Leases (Lease Cost and Additional Information) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Lessee, Lease, Description [Line Items]
Operating lease cost	$ 396	$ 371
Short-term lease cost	3	8
Variable lease cost	138	150
Sublease income	(19)	(18)
Net lease cost	518	511
Operating cash outflows from operating leases	420	392
Right of use assets obtained in exchange for new operating lease liabilities	$ 261	$ 140
Real Estate Lease
Lessee, Lease, Description [Line Items]
Weighted-average remaining lease term – real estate	8 years 5 months 1 day	8 years 9 months 10 days
Weighted-average discount rate – real estate leases	2.94%	3.10%