Leases (Future Minimum Maturities of Lease Liabilities) (Details) $ in Millions	Mar. 31, 2019 USD ($)
Operating Leases [Abstract]
Remainder of 2019	$ 3,201
2020	3,981
2021	3,533
2022	3,231
2023	2,893
Thereafter	9,633
Total lease payments	26,472
Less imputed interest	(5,147)
Total	21,325
Finance Leases [Abstract]
Remainder of 2019	246
2020	290
2021	279
2022	263
2023	254
Thereafter	1,828
Total lease payments	3,160
Less inputed interest	(1,173)
Total	$ 1,987