Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Effective income tax rate (as a percent)	21.60%	29.50%
Unrecognized tax benefits	$ 210		$ 212
Accrued interest and penalties associated with unrecognized tax benefits	56		$ 59
Reduction of unrecognized tax benefits is reasonably possible	$ 90