Leases (Schedule of Future Minimum Lease Payments for Capital and Operating Leases) (Details) $ in Millions	Dec. 31, 2017 USD ($)
Leases [Abstract]
Operating Leases, Future Minimum Payments Due, Next Twelve Months	$ 208	[1]
Operating Leases, Future Minimum Payments, Due in Two Years	161	[1]
Operating Leases, Future Minimum Payments, Due in Three Years	130	[1]
Operating Leases, Future Minimum Payments, Due in Four Years	91	[1]
Operating Leases, Future Minimum Payments, Due in Five Years	70	[1]
Operating Leases, Future Minimum Payments, Due Thereafter	246	[1]
Operating Leases, Future Minimum Payments Due, Total	906	[1]
Capital Leases, Future Minimum Payments Due, Next Twelve Months	32
Capital Leases, Future Minimum Payments Due in Two Years	28
Capital Leases, Future Minimum Payments Due in Three Years	23
Capital Leases, Future Minimum Payments Due in Four Years	23
Capital Leases, Future Minimum Payments Due in Five Years	22
Capital Leases, Future Minimum Payments Due Thereafter	299
Capital Leases, Future Minimum Payments Due, Total	427
Capital Leases, Future Minimum Payments, Interest Included in Payments	(123)
Capital Lease Obligations, Total	304
Capital Lease Obligations, Current	(17)
Capital Lease Obligations, Noncurrent	$ 287

[1] Minimum lease payments have not been reduced by minimum sublease rentals due in the future of $6 million.