Business, Basis of Presentation and Summary of Significant Accounting Policies - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) Segment	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of segments | Segment	5
Property, Plant and Equipment [Line Items]
Cost basis of property, equipment and leasehold improvements	$ 2,600	$ 2,500
Accumulated depreciation and amortization of property, equipment and leasehold improvements	1,200	1,100
Depreciation and amortization expense	$ 191	207	$ 206
Minimum
Real Estate Held-for-investment And Accumulated Depreciation [Line Items]
Real Estate Held-for-investment And Accumulated Depreciation Life Used For Depreciation	20 years
Maximum
Real Estate Held-for-investment And Accumulated Depreciation [Line Items]
Real Estate Held-for-investment And Accumulated Depreciation Life Used For Depreciation	55 years
Building
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	40 years
Leasehold Improvements | Minimum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	1 year
Leasehold Improvements | Maximum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	25 years
Other Capitalized Property Plant and Equipment | Minimum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	3 years
Other Capitalized Property Plant and Equipment | Maximum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	7 years
VODA and VOCRA | Minimum
Finite-Lived Intangible Assets [Line Items]
Amortization period	10 years
VODA and VOCRA | Maximum
Finite-Lived Intangible Assets [Line Items]
Amortization period	40 years
Computer Software, Intangible Asset
Finite-Lived Intangible Assets [Line Items]
Amortization period	4 years
Cost basis of computer software	$ 3,100	2,800
Accumulated amortization of computer software	2,200	2,000
Amortization expense related to computer software	$ 276	$ 250	$ 208