Summary of Significant Accounting Policies (Components of Fixed Assets) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 2,408	$ 2,394
Less-accumulated depreciation and amortization	(1,683)	(1,621)
Property, plant and equipment, net	725	773	$ 809
Furniture and Equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	1,113	1,133
Land and Building
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	389	396
Leasehold Improvements
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 906	$ 865
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