Leased Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Leases [Abstract]
Gross rental expense	$ 332	$ 332	$ 324
Sublease rental income	(17)	(27)	(33)
Rent expense, net	315	$ 305	$ 291
Future Minimum Payments Due, Fiscal Year Maturity
2018	10
2019	7
2020	6
2021	18
2022	2
2023 and Beyond	24
Total	67
Imputed Interest, Fiscal Year Maturity
2018	(3)
2019	(3)
2020	(3)
2021	(3)
2022	(1)
2023 and Beyond	(14)
Total	(27)
Present Value, Fiscal Year Maturity
2018	7
2019	4
2020	3
2021	15
2022	1
2023 and Beyond	10
Total	40
Future Minimum Payments Due, Fiscal Year Maturity
2018	265
2019	202
2020	156
2021	110
2022	63
2023 and Beyond	250
Total	1,046
Minimum Sublease Rentals, Fiscal Year Maturity
2018	(12)
2019	(8)
2020	(6)
2021	(3)
2022	(2)
2023 and Beyond	(7)
Total	(38)
Future Minimum Payments Due, Net of Sublease Rentals, Fiscal Year Maturity
2018	253
2019	194
2020	150
2021	107
2022	61
2023 and Beyond	243
Total	1,008
Imputed interest	(161)
Present value	$ 847