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
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 18	$ 204	$ 483	$ 489	$ 10,860	$ 1,865	$ 3,834	$ 4,200	$ 1,194	$ 20,759	$ 7,669
Operating expenses	19,762	19,396	21,671	21,867	18,221	15,210	25,015	17,071	82,696	75,517	52,579
Net loss	$ (20,020)	$ (19,519)	$ (21,090)	$ (21,207)	$ (7,226)	$ (13,000)	$ (21,035)	$ (14,487)	$ (81,836)	$ (55,748)	$ (56,680)
Basic and diluted net loss per share (dollars per share)	$ (0.38)	$ (0.37)	$ (0.40)	$ (0.40)					$ (1.55)	$ (1.08)	$ (1.29)
Basic net loss per share (dollars per share)					$ (0.14)	$ (0.25)	$ (0.41)	$ (0.29)