Summary of Significant Accounting Policies (Components of Fixed Assets) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 2,543	$ 2,538
Less-accumulated depreciation and amortization	(1,842)	(1,826)
Fixed assets, net	701	712	$ 725
Furniture and Equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	1,159	1,179
Land and Building
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	377	385
Leasehold Improvements
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 1,007	$ 974
Minimum | Furniture and Equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	3 years
Minimum | Building
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	30 years
Maximum | Furniture and Equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	10 years
Maximum | Building
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	40 years