Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	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
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 18	$ 18	$ 18	$ 18	$ 18	$ 204	$ 483	$ 489	$ 72	$ 1,194	$ 20,759
Total operating expenses	13,724	15,433	21,335	19,711	19,762	19,396	21,671	21,867	70,203	82,696	75,517
Net loss	$ (14,448)	$ (15,828)	$ (21,608)	$ (20,021)	$ (20,020)	$ (19,519)	$ (21,090)	$ (21,207)	$ (71,905)	$ (81,836)	$ (55,748)
Basic and diluted net loss per share (dollars per share)	$ (0.14)	$ (0.18)	$ (0.41)	$ (0.38)					$ (0.96)	$ (1.55)	$ (1.08)
Basic net loss per share (dollars per share)					$ (0.38)	$ (0.37)	$ (0.40)	$ (0.40)