Net Loss per Share - Loss per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Numerator:
Net loss, basic and diluted	$ (32,416)	$ (33,824)	$ (66,507)	$ (66,697)
Denominator:
Weighted-average number of shares used in computing net loss per share, basic and diluted	68,224	54,551	65,542	54,342
Net loss per share, basic (in dollars per share)	$ (0.48)	$ (0.62)	$ (1.01)	$ (1.23)
Net loss per share, diluted (in dollars per share)	$ (0.48)	$ (0.62)	$ (1.01)	$ (1.23)