Investments - Investments - Mortgage Loans by Region and Property Type (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Mortgage Loans on Real Estate [Line Items]
Carrying Value	$ 3,355	$ 3,175
Industrial
Mortgage Loans on Real Estate [Line Items]
Carrying Value	964	817
Multifamily
Mortgage Loans on Real Estate [Line Items]
Carrying Value	1,022	1,006
Office
Mortgage Loans on Real Estate [Line Items]
Carrying Value	762	751
Retail
Mortgage Loans on Real Estate [Line Items]
Carrying Value	363	367
Other
Mortgage Loans on Real Estate [Line Items]
Carrying Value	244	234
East North Central
Mortgage Loans on Real Estate [Line Items]
Carrying Value	251	251
Middle Atlantic
Mortgage Loans on Real Estate [Line Items]
Carrying Value	271	272
Mountain
Mortgage Loans on Real Estate [Line Items]
Carrying Value	31	31
New England
Mortgage Loans on Real Estate [Line Items]
Carrying Value	291	293
Pacific
Mortgage Loans on Real Estate [Line Items]
Carrying Value	803	760
South Atlantic
Mortgage Loans on Real Estate [Line Items]
Carrying Value	691	710
West North Central
Mortgage Loans on Real Estate [Line Items]
Carrying Value	148	149
West South Central
Mortgage Loans on Real Estate [Line Items]
Carrying Value	383	278
Other
Mortgage Loans on Real Estate [Line Items]
Carrying Value	$ 486	$ 431
Mortgage loans
Mortgage Loans on Real Estate [Line Items]
Percent of Total	100.00%	100.00%
Mortgage loans | Industrial
Mortgage Loans on Real Estate [Line Items]
Percent of Total	28.70%	25.70%
Mortgage loans | Multifamily
Mortgage Loans on Real Estate [Line Items]
Percent of Total	30.50%	31.70%
Mortgage loans | Office
Mortgage Loans on Real Estate [Line Items]
Percent of Total	22.70%	23.70%
Mortgage loans | Retail
Mortgage Loans on Real Estate [Line Items]
Percent of Total	10.80%	11.50%
Mortgage loans | Other
Mortgage Loans on Real Estate [Line Items]
Percent of Total	7.30%	7.40%
Mortgage loans | East North Central
Mortgage Loans on Real Estate [Line Items]
Percent of Total	7.50%	7.90%
Mortgage loans | Middle Atlantic
Mortgage Loans on Real Estate [Line Items]
Percent of Total	8.10%	8.60%
Mortgage loans | Mountain
Mortgage Loans on Real Estate [Line Items]
Percent of Total	0.90%	1.00%
Mortgage loans | New England
Mortgage Loans on Real Estate [Line Items]
Percent of Total	8.70%	9.20%
Mortgage loans | Pacific
Mortgage Loans on Real Estate [Line Items]
Percent of Total	23.90%	23.90%
Mortgage loans | South Atlantic
Mortgage Loans on Real Estate [Line Items]
Percent of Total	20.60%	22.40%
Mortgage loans | West North Central
Mortgage Loans on Real Estate [Line Items]
Percent of Total	4.40%	4.70%
Mortgage loans | West South Central
Mortgage Loans on Real Estate [Line Items]
Percent of Total	11.40%	8.70%
Mortgage loans | Other
Mortgage Loans on Real Estate [Line Items]
Percent of Total	14.50%	13.60%