Investments - Mortgage Loans by Region and Property Type (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Mortgage Loans on Real Estate [Line Items]
Carrying Value	$ 3,175	$ 2,886
Industrial
Mortgage Loans on Real Estate [Line Items]
Carrying Value	817	675
Multifamily
Mortgage Loans on Real Estate [Line Items]
Carrying Value	1,006	830
Office
Mortgage Loans on Real Estate [Line Items]
Carrying Value	751	756
Retail
Mortgage Loans on Real Estate [Line Items]
Carrying Value	367	425
Other
Mortgage Loans on Real Estate [Line Items]
Carrying Value	$ 234	$ 200
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	25.70%	23.40%
Mortgage loans | Multifamily
Mortgage Loans on Real Estate [Line Items]
Percent of Total	31.70%	28.80%
Mortgage loans | Office
Mortgage Loans on Real Estate [Line Items]
Percent of Total	23.70%	26.20%
Mortgage loans | Retail
Mortgage Loans on Real Estate [Line Items]
Percent of Total	11.50%	14.70%
Mortgage loans | Other
Mortgage Loans on Real Estate [Line Items]
Percent of Total	7.40%	6.90%
East North Central
Mortgage Loans on Real Estate [Line Items]
Carrying Value	$ 251	$ 239
East North Central | Mortgage loans
Mortgage Loans on Real Estate [Line Items]
Percent of Total	7.90%	8.30%
Middle Atlantic
Mortgage Loans on Real Estate [Line Items]
Carrying Value	$ 272	$ 297
Middle Atlantic | Mortgage loans
Mortgage Loans on Real Estate [Line Items]
Percent of Total	8.60%	10.30%
Mountain
Mortgage Loans on Real Estate [Line Items]
Carrying Value	$ 31	$ 61
Mountain | Mortgage loans
Mortgage Loans on Real Estate [Line Items]
Percent of Total	1.00%	2.10%
New England
Mortgage Loans on Real Estate [Line Items]
Carrying Value	$ 293	$ 252
New England | Mortgage loans
Mortgage Loans on Real Estate [Line Items]
Percent of Total	9.20%	8.70%
Pacific
Mortgage Loans on Real Estate [Line Items]
Carrying Value	$ 760	$ 795
Pacific | Mortgage loans
Mortgage Loans on Real Estate [Line Items]
Percent of Total	23.90%	27.50%
South Atlantic
Mortgage Loans on Real Estate [Line Items]
Carrying Value	$ 710	$ 585
South Atlantic | Mortgage loans
Mortgage Loans on Real Estate [Line Items]
Percent of Total	22.40%	20.30%
West North Central
Mortgage Loans on Real Estate [Line Items]
Carrying Value	$ 149	$ 40
West North Central | Mortgage loans
Mortgage Loans on Real Estate [Line Items]
Percent of Total	4.70%	1.40%
West South Central
Mortgage Loans on Real Estate [Line Items]
Carrying Value	$ 278	$ 210
West South Central | Mortgage loans
Mortgage Loans on Real Estate [Line Items]
Percent of Total	8.70%	7.30%
Other
Mortgage Loans on Real Estate [Line Items]
Carrying Value	$ 431	$ 407
Other | Mortgage loans
Mortgage Loans on Real Estate [Line Items]
Percent of Total	13.60%	14.10%