Leases Leases - Maturity Table (Details) - USD ($) $ in Millions		Jun. 30, 2019	Dec. 31, 2018
Operating Lease Liabilities, Payments Due [Abstract]
Lessee, Operating Lease, Liability, Payments, Remainder of Fiscal Year		$ 77
Lessee, Operating Lease, Liability, Payments, Due Year Two		139
Lessee, Operating Lease, Liability, Payments, Due Year Three		117
Lessee, Operating Lease, Liability, Payments, Due Year Four		93
Lessee, Operating Lease, Liability, Payments, Due Year Five		66
Lessee, Operating Lease, Liability, Payments, Due after Year Five		236
Lessee, Operating Lease, Liability, Payments, Due		728
Lessee, Operating Lease, Liability, Undiscounted Excess Amount		(131)
Operating Lease, Liability		597
Finance Lease Liabilities, Payments, Due [Abstract]
Finance Lease, Liability, Payments, Remainder of Fiscal Year		39
Finance Lease, Liability, Payments, Due Year Two		69
Finance Lease, Liability, Payments, Due Year Three		75
Finance Lease, Liability, Payments, Due Year Four		68
Finance Lease, Liability, Payments, Due Year Five		57
Finance Lease, Liability, Payments, Due after Year Five		776
Finance Lease, Liability, Payments, Due		1,084
Finance Lease, Liability		523
Finance Lease, Liability, Undiscounted Excess Amount		(561)
Lease, Liability, Payments, Remainder of Fiscal Year		116	$ 216	[1]
Lease, Liability, Payments, Due Year Two		208	196	[1]
Lease, Liability, Payments, Due Year Three		192	183	[1]
Lease, Liability, Payments, Due Year Four		161	154	[1]
Lease, Liability, Payments, Due Year Five		123	117	[1]
Lease, Liability, Payments, Due after Year Five		1,012	931	[1]
Lease, Liability, Payments, Due		1,812	1,797	[1]
Lease, Liability, Undiscounted Excess Amount		(692)
Total Lease Liabilities		$ 1,120
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".