Selected Quarterly Financial Data (Unaudited) - Selected Unaudited Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Loss from operations	$ (19,832)	$ (15,473)	$ (14,203)	$ (10,538)	$ (10,032)	$ (6,999)	$ (5,532)	$ (4,163)	$ (60,046)	$ (26,726)	$ (10,950)
Net loss	$ (19,676)	$ (15,369)	$ (14,159)	$ (10,464)	$ (9,959)	$ (7,031)	$ (5,413)	$ (4,055)	$ (59,668)	$ (26,458)	$ (10,819)
Net loss per share, basic and diluted	$ (0.91)	$ (0.94)	$ (6.43)	$ (4.85)	$ (4.97)	$ (5.48)	$ (8.46)	$ (6.63)	$ (5.59)	$ (23.03)	$ (21.56)
Weighted-average number of shares used in computing net loss per share, basic and diluted	21,726,223	16,423,894	2,200,406	2,159,065	2,004,860	1,283,882	640,001	612,039	10,673,559	1,148,827	501,707