Net loss per share - Basic and Diluted Net Income per Share Attributable to Common Stockholders (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Numerator:
Net loss	$ (11,287)	$ (24,365)	$ (37,269)	$ (76,347)	$ (35,652)	$ (113,616)
Denominator:
Weighted-average common shares—basic for Class A and Class B common stock (shares)	144,668		139,166		143,640	138,515
Earnings Per Share, Basic and Diluted	$ (0.08)		$ (0.27)		$ (0.25)	$ (0.82)