CONDENSED 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, 2017	$ 103,797	$ 160	$ 1,089,684	$ (985,840)	$ (207)
Balances (in shares) at Dec. 31, 2017		159,877,239
Increase (Decrease) in Stockholders' Equity
Cumulative effect of accounting principle change	1,393			1,393
Net loss	(7,186)			(7,186)
Other comprehensive income (loss)	(124)				(124)
Issuance of common stock in connection with at market offering, net of issuance costs	1,553	$ 1	1,552
Issuance of common stock in connection with at market offering, net of issuance costs (in shares)		776,788
Stock-based compensation related to issuance of common stock and options in exchange for services	71		71
Stock-based compensation related to issuance of common stock and options in exchange for services (in shares)		8,308
Stock-based compensation for equity-based awards to employees and directors	1,614		1,614
401(k) contribution	10		10
Balances at Mar. 31, 2018	101,128	$ 161	1,092,931	(991,633)	(331)
Balances (in shares) at Mar. 31, 2018		160,662,335
Balances at Dec. 31, 2017	103,797	$ 160	1,089,684	(985,840)	(207)
Balances (in shares) at Dec. 31, 2017		159,877,239
Increase (Decrease) in Stockholders' Equity
Net loss	(14,120)
Other comprehensive income (loss)	130
Balances at Jun. 30, 2018	178,362	$ 183	1,176,823	(998,567)	(77)
Balances (in shares) at Jun. 30, 2018		182,547,071
Balances at Mar. 31, 2018	101,128	$ 161	1,092,931	(991,633)	(331)
Balances (in shares) at Mar. 31, 2018		160,662,335
Increase (Decrease) in Stockholders' Equity
Net loss	(6,934)			(6,934)
Other comprehensive income (loss)	254				254
Issuance of common stock in connection with at market offering, net of issuance costs	82,306	$ 22	82,284
Issuance of common stock in connection with at market offering, net of issuance costs (in shares)		21,865,344
Stock-based compensation related to issuance of common stock and options in exchange for services	49		49
Stock-based compensation related to issuance of common stock and options in exchange for services (in shares)		10,295
Stock-based compensation for equity-based awards to employees and directors	1,545		1,545
Issuance of common stock under equity plans	14		14
Issuances of common stock under equity plans (in shares)		9,097
Balances at Jun. 30, 2018	178,362	$ 183	1,176,823	(998,567)	(77)
Balances (in shares) at Jun. 30, 2018		182,547,071
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 income (loss)	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 income (loss)	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 income (loss)	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