Maturities of Lease Liabilities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2019
Lessee, Operating Lease, Liability, Payment, Due [Abstract]
Operating Leases, 2020			$ 498
Operating Leases, 2021			426
Operating Leases, 2022			340
Operating Leases, 2023			299
Operating Leases, 2024			249
Operating Leases, Thereafter			569
Operating Leases, Total lease payments			2,381
Operating Leases, Less amount representing interest			(294)
Operating Leases, Total			2,087
Finance Lease, Liability, Payment, Due [Abstract]
Finance Lease, 2020			70
Finance Lease, 2021			200
Finance Lease, 2022			35
Finance Lease, 2023			28
Finance Lease, 2024			24
Finance Lease, Thereafter			77
Finance Lease, Total lease payments			434
Finance Lease, Less amount representing interest			(62)
Total finance lease liabilities			$ 372
Operating and Capital Leases, Future Minimum Payments, Net Minimum Payments, Fiscal Year Maturity [Abstract]
Operating Leases, 2019	$ 400
Operating Leases, 2020	496
Operating Leases, 2021	421
Operating Leases, 2022	328
Operating Leases, 2023	289
Operating Leases, Thereafter	787
Operating Leases, Total Due	2,721
Capital Leases, 2019	72
Capital Leases, 2020	69
Capital Leases, 2021	200
Capital Leases, 2022	35
Capital Leases, 2023	28
Capital Leases, Thereafter	101
Capital Leases, Total Due	505
Capital Leases, Less amount representing interest	(86)
Capital Leases, Total	419
Less current obligations	47
Long-term lease obligations	372
Lease rental expense	$ 555	$ 587