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Property and Equipment (Schedule of Property and Equipment) (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Property, Plant and Equipment [Line Items]    
Property and equipment, gross $ 1,657 $ 1,488
Accumulated depreciation and amortization (1,047) (862)
Property and equipment, net 610 626
Land [Member]    
Property, Plant and Equipment [Line Items]    
Property and equipment, gross 31 31
Building [Member]    
Property, Plant and Equipment [Line Items]    
Property and equipment, gross 228 204
Leasehold Improvements [Member]    
Property, Plant and Equipment [Line Items]    
Property and equipment, gross 158 137
Computer Equipment [Member]    
Property, Plant and Equipment [Line Items]    
Property and equipment, gross 1,073 909
Furniture, Fixture and Other Equipment [Member]    
Property, Plant and Equipment [Line Items]    
Property and equipment, gross $ 167 $ 207