Condensed Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	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
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	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