Leases (Operating Lease Assets and Liabilities ) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
Right-of-use assets	$ 1,876	$ 1,982
Operating lease, right-of-use asset, statement of financial position [Extensible List]	us-gaap:OtherAssets	us-gaap:OtherAssets
Lease liabilities	$ 2,097	$ 2,170
Operating lease, liability, statement of financial position [Extensible List]	us-gaap:AccruedLiabilitiesAndOtherLiabilities	us-gaap:AccruedLiabilitiesAndOtherLiabilities