Income Taxes - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Effective income tax rate (as a percent)	20.40%	27.30%	21.90%	28.50%
Unrecognized tax benefits	$ 152		$ 152		$ 212
Accrued interest and penalties associated with unrecognized tax benefits	32		32		$ 59
Reduction of unrecognized tax benefits is reasonably possible	$ 40		$ 40