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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Numerator:
Net loss used to compute net loss per share, basic	$ (29,616)	$ (27,865)	$ (96,313)	$ (90,411)
Denominator:
Weighted-average number of shares used in computing net loss per share, basic	60,570	53,447	56,462	53,100
Net loss per share, basic (in dollars per share)	$ (0.49)	$ (0.52)	$ (1.71)	$ (1.70)
Net loss per share, diluted (in dollars per share)	$ (0.49)	$ (0.52)	$ (1.71)	$ (1.70)