Selected Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended									12 Months Ended
	Dec. 31, 2017	[1]	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Selected Quarterly Financial Information [Line Items]
Loss from operations	$ (5,783)		$ (5,842)	$ (6,475)	$ (8,149)	$ (10,204)	$ (9,156)	$ (9,311)	$ (10,581)
Net (loss) income	$ 2,812		$ (10,930)	$ (4,315)	$ (3,058)	$ (2,472)	$ (8,489)	$ (5,764)	$ (11,078)	$ (15,174)	$ (27,255)	$ (43,737)
Net (loss) income per share - basic (in dollars per share)	$ 0.02		$ (0.14)	$ (0.03)	$ (0.02)	$ (0.02)	$ (0.09)	$ (0.06)	$ (0.12)
Net (loss) income per share - dilutive (in dollars per share)	$ 0.02		$ (0.14)	$ (0.03)	$ (0.02)	$ (0.02)	$ (0.09)	$ (0.10)	$ (0.12)
Weighted average common share - basic (in shares)	128,566,883		128,279,674	123,005,220	117,447,260	103,804,308	91,441,687	91,015,733	90,826,752
Weighted average common share - dilutive (in shares)	150,847,262		128,279,674	123,005,220	117,447,260	103,804,308	91,441,687	93,930,540	90,826,752

[1] Net Income due to gain on remeasurement of the warrant liabilities in excess of the quarter to date loss.