Condensed Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Public Offering	Common Stock	Common Stock Public Offering	Additional Paid-in Capital	Additional Paid-in Capital Public Offering	Accumulated Deficit
Balance at Dec. 31, 2019	$ 216,174		$ 1,462		$ 733,888		$ (519,176)
Balance, shares at Dec. 31, 2019			147,191,695
Issuance of common stock from option exercises	2				2
Issuance of common stock from option exercises, shares			2,870
Stock-based compensation	5,501				5,501
Issuance of common stock from releases of restricted stock units			$ 2		(2)
Issuance of common stock from releases of restricted stock units, shares			202,420
Tax withholding paid on behalf of employees for stock-based awards	(131)				(131)
Net loss	(27,532)						(27,532)
Balance at Mar. 31, 2020	194,014		$ 1,464		739,258		(546,708)
Balance, shares at Mar. 31, 2020			147,396,985
Balance at Dec. 31, 2019	216,174		$ 1,462		733,888		(519,176)
Balance, shares at Dec. 31, 2019			147,191,695
Net loss	(53,684)
Balance at Jun. 30, 2020	174,024		$ 1,466		745,418		(572,860)
Balance, shares at Jun. 30, 2020			147,616,373
Balance at Dec. 31, 2019	216,174		$ 1,462		733,888		(519,176)
Balance, shares at Dec. 31, 2019			147,191,695
Balance at Dec. 31, 2020	130,266		$ 1,476		755,874		(627,084)
Balance, shares at Dec. 31, 2020			148,615,351
Balance at Mar. 31, 2020	194,014		$ 1,464		739,258		(546,708)
Balance, shares at Mar. 31, 2020			147,396,985
Issuance of common stock from option exercises	13				13
Issuance of common stock from option exercises, shares			17,709
Stock-based compensation	5,802				5,802
Issuance of common stock from releases of restricted stock units			$ 1		(1)
Issuance of common stock from releases of restricted stock units, shares			119,447
Issuance of common stock from employee stock purchase plan	156		$ 1		155
Issuance of common stock from employee stock purchase plan, shares			82,232
Write-down of offering costs related to previous issuance of common stock upon public offering		$ 191				$ 191
Net loss	(26,152)						(26,152)
Balance at Jun. 30, 2020	174,024		$ 1,466		745,418		(572,860)
Balance, shares at Jun. 30, 2020			147,616,373
Balance at Dec. 31, 2020	130,266		$ 1,476		755,874		(627,084)
Balance, shares at Dec. 31, 2020			148,615,351
Issuance of common stock from option exercises	19				19
Issuance of common stock from option exercises, shares			6,021
Stock-based compensation	8,494				8,494
Issuance of common stock from releases of restricted stock units			$ 12		(12)
Issuance of common stock from releases of restricted stock units, shares			1,209,870
Tax withholding paid on behalf of employees for stock-based awards	(1,473)				(1,473)
Issuance of common stock upon public offering		$ 53,513		$ 119		$ 53,394
Issuance of common stock upon public offering, shares				11,856,500
Issuance of common stock from warrant exercises	2				2
Issuance of common stock from warrant exercises, shares			42,621
Reclassification of warrant liability to additional paid-in capital upon warrant exercises	327				327
Net loss	(26,743)						(26,743)
Balance at Mar. 31, 2021	164,405		$ 1,607		816,625		(653,827)
Balance, shares at Mar. 31, 2021			161,730,363
Balance at Dec. 31, 2020	$ 130,266		$ 1,476		755,874		(627,084)
Balance, shares at Dec. 31, 2020			148,615,351
Issuance of common stock from option exercises, shares	43,651
Net loss	$ (57,719)
Balance at Jun. 30, 2021	139,051		$ 1,624		822,230		(684,803)
Balance, shares at Jun. 30, 2021			163,590,744
Balance at Mar. 31, 2021	164,405		$ 1,607		816,625		(653,827)
Balance, shares at Mar. 31, 2021			161,730,363
Issuance of common stock from option exercises	165				165
Issuance of common stock from option exercises, shares			37,630
Stock-based compensation	6,455				6,455
Issuance of common stock from releases of restricted stock units			$ 17		(17)
Issuance of common stock from releases of restricted stock units, shares			1,738,516
Tax withholding paid on behalf of employees for stock-based awards	(1,286)				(1,286)
Issuance of common stock from warrant exercises, 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	264				264
Issuance of common stock from employee stock purchase plan, shares			81,804
Net loss	(30,976)						(30,976)
Balance at Jun. 30, 2021	$ 139,051		$ 1,624		$ 822,230		$ (684,803)
Balance, shares at Jun. 30, 2021			163,590,744