Leases Leases - Maturity Table (Details) - USD ($) $ in Millions		Mar. 31, 2019	Dec. 31, 2018
Operating Lease Liabilities, Payments Due [Abstract]
Lessee, Operating Lease, Liability, Payments, Remainder of Fiscal Year		$ 115
Lessee, Operating Lease, Liability, Payments, Due Year Two		138
Lessee, Operating Lease, Liability, Payments, Due Year Three		116
Lessee, Operating Lease, Liability, Payments, Due Year Four		92
Lessee, Operating Lease, Liability, Payments, Due Year Five		66
Lessee, Operating Lease, Liability, Payments, Due after Year Five		198
Lessee, Operating Lease, Liability, Payments, Due		725
Lessee, Operating Lease, Liability, Undiscounted Excess Amount		(139)
Operating Lease, Liability		586
Finance Lease Liabilities, Payments, Due [Abstract]
Finance Lease, Liability, Payments, Remainder of Fiscal Year		53
Finance Lease, Liability, Payments, Due Year Two		68
Finance Lease, Liability, Payments, Due Year Three		74
Finance Lease, Liability, Payments, Due Year Four		67
Finance Lease, Liability, Payments, Due Year Five		56
Finance Lease, Liability, Payments, Due after Year Five		775
Finance Lease, Liability, Payments, Due		1,093
Finance Lease, Liability		522
Finance Lease, Liability, Undiscounted Excess Amount		(571)
Lease, Liability, Payments, Remainder of Fiscal Year		168	$ 216	[1]
Lease, Liability, Payments, Due Year Two		206	196	[1]
Lease, Liability, Payments, Due Year Three		190	183	[1]
Lease, Liability, Payments, Due Year Four		159	154	[1]
Lease, Liability, Payments, Due Year Five		122	117	[1]
Lease, Liability, Payments, Due after Year Five		973	931	[1]
Lease, Liability, Payments, Due		1,818	1,797	[1]
Lease, Liability, Undiscounted Excess Amount		(710)
Total Lease Liabilities		$ 1,108
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Operating Leases, Future Minimum Payments Due, Next Twelve Months	[1]		147
Operating Leases, Future Minimum Payments, Due in Two Years	[1]		128
Operating Leases, Future Minimum Payments, Due in Three Years	[1]		110
Operating Leases, Future Minimum Payments, Due in Four Years	[1]		87
Operating Leases, Future Minimum Payments, Due in Five Years	[1]		61
Operating Leases, Future Minimum Payments, Due Thereafter	[1]		159
Operating Leases, Future Minimum Payments Due	[1]		692
Capital Leases, Future Minimum Payments, Net Present Value [Abstract]
Capital Leases, Future Minimum Payments Due, Next Twelve Months	[1]		69
Capital Leases, Future Minimum Payments Due in Two Years	[1]		68
Capital Leases, Future Minimum Payments Due in Three Years	[1]		73
Capital Leases, Future Minimum Payments Due in Four Years	[1]		67
Capital Leases, Future Minimum Payments Due in Five Years	[1]		56
Capital Leases, Future Minimum Payments Due Thereafter	[1]		772
Capital Leases, Future Minimum Payments Due	[1]		$ 1,105

[1] (1) Amounts included for comparability and accounted for in accordance with ASC 840, "Leases".