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	2,884	$ 7	2,877
Issuance of common stock from option exercises, shares		588,120
Stock-based compensation	4,552		4,552
Issuance of common stock from warrant exercises, shares		12,955
Reclassification of warrant liability to additional paid-in capital upon warrant exercises	92		92
Net loss	(33,361)			(33,361)
Balance at Mar. 31, 2019	141,476	$ 959	572,855	(432,338)
Balance, shares at Mar. 31, 2019		96,933,098
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	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