Quarterly Financial Information (unaudited) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]
Total revenue	$ 410	$ 647	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 1,057	$ 0	$ 0
Loss from operations	(11,858)	(10,487)	(10,658)	(8,966)	(6,863)	(5,061)	(4,286)	(5,130)	(41,969)	(21,340)	(17,941)
Net loss	$ (11,833)	$ (10,474)	$ (10,650)	$ (8,955)	$ (6,849)	$ (5,052)	$ (4,284)	$ (5,128)	$ (41,912)	$ (21,313)	$ (17,935)
Basic and diluted net loss per share (usd per share)	$ (0.69)	$ (0.70)	$ (0.70)	$ (0.59)	$ (0.45)	$ (0.37)	$ (0.53)	$ (0.93)	$ (2.68)	$ (2.00)	$ (12.16)
Shares used in computing basic and diluted net loss per share (in shares)	17,200	14,899	15,251	15,242	15,226	13,646	8,122	5,535	15,651	10,667	1,540