Summary of Accounting Policies - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Redeemable Noncontrolling Interest [Line Items]
Increase from acquisition	$ 96	$ 0	$ 0
Redeemable noncontrolling interests	138	42	$ 40	$ 41
Certain Subsidiaries
Redeemable Noncontrolling Interest [Line Items]
Increase from acquisition	10
Subsidiary In India
Redeemable Noncontrolling Interest [Line Items]
Increase from acquisition	86
As Reported
Redeemable Noncontrolling Interest [Line Items]
Redeemable noncontrolling interests	$ 35	$ 42