Leases - Maturities of Lease Liabilities (Details) $ in Millions	Jun. 30, 2019 USD ($)
Operating Leases
The remainder of 2019	$ 409
2020	1,004
2021	956
2022	877
2023	840
Thereafter	6,222
Total undiscounted cash flows	10,308
Less imputed interest	(2,498)
Present value of lease liabilities	7,810
Finance Leases
The remainder of 2019	30
2020	43
2021	34
2022	30
2023	30
Thereafter	334
Total undiscounted cash flows	501
Less imputed interest	(107)
Present value of lease liabilities	$ 394