Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Selected Quarterly Financial Information [Line Items]
Loss from operations	$ (11,393)	$ (11,650)	$ (9,730)	$ (8,207)	$ (7,101)	$ (4,911)	$ (4,651)	$ (3,839)
Net loss	$ (11,296)	$ (7,507)	$ (13,623)	$ (12,359)	$ (6,480)	$ (4,910)	$ (4,556)	$ (3,838)	$ (43,737)	$ (19,784)	$ (12,317)
Net loss per share - basic (in dollars per share)	$ (0.12)	$ (0.08)	$ (0.19)	$ (0.17)
Net loss per share - dilutive (in dollars per share)	$ (0.12)	$ (0.12)	$ (0.19)	$ (0.17)
Weighted average common share - basic (in shares)	90,810,629	85,974,751	72,736,829	72,673,959
Weighted average common share - dilutive (in shares)	90,810,629	87,585,103	72,736,829	72,673,959
Net loss per share - basic and dilutive (in dollars per share)					$ (0.10)	$ (0.08)	$ (0.08)	$ (0.07)	$ (0.54)	$ (0.32)	$ (0.27)
Weighted average common share - basic and dilutive (in shares)					65,483,336	58,543,528	58,543,528	58,324,260	80,705,692	61,945,356	45,667,813