common stock from releases of restricted stock units (in shares)		119,447
Issuance of common stock from releases of restricted stock units	0	$ 1	(1)
Issuance of common stock from employee stock purchase plan (in shares)		82,232
Issuance of common stock from employee stock purchase plan	156	$ 1	155
Write-down of offering costs related to previous issuance of common stock upon public offering	191		191
Net loss	(26,152)			(26,152)
Ending balance (in shares) at Jun. 30, 2020		147,616,373
Ending balance at Jun. 30, 2020	174,024	$ 1,466	745,418	(572,860)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	5,759		5,759
Issuance of common stock from releases of restricted stock units (in shares)		338,053
Issuance of common stock from releases of restricted stock units	0	$ 3	(3)
Tax withholding paid on behalf of employees for stock-based awards	(622)		(622)
Net loss	(28,125)			(28,125)
Ending balance (in shares) at Sep. 30, 2020		147,954,426
Ending balance at Sep. 30, 2020	151,036	$ 1,469	750,552	(600,985)
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, 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)	127,913
Net loss	$ (82,980)
Ending balance (in shares) at Sep. 30, 2021		164,382,033
Ending balance at Sep. 30, 2021	117,748	$ 1,634	826,178	(710,064)
Beginning balance (in shares) at Mar. 31, 2021		161,730,363
Beginning balance at Mar. 31, 2021	164,405	$ 1,607	816,625	(653,827)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock from option exercises (in shares)		37,630
Issuance of common stock from option exercises	165		165
Stock-based compensation	6,455		6,455
Issuance of common stock from releases of restricted stock units (in shares)		1,738,516
Issuance of common stock from releases of restricted stock units	0	$ 17	(17)
Tax withholding paid on behalf of employees for stock-based awards	(1,286)		(1,286)
Issuance of common stock from warrant exercises (in shares)		2,431
Reclassification of warrant liability to additional paid-in capital upon warrant exercises	24		24
Issuance of common stock from employee stock purchase plan (in shares)		81,804
Issuance of common stock from employee stock purchase plan	264		264
Net loss	(30,976)			(30,976)
Ending balance (in shares) at Jun. 30, 2021		163,590,744
Ending balance at Jun. 30, 2021	139,051	$ 1,624	822,230	(684,803)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock from option exercises (in shares)		84,262
Issuance of common stock from option exercises	427	$ 3	424
Stock-based compensation	4,634		4,634
Issuance of common stock from releases of restricted stock units (in shares)		707,027
Issuance of common stock from releases of restricted stock units	0	$ 7	(7)
Tax withholding paid on behalf of employees for stock-based awards	(1,103)		(1,103)
Net loss	(25,261)			(25,261)
Ending balance (in shares) at Sep. 30, 2021		164,382,033
Ending balance at Sep. 30, 2021	$ 117,748	$ 1,634	$ 826,178	$ (710,064)