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								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Numerator:
Net loss									$ (514,021)	$ (384,734)	$ (255,916)
Premium paid on repurchase of preferred stock									0	(4,127)	0
Cumulative dividends on redeemable convertible preferred stock									0	(12,996)	(13,925)
Net loss attributable to common stockholders									$ (514,021)	$ (401,857)	$ (269,841)
Denominator:
Weighted average common shares used in net loss per share attributable to common stockholders, basic and diluted (in shares)									330,802,136	81,114,183	64,497,544
Net loss per share attributable to common stockholders, basic and diluted (usd per share)	$ (0.37)	$ (0.37)	$ (0.41)	$ (0.40)	$ (1.14)	$ (1.32)	$ (1.43)	$ (1.16)	$ (1.55)	$ (4.95)	$ (4.18)