Net Loss per Share - Loss per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ 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
Basic and diluted loss per share:
Net loss	$ (37,907)	$ (26,017)	$ (23,156)	$ (8,685)	$ (22,960)	$ (17,630)	$ (19,681)	$ (10,004)	$ (95,765)	$ (70,275)	$ (5,152)
Weighted-average common shares outstanding									51,667	27,687	6,670
Less: weighted-average unvested common shares subject to repurchase									(91)	(1,483)	(1,870)
Weighted-average number of shares used in computing net loss per share, basic and diluted									51,576	26,204	4,800
Basic and diluted net loss per share (in dollars per share)	$ (0.72)	$ (0.50)	$ (0.46)	$ (0.17)	$ (0.47)	$ (0.39)	$ (3.58)	$ (1.89)	$ (1.86)	$ (2.68)	$ (1.07)