Summary of Significant Accounting Policies - Property and Equipment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 1,227	$ 1,176
Less: accumulated depreciation	(829)	(786)
Total	398	390
Depreciation expense	92	92	$ 90
Land
Property, Plant and Equipment [Line Items]
Property and equipment, gross	10	13
Data processing equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 775	726
Data processing equipment | Minimum
Property, Plant and Equipment [Line Items]
Estimated Useful Lives	3 years
Data processing equipment | Maximum
Property, Plant and Equipment [Line Items]
Estimated Useful Lives	5 years
Buildings and leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 256	255
Buildings and leasehold improvements | Minimum
Property, Plant and Equipment [Line Items]
Estimated Useful Lives	5 years
Buildings and leasehold improvements | Maximum
Property, Plant and Equipment [Line Items]
Estimated Useful Lives	40 years
Furniture and equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 186	$ 182
Furniture and equipment | Minimum
Property, Plant and Equipment [Line Items]
Estimated Useful Lives	5 years
Furniture and equipment | Maximum
Property, Plant and Equipment [Line Items]
Estimated Useful Lives	8 years