CONDENSED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Public Offering of Common Stock and Warrants	Common Stock	Common Stock Public Offering of Common Stock and Warrants	Additional Paid-In Capital	Additional Paid-In Capital Public Offering of Common Stock and Warrants	Accumulated Deficit	Accumulated Other Comprehensive Gain (Loss)
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	(16,355)						(16,355)
Other comprehensive (loss) income	(256)							(256)
Issuance of common stock in connection with at market offering, net of issuance costs	686				686
Issuance of common stock in connection with at market offering, net of issuance costs (in shares)			530,228
Stock-based compensation related to issuance of common stock and options in exchange for services	16				16
Stock-based compensation related to issuance of common stock and options in exchange for services (in shares)			6,039
Stock-based compensation for equity-based awards to employees and directors	1,568				1,568
Balances at Mar. 31, 2020	120,814		$ 200		1,217,105		(1,096,367)	(124)
Balances (in shares) at Mar. 31, 2020			200,350,848
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	(51,830)
Other comprehensive (loss) income	38
Balances at Sep. 30, 2020	232,832		$ 310		1,364,194		(1,131,842)	170
Balances (in shares) at Sep. 30, 2020			310,471,074
Balances at Mar. 31, 2020	120,814		$ 200		1,217,105		(1,096,367)	(124)
Balances (in shares) at Mar. 31, 2020			200,350,848
Increase (Decrease) in Stockholders' Equity
Net loss	(15,824)						(15,824)
Other comprehensive (loss) income	313							313
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	3,389		$ 3		3,386
Issuance of common stock in connection with at market offering, net of issuance costs (in shares)			2,966,388
Stock-based compensation related to issuance of common stock and options in exchange for services	16				16
Stock-based compensation related to issuance of common stock and options in exchange for services (in shares)			3,297
Issuance of common stock under equity plans	82				82
Issuances of common stock under equity plans (in shares)			72,500
Stock-based compensation for equity-based awards to employees and directors	1,707				1,707
Balances at Jun. 30, 2020	250,681		$ 310		1,362,373		(1,112,191)	189
Balances (in shares) at Jun. 30, 2020			310,442,408
Increase (Decrease) in Stockholders' Equity
Net loss	(19,651)						(19,651)
Other comprehensive (loss) income	(19)							(19)
Stock-based compensation related to issuance of common stock and options in exchange for services	22				22
Stock-based compensation related to issuance of common stock and options in exchange for services (in shares)			4,130
Issuance of common stock in connection with exercise of warrants	16				16
Issuance of common stock in connection exercise of warrants (in shares)			12,500
Issuance of common stock under equity plans	15				15
Issuances of common stock under equity plans (in shares)			12,036
Stock-based compensation for equity-based awards to employees and directors	1,768				1,768
Balances at Sep. 30, 2020	232,832		$ 310		1,364,194		(1,131,842)	170
Balances (in shares) at Sep. 30, 2020			310,471,074
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
Increase (Decrease) in Stockholders' Equity
Net loss	(27,824)						(27,824)
Other comprehensive (loss) income	(43)							(43)
Issuance of common stock in connection with at market offering, net of issuance costs	16,234		$ 8		16,226
Issuance of common stock in connection with at market offering, net of issuance costs (in shares)			7,948,505
Stock-based compensation related to issuance of common stock and options in exchange for services	25				25
Stock-based compensation related to issuance of common stock and options in exchange for services (in shares)			4,549
Issuance of common stock in connection with exercise of warrants	12				12
Issuance of common stock in connection exercise of warrants (in shares)			8,869
Issuance of common stock under equity plans	17				17
Issuances of common stock under equity plans (in shares)			16,232
Stock-based compensation for equity-based awards to employees and directors	1,794				1,794
Balances at Mar. 31, 2021	201,162		$ 318		1,384,262		(1,183,453)	35
Balances (in shares) at Mar. 31, 2021			318,545,008
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
Increase (Decrease) in Stockholders' Equity
Net loss	(84,095)
Other comprehensive (loss) income	(62)
Balances at Sep. 30, 2021	151,553		$ 321		1,390,940		(1,239,724)	16
Balances (in shares) at Sep. 30, 2021			320,604,441
Balances at Mar. 31, 2021	201,162		$ 318		1,384,262		(1,183,453)	35
Balances (in shares) at Mar. 31, 2021			318,545,008
Increase (Decrease) in Stockholders' Equity
Net loss	(29,574)						(29,574)
Other comprehensive (loss) income	(32)							(32)
Stock-based compensation related to issuance of common stock and options in exchange for services	23				23
Stock-based compensation related to issuance of common stock and options in exchange for services (in shares)			5,097
Issuance of common stock in connection with exercise of warrants	2,467		$ 2		2,465
Issuance of common stock in connection exercise of warrants (in shares)			1,897,472
Issuance of common stock under equity plans	187		$ 1		186
Issuances of common stock under equity plans (in shares)			151,618
Stock-based compensation for equity-based awards to employees and directors	1,973				1,973
Balances at Jun. 30, 2021	176,206		$ 321		1,388,909		(1,213,027)	3
Balances (in shares) at Jun. 30, 2021			320,599,195
Increase (Decrease) in Stockholders' Equity
Net loss	(26,697)						(26,697)
Other comprehensive (loss) income	13							13
Stock-based compensation related to issuance of common stock and options in exchange for services	23				23
Stock-based compensation related to issuance of common stock and options in exchange for services (in shares)			5,246
Stock-based compensation for equity-based awards to employees and directors	2,008				2,008
Balances at Sep. 30, 2021	$ 151,553		$ 321		$ 1,390,940		$ (1,239,724)	$ 16
Balances (in shares) at Sep. 30, 2021			320,604,441