Net Loss per Share - Loss per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Basic and diluted loss per share:
Net loss	$ (26,017)	$ (17,630)	$ (57,858)	$ (47,315)
Weighted-average common shares outstanding	52,012	46,380	51,422	20,252
Less: weighted-average unvested common shares subject to repurchase		(1,436)	(121)	(1,531)
Weighted-average number of shares used in computing net loss per share, basic and diluted	52,012	44,944	51,301	18,721
Basic and diluted net loss per share (in dollars per share)	$ (0.50)	$ (0.39)	$ (1.13)	$ (2.53)