Quarterly Financial Information (unaudited) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended							9 Months Ended	12 Months Ended
	Dec. 31, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Total revenue	$ 85	$ 253	$ 234	$ 410	$ 647	$ 0	$ 0	$ 44	$ 616	$ 1,057	$ 0
Loss from operations	(24,924)	(15,617)	(16,534)	(11,858)	(10,487)	(10,658)	(8,966)	(21,297)	(78,372)	(41,969)	(21,340)
Net loss	$ (24,887)	$ (15,566)	$ (16,481)	$ (11,833)	$ (10,474)	$ (10,650)	$ (8,955)	$ (21,251)	$ (78,185)	$ (41,912)	$ (21,313)
Basic and diluted net loss per share (usd per share)	$ (1.44)	$ (0.90)	$ (0.96)	$ (0.69)	$ (0.70)	$ (0.70)	$ (0.59)	$ (1.23)	$ (4.54)	$ (2.68)	$ (2.00)
Shares used in computing basic and diluted net loss per share (in shares)	17,280	17,211	17,210	17,200	14,899	15,251	15,242	17,211	17,228	15,651	10,667