Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity - USD ($) $ in Thousands	Total	Public Offering	Direct Registered Offering	At The Market Offering Program	Convertible Preferred Stock Direct Registered Offering	Additional Paid-in Capital	Additional Paid-in Capital Public Offering	Additional Paid-in Capital Direct Registered Offering	Additional Paid-in Capital At The Market Offering Program	Additional Paid-in Capital Convertible Preferred Stock Direct Registered Offering	Common Stock	Common Stock Public Offering	Common Stock Direct Registered Offering	Common Stock At The Market Offering Program	Accumulated Deficit	Accumulated Deficit Direct Registered Offering
Balance at Dec. 31, 2017	$ 1,987					$ 321,174					$ 666				$ (319,853)
Balance, shares at Dec. 31, 2017											67,653,974
Issuance of common stock from option exercises	5,285					5,259					$ 26
Issuance of common stock from option exercises, shares											2,608,812
Issuance of common stock from releases of restricted stock units, shares											59,437
Stock-based compensation	14,169					14,169
Issuance of common stock, value		$ 161,869	$ 30,093	$ 272			$ 161,682	$ 30,052	$ 272			$ 187	$ 41
Issuance of common stock, shares												18,648,649	4,090,000	33,097
Issuance of preferred Series A stock upon direct registered offering, value			22,207		$ 30			2,728		$ 22,177						$ (2,728)
Issuance of preferred Series A stock upon direct registered offering, share					3,000,581
Issuance of common stock warrants in connection with direct registered offering			$ 6,623					6,623
Conversion of Series A preferred stock into common stock, value					$ (30)			$ 22,177		$ (22,177)			$ 30
Conversion of Series A preferred stock into common stock, share					(3,000,581)								3,000,581
Issuance of common stock from warrant exercises	3					1					$ 2
Issuance of common stock from warrant exercises, shares											237,473
Reclassification of warrant liability to additional paid-in capital upon warrant exercises	1,197					1,197
Net loss	(76,396)														(76,396)
Balance at Dec. 31, 2018	167,309					565,334					$ 952				(398,977)
Balance, shares at Dec. 31, 2018											96,332,023
Issuance of common stock from option exercises	9,641					9,618					$ 23
Issuance of common stock from option exercises, shares											2,219,251
Issuance of common stock from releases of restricted stock units						(4)					$ 4
Issuance of common stock from releases of restricted stock units, shares											393,722
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, value		138,413					137,935					$ 478
Issuance of common stock, shares												47,782,500
Issuance of common stock from warrant exercises						(5)					$ 5
Issuance of common stock from warrant exercises, shares											464,199
Reclassification of warrant liability to additional paid-in capital upon warrant exercises	3,976					3,976
Net loss	(120,199)														(120,199)
Balance at Dec. 31, 2019	216,174					733,888					$ 1,462				(519,176)
Balance, shares at Dec. 31, 2019											147,191,695
Issuance of common stock from option exercises	$ 15					15
Issuance of common stock from option exercises, shares	20,579										20,579
Issuance of common stock from releases of restricted stock units						(12)					$ 12
Issuance of common stock from releases of restricted stock units, shares											1,214,786
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	365					363					$ 2
Issuance of common stock from employee stock purchase plan, shares											188,291
Write-down of offering costs related to previous issuance of common stock upon public offering		$ 191					$ 191
Net loss	(107,908)														(107,908)
Balance at Dec. 31, 2020	$ 130,266					$ 755,874					$ 1,476				$ (627,084)
Balance, shares at Dec. 31, 2020											148,615,351