STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Deficit	Accumulated Other Comprehensive Gain (Loss)
Balances at Dec. 31, 2018	$ 177,733	$ 186	$ 1,189,194	$ (1,011,464)	$ (183)
Balances (in shares) at Dec. 31, 2018		186,392,682
Increase (Decrease) in Stockholders' Equity
Net loss	(68,548)			(68,548)
Other comprehensive income (loss)	315				315
Issuance of common stock in connection with at market offering, net of issuance costs	19,295	$ 14	19,281
Issuance of common stock in connection with at market offering, net of issuance costs (in shares)		13,214,867
Stock-based compensation related to issuance of common stock and options in exchange for services	68		68
Stock-based compensation related to issuance of common stock and options in exchange for services (in shares)		29,150
Issuances of common stock under equity plans	204		204
Issuances of common stock under equity plans (in shares)		177,882
Stock-based compensation for equity-based awards to employees and directors	6,079		6,079
401(k) contribution	9		9
Balances at Dec. 31, 2019	135,155	$ 200	1,214,835	(1,080,012)	132
Balances (in shares) at Dec. 31, 2019		199,814,581
Increase (Decrease) in Stockholders' Equity
Net loss	(75,617)			(75,617)
Other comprehensive income (loss)	(54)				(54)
Issuance of common stock, pre-funded warrant and warrants to purchase common stock in public offering, net of issuance costs	140,184	$ 107	140,077
Issuance of common stock, pre-funded warrant and warrants to purchase common stock in public offering, net of issuance costs (in shares)		107,049,375
Issuance of common stock in connection with at market offering, net of issuance costs	4,075	$ 3	4,072
Issuance of common stock in connection with at market offering, net of issuance costs (in shares)		3,496,616
Issuance of common stock in connection exercise of warrants	16		16
Issuance of common stock in connection exercise of warrants (in shares)		12,500
Stock-based compensation related to issuance of common stock and options in exchange for services	85		85
Stock-based compensation related to issuance of common stock and options in exchange for services (in shares)		17,986
Issuances of common stock under equity plans	208		208
Issuances of common stock under equity plans (in shares)		175,795
Stock-based compensation for equity-based awards to employees and directors	6,895		6,895
Balances at Dec. 31, 2020	$ 210,947	$ 310	1,366,188	(1,155,629)	78
Balances (in shares) at Dec. 31, 2020	310,566,853	310,566,853
Increase (Decrease) in Stockholders' Equity
Net loss	$ (116,112)			(116,112)
Other comprehensive income (loss)	(251)				(251)
Issuance of common stock in connection with at market offering, net of issuance costs	20,385	$ 11	20,374
Issuance of common stock in connection with at market offering, net of issuance costs (in shares)		10,571,556
Issuance of common stock in connection exercise of warrants	2,479	$ 2	2,477
Issuance of common stock in connection exercise of warrants (in shares)		1,906,341
Stock-based compensation related to issuance of common stock and options in exchange for services	91		91
Stock-based compensation related to issuance of common stock and options in exchange for services (in shares)		20,783
Issuances of common stock under equity plans	797	$ 1	796
Issuances of common stock under equity plans (in shares)		666,058
Stock-based compensation for equity-based awards to employees and directors	8,080		8,080
Balances at Dec. 31, 2021	$ 126,416	$ 324	$ 1,398,006	$ (1,271,741)	$ (173)
Balances (in shares) at Dec. 31, 2021	323,731,591	323,731,591