Net Loss Per Share Attributable to Common Stockholders - Basic and Diluted (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Numerator:
Net loss attributable to common stockholders	$ (41,420)	$ (52,709)
Denominator:
Weighted-average shares used in computing net loss per share attributable to common stockholders, basic and diluted (in shares)	127,346	126,857
Net loss per share attributable to common stockholders, basic and diluted (in dollars per share)	$ (0.33)	$ (0.42)