Equipment, Office Facilities, and Property (Schedule of Equipment, Office Facilities, and Property) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Total equipment, office facilities, and property	$ 3,551	$ 3,234
Accumulated depreciation and amortization	(2,080)	(1,935)
Total equipment, office facilities, and property — net	1,471	1,299
Software [Member]
Property, Plant and Equipment [Line Items]
Total equipment, office facilities, and property	1,490	1,335
Buildings [Member]
Property, Plant and Equipment [Line Items]
Total equipment, office facilities, and property	810	807
Leasehold improvements [Member]
Property, Plant and Equipment [Line Items]
Total equipment, office facilities, and property	357	342
Information technology equipment [Member]
Property, Plant and Equipment [Line Items]
Total equipment, office facilities, and property	326	299
Furniture and equipment [Member]
Property, Plant and Equipment [Line Items]
Total equipment, office facilities, and property	193	190
Land [Member]
Property, Plant and Equipment [Line Items]
Total equipment, office facilities, and property	167	168
Construction in progress [Member]
Property, Plant and Equipment [Line Items]
Total equipment, office facilities, and property	142	26
Telecommunications equipment [Member]
Property, Plant and Equipment [Line Items]
Total equipment, office facilities, and property	$ 66	$ 67