Summary of Significant Accounting Policies (Components of Fixed Assets) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Fixed assets, gross	$ 3,015	$ 2,859
Less-accumulated depreciation and amortization	(2,159)	(2,001)
Fixed assets, net	856	858	$ 701
Furniture and equipment
Property, Plant and Equipment [Line Items]
Fixed assets, gross	1,326	1,268
Land and buildings
Property, Plant and Equipment [Line Items]
Fixed assets, gross	379	377
Leasehold and building improvements
Property, Plant and Equipment [Line Items]
Fixed assets, gross	$ 1,310	$ 1,214