Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Selected Quarterly Financial Information [Line Items]
Loss from operations	$ (10,204)	$ (9,156)	$ (9,311)	$ (10,581)	$ (11,393)	$ (11,650)	$ (9,730)	$ (8,207)
Net loss	$ (2,472)	$ (8,489)	$ (5,764)	$ (11,078)	$ (11,296)	$ (7,507)	$ (13,623)	$ (12,359)	$ (27,255)	$ (43,737)	$ (19,784)
Net loss per share - basic (in dollars per share)	$ (0.02)	$ (0.09)	$ (0.06)	$ (0.12)	$ (0.12)	$ (0.08)	$ (0.19)	$ (0.17)
Net loss per share - dilutive (in dollars per share)	$ (0.02)	$ (0.09)	$ (0.10)	$ (0.12)	$ (0.12)	$ (0.12)	$ (0.19)	$ (0.17)
Weighted average common share - basic (in shares)	103,804,308	91,441,687	91,015,733	90,826,752	90,810,629	85,974,751	72,736,829	72,673,959
Weighted average common share - dilutive (in shares)	103,804,308	91,441,687	93,930,540	90,826,752	90,810,629	87,585,103	72,736,829	72,673,959