Condensed Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Deficit
Balance at Dec. 31, 2018	$ 167,309	$ 952	$ 565,334	$ (398,977)
Balance, shares at Dec. 31, 2018		96,332,023
Issuance of common stock from option exercises	9,641	$ 23	9,618
Issuance of common stock from option exercises, shares		2,219,251
Stock-based compensation	14,369		14,369
Issuance of common stock from releases of restricted stock units		$ 4	(4)
Issuance of common stock from releases of restricted stock units, shares		390,332
Tax withholding paid on behalf of employees for stock-based awards	(2,410)		(2,410)
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	(84,990)			(84,990)
Balance at Sep. 30, 2019	107,895	$ 984	590,878	(483,967)
Balance, shares at Sep. 30, 2019		99,405,805
Balance at Jun. 30, 2019	122,917	$ 973	585,080	(463,136)
Balance, shares at Jun. 30, 2019		98,259,203
Issuance of common stock from option exercises	1,438	$ 5	1,433
Issuance of common stock from option exercises, shares		517,624
Stock-based compensation	4,910		4,910
Issuance of common stock from releases of restricted stock units		$ 3	(3)
Issuance of common stock from releases of restricted stock units, shares		335,863
Tax withholding paid on behalf of employees for stock-based awards	(2,410)		(2,410)
Issuance of common stock from warrant exercises		$ 3	(3)
Issuance of common stock from warrant exercises, shares		293,115
Reclassification of warrant liability to additional paid-in capital upon warrant exercises	1,871		1,871
Net loss	(20,831)			(20,831)
Balance at Sep. 30, 2019	107,895	$ 984	590,878	(483,967)
Balance, shares at Sep. 30, 2019		99,405,805
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	$ 15		15
Issuance of common stock from option exercises, shares	20,579	20,579
Stock-based compensation	$ 17,063		17,063
Issuance of common stock from releases of restricted stock units		$ 6	(6)
Issuance of common stock from releases of restricted stock units, shares		659,920
Tax withholding paid on behalf of employees for stock-based awards	(754)		(754)
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	(81,809)			(81,809)
Balance at Sep. 30, 2020	151,036	$ 1,469	750,552	(600,985)
Balance, shares at Sep. 30, 2020		147,954,426
Balance at Jun. 30, 2020	174,024	$ 1,466	745,418	(572,860)
Balance, shares at Jun. 30, 2020		147,616,373
Stock-based compensation	5,759		5,759
Issuance of common stock from releases of restricted stock units		$ 3	(3)
Issuance of common stock from releases of restricted stock units, shares		338,053
Tax withholding paid on behalf of employees for stock-based awards	(622)		(622)
Net loss	(28,125)			(28,125)
Balance at Sep. 30, 2020	$ 151,036	$ 1,469	$ 750,552	$ (600,985)
Balance, shares at Sep. 30, 2020		147,954,426