Statements of Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Common stock [Member]	Additional Paid-In Capital [Member]	Accumulated Deficit [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Balance at Dec. 31, 2019	$ 277,119	$ 1	$ 405,803	$ (128,697)	$ 12
Balance (in shares) at Dec. 31, 2019		72,413,602
Employee stock-based compensation	1,652		1,652
Shares withheld related to employee tax withholding	(127)		(127)
Shares withheld related to employee tax withholding (in shares)		(17,124)
Issuance of common stock under employee stock plans	12		12
Issuance of common stock under employee stock plans (in shares)		61,027
Issuance of common stock from warrant exercises	138		138
Issuance of common stock from warrant exercises (in shares)		105,358
Unrealized loss on investments	(824)				(824)
Net loss	(9,687)			(9,687)
Balance at Mar. 31, 2020	268,283	$ 1	407,478	(138,384)	(812)
Balance (in shares) at Mar. 31, 2020		72,562,863
Balance at Dec. 31, 2020	244,344	$ 1	412,589	(168,192)	(54)
Balance (in shares) at Dec. 31, 2020		73,215,940
Employee stock-based compensation	1,577		1,577
Shares withheld related to employee tax withholding	(473)		(473)
Shares withheld related to employee tax withholding (in shares)		(81,016)
Issuance of common stock under employee stock plans	156		156
Issuance of common stock under employee stock plans (in shares)		236,932
Issuance of common stock from warrant exercises	5,679		5,679
Issuance of common stock from warrant exercises (in shares)		3,806,225
Unrealized loss on investments	(50)				(50)
Net loss	(14,010)			(14,010)
Balance at Mar. 31, 2021	$ 237,223	$ 1	$ 419,528	$ (182,202)	$ (104)
Balance (in shares) at Mar. 31, 2021		77,178,081