STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Accumulated Other Comprehensive (Loss)/Income	Total
Beginning balance at Dec. 31, 2014	$ 95	$ 494,850	$ (451,526)	$ (17)	$ 43,402
Beginning balance, shares at Dec. 31, 2014	94,829
Sale of common stock and warrants, net of issuance costs	$ 23	80,576			80,599
Sale of common stock and warrants, shares	23,000
Exercise of common stock options, warrants and employee stock purchases	$ 3	2,543			2,546
Exercise of common stock options, warrants and employee stock purchases, shares	3,402
Issuance of common stock for services		122			122
Issuance of common stock for services, shares	34
Non-employee stock option expense		143			143
Stock-based compensation		5,442			5,442
Unrealized gain (loss) on marketable securities				(117)	(117)
Net loss			(48,555)		(48,555)
Ending balance at Dec. 31, 2015	$ 121	583,676	(500,081)	(134)	83,582
Ending balance, shares at Dec. 31, 2015	121,265
Sale of common stock and warrants, net of issuance costs	$ 26	48,822			48,848
Sale of common stock and warrants, shares	26,225
Exercise of common stock options, warrants and employee stock purchases	$ 2	2,170			2,172
Exercise of common stock options, warrants and employee stock purchases, shares	1,491
Issuance of common stock for services		172			172
Issuance of common stock for services, shares	84
Stock-based compensation		6,847			6,847
Unrealized gain (loss) on marketable securities				117	117
Net loss			(38,389)		(38,389)
Ending balance at Dec. 31, 2016	$ 149	641,687	(538,470)	(17)	$ 103,349
Ending balance, shares at Dec. 31, 2016	149,065				149,065
Cumulative effect from adoption of new accounting standard (Note 2)		40	(40)
Sale of common stock and warrants, net of issuance costs	$ 38	53,708			$ 53,746
Sale of common stock and warrants, shares	38,333
Exercise of common stock options, warrants and employee stock purchases	$ 9	5,688			5,697
Exercise of common stock options, warrants and employee stock purchases, shares	8,146
Issuance of common stock for services		150			150
Issuance of common stock for services, shares	81
Stock-based compensation		10,720			10,720
Unrealized gain (loss) on marketable securities				$ 17	17
Net loss			(65,984)		(65,984)
Ending balance at Dec. 31, 2017	$ 196	$ 711,993	$ (604,494)		$ 107,695
Ending balance, shares at Dec. 31, 2017	195,625				195,625