Summary of Significant Accounting Policies (Components of Fixed Assets) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Fixed assets, gross	$ 2,859	$ 2,543
Less-accumulated depreciation and amortization	(2,001)	(1,842)
Fixed assets, net	858	701	$ 712
Furniture and equipment
Property, Plant and Equipment [Line Items]
Fixed assets, gross	1,268	1,159
Land and buildings
Property, Plant and Equipment [Line Items]
Fixed assets, gross	377	377
Leasehold and building improvements
Property, Plant and Equipment [Line Items]
Fixed assets, gross	$ 1,214	$ 1,007