Condensed Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional paid-in capital [Member]	Accumulated other comprehensive income (loss) [Member]	Accumulated deficit [Member]
Beginning balance at Dec. 31, 2019	$ 22,860	$ 19	$ 512,046	$ (3)	$ (489,202)
Beginning balance, shares at Dec. 31, 2019		195,257,000
Stock-based compensation expense from stock options and ESPP shares	761		761
Fully vested options issued to settle accrued liabilities	416		416
Net loss	(9,948)				(9,948)
Unrealized loss on available-for-sale securities, net of tax	(15)			(15)
Ending balance at Mar. 31, 2020	14,074	$ 19	513,223	(18)	(499,150)
Ending balance, shares at Mar. 31, 2020		195,834,000
Stock-based compensation expense from stock options and ESPP shares, shares		577,000
Beginning balance at Dec. 31, 2020	40,115	$ 20	529,884	(5)	(489,784)
Beginning balance, shares at Dec. 31, 2020		203,533,000
Issuance of common stock upon exercise of stock options	3,263	$ 1	3,262
Issuance of common stock upon exercise of stock options, Shares		2,502,000
Issuance of common stock upon equity financings, net of issuance costs of $269	47,786	$ 2	47,784
Issuance of common stock upon equity financings, net of issuance cost, shares		21,315,000
Stock-based compensation expense from stock options and ESPP shares	702		702
Net loss	(10,134)				(10,134)
Unrealized loss on available-for-sale securities, net of tax	(9)			(9)
Ending balance at Mar. 31, 2021	$ 81,723	$ 23	$ 581,632	$ (14)	$ (499,918)
Ending balance, shares at Mar. 31, 2021		227,350,000