Net Loss per Share - Basic and Diluted Net Loss per Share Attributable to Common Stockholders (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Numerator:
Net loss	$ (123,194)	$ (80,331)	$ (390,905)	$ (243,308)
Premium paid on repurchase of redeemable convertible preferred stock	0	(4,127)	0	(4,127)
Cumulative dividends on redeemable convertible preferred stock	0	(3,361)	0	(10,323)
Net loss	$ (123,194)	$ (87,819)	$ (390,905)	$ (257,758)
Denominator:
Weighted average common shares used in net loss per share attributable to common stockholders, basic and diluted	330,769,341	66,283,040	329,592,322	65,887,511
Net loss per share attributable to common stockholders, basic and diluted (usd per share)	$ (0.37)	$ (1.32)	$ (1.19)	$ (3.91)