Quarterly Financial Information (unaudited) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended						6 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Jun. 30, 2016	Sep. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Total revenue	$ 0	$ 0	$ 0	$ 0	$ 85	$ 234	$ 253	$ 44	$ 0	$ 616	$ 1,057
Loss from operations	(38,846)	(26,483)	(26,830)	(23,268)	(24,924)	(16,534)	(15,617)	(21,297)	(115,427)	(78,372)	(41,969)
Net loss	$ (38,546)	$ (26,288)	$ (26,729)	$ (23,240)	$ (24,887)	$ (16,481)	$ (15,566)	$ (21,251)	$ (114,803)	$ (78,185)	$ (41,912)
Basic and diluted net loss per share (usd per share)	$ (1.74)	$ (1.19)	$ (1.21)	$ (1.26)	$ (1.44)	$ (0.96)	$ (0.90)	$ (1.23)	$ (5.41)	$ (4.54)	$ (2.68)
Weighted-average common shares outstanding - basic and diluted (in shares)	22,151	22,125	22,123	18,410	17,280	17,210	17,211	17,211	21,215	17,228	15,651