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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Numerator:
Net loss, basic and diluted	$ (23,137)	$ (29,616)	$ (89,644)	$ (96,313)
Denominator:
Weighted-average number of shares used in computing net loss per share, basic and diluted	70,456	60,570	67,198	56,462
Net loss per share, basic (in dollars per share)	$ (0.33)	$ (0.49)	$ (1.33)	$ (1.71)
Net loss per share, diluted (in dollars per share)	$ (0.33)	$ (0.49)	$ (1.33)	$ (1.71)