Leases (Supplemental Balance Sheet Information) (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Property leases
Operating lease ROU assets	$ 2,510	$ 2,560
Short-term operating lease liabilities	519	514
Long-term operating lease liabilities	2,025	2,078
Operating lease liabilities	$ 2,544	$ 2,592
Weighted average remaining lease term	8 years 3 months 18 days	8 years 4 months 24 days
Weighted average discount rate	3.85%	3.86%