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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Numerator:
Net loss, basic and diluted	$ (27,119)	$ (26,017)	$ (90,812)	$ (57,858)
Denominator:
Weighted-average shares outstanding	53,447	52,012	53,100	51,422
Less: weighted-average unvested shares subject to repurchase				(121)
Weighted-average number of shares used in computing net loss per share, basic and diluted	53,447	52,012	53,100	51,301
Net loss per share, basic and diluted (in dollars per share)	$ (0.51)	$ (0.50)	$ (1.71)	$ (1.13)