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, 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	(10,059)						(10,059)
Other comprehensive (loss) income	282							282
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)			13,365
Stock-based compensation for equity- based awards to employees and directors	1,426				1,426
401(k) contribution	9				9
Balances at Mar. 31, 2019	169,413		$ 186		1,190,651		(1,021,523)	99
Balances (in shares) at Mar. 31, 2019			186,406,047
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	(24,298)
Other comprehensive (loss) income	464
Balances at Jun. 30, 2019	157,044		$ 187		1,192,338		(1,035,762)	281
Balances (in shares) at Jun. 30, 2019			186,638,401
Balances at Mar. 31, 2019	169,413		$ 186		1,190,651		(1,021,523)	99
Balances (in shares) at Mar. 31, 2019			186,406,047
Increase (Decrease) in Stockholders' Equity
Net loss	(14,239)						(14,239)
Other comprehensive (loss) income	182							182
Issuance of common stock in connection with at market offering, net of issuance costs	101				101
Issuance of common stock in connection with at market offering, net of issuance costs (in shares)			108,386
Stock-based compensation related to issuance of common stock and options in exchange for services	11				11
Stock-based compensation related to issuance of common stock and options in exchange for services (in shares)			5,097
Stock-based compensation for equity- based awards to employees and directors	1,434				1,434
Issuance of common stock under equity plans	142		$ 1		141
Issuances of common stock under equity plans (in shares)			118,871
Balances at Jun. 30, 2019	157,044		$ 187		1,192,338		(1,035,762)	281
Balances (in shares) at Jun. 30, 2019			186,638,401
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	(32,179)
Other comprehensive (loss) income	57
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
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
Stock-based compensation for equity- based awards to employees and directors	1,707				1,707
Issuance of common stock under equity plans	82				82
Issuances of common stock under equity plans (in shares)			72,500
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