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	8,203	$ 18	8,185
Issuance of common stock from option exercises, shares		1,701,627
Stock-based compensation	9,459		9,459
Issuance of common stock from releases of restricted stock units		$ 1	(1)
Issuance of common stock from releases of restricted stock units, shares		54,469
Issuance of common stock from warrant exercises		$ 2	(2)
Issuance of common stock from warrant exercises, shares		171,084
Reclassification of warrant liability to additional paid-in capital upon warrant exercises	2,105		2,105
Net loss	(64,159)			(64,159)
Balance at Jun. 30, 2019	122,917	$ 973	585,080	(463,136)
Balance, shares at Jun. 30, 2019		98,259,203
Balance at Mar. 31, 2019	141,476	$ 959	572,855	(432,338)
Balance, shares at Mar. 31, 2019		96,933,098
Issuance of common stock from option exercises	5,319	$ 11	5,308
Issuance of common stock from option exercises, shares		1,113,507
Stock-based compensation	4,907		4,907
Issuance of common stock from releases of restricted stock units		$ 1	(1)
Issuance of common stock from releases of restricted stock units, shares		54,469
Issuance of common stock from warrant exercises		$ 2	(2)
Issuance of common stock from warrant exercises, shares		158,129
Reclassification of warrant liability to additional paid-in capital upon warrant exercises	2,013		2,013
Net loss	(30,798)			(30,798)
Balance at Jun. 30, 2019	122,917	$ 973	585,080	(463,136)
Balance, shares at Jun. 30, 2019		98,259,203
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	$ 11,303		11,303
Issuance of common stock from releases of restricted stock units		$ 3	(3)
Issuance of common stock from releases of restricted stock units, shares		321,867
Tax withholding paid on behalf of employees for stock-based awards	(131)		(131)
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	(53,684)			(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 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