Leases (Lease Cost and Additional Information) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Lessee, Lease, Description [Line Items]
Operating lease cost	$ 94	$ 92
Short-term lease cost	1	1
Variable lease cost	37	37
Sublease income	(8)	(5)
Net lease cost	124	125
Operating cash outflows from operating leases	99	103
Right of use assets obtained in exchange for new operating lease liabilities	$ 22	$ 79
Real Estate Lease
Lessee, Lease, Description [Line Items]
Weighted-average remaining lease term	8 years 3 months 18 days	8 years 8 months 12 days
Weighted-average discount rate	2.93%	3.06%