Selected Quarterly Financial Data (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
Selected Quarterly Financial Data
Total revenue	$ 3,667	$ 6,282	$ 3,097	$ 682	$ 898	$ 962	$ 3,388	$ 805	$ 13,728	$ 6,053	$ 1,629
Total operating expenses	40,109	33,031	29,212	31,309	39,882	22,127	22,588	18,644	133,661	103,241	36,941
Total other income (expense), net	253	150	(324)	(470)	(392)	145	158	94	(391)	5	(37,588)
Net loss	(36,189)	(26,599)	(26,439)	(31,097)	(39,376)	(21,020)	(19,042)	(17,745)	(120,324)	(97,183)	(72,900)
Net loss applicable to common stockholders	$ (36,189)	$ (26,599)	$ (26,439)	$ (31,097)	$ (39,376)	$ (21,020)	$ (19,042)	$ (17,792)	$ (120,324)	$ (97,230)	$ (73,294)
Net loss per share applicable to common stockholders - basic and diluted	$ (0.84)	$ (0.64)	$ (0.65)	$ (0.85)	$ (1.10)	$ (0.59)	$ (0.54)	$ (0.80)