Investments - Mortgage Loans by Region and Property Type (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Mortgage Loans on Real Estate [Line Items]
Carrying Value	$ 2,839	$ 2,872
Industrial
Mortgage Loans on Real Estate [Line Items]
Carrying Value	750	743
Hotel [Member]
Mortgage Loans on Real Estate [Line Items]
Carrying Value	24	24
Multifamily
Mortgage Loans on Real Estate [Line Items]
Carrying Value	644	662
Office
Mortgage Loans on Real Estate [Line Items]
Carrying Value	667	685
Retail
Mortgage Loans on Real Estate [Line Items]
Carrying Value	554	557
Other
Mortgage Loans on Real Estate [Line Items]
Carrying Value	$ 200	$ 201
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	26.40%	25.90%
Mortgage loans | Hotel [Member]
Mortgage Loans on Real Estate [Line Items]
Percent of Total	0.90%	0.80%
Mortgage loans | Multifamily
Mortgage Loans on Real Estate [Line Items]
Percent of Total	22.70%	23.00%
Mortgage loans | Office
Mortgage Loans on Real Estate [Line Items]
Percent of Total	23.50%	23.90%
Mortgage loans | Retail
Mortgage Loans on Real Estate [Line Items]
Percent of Total	19.50%	19.40%
Mortgage loans | Other
Mortgage Loans on Real Estate [Line Items]
Percent of Total	7.00%	7.00%
East North Central
Mortgage Loans on Real Estate [Line Items]
Carrying Value	$ 62	$ 62
East North Central | Mortgage loans
Mortgage Loans on Real Estate [Line Items]
Percent of Total	2.20%	2.10%
East South Central [Member] [Domain]
Mortgage Loans on Real Estate [Line Items]
Carrying Value	$ 14	$ 14
East South Central [Member] [Domain] | Mortgage loans
Mortgage Loans on Real Estate [Line Items]
Percent of Total	0.50%	0.50%
Middle Atlantic
Mortgage Loans on Real Estate [Line Items]
Carrying Value	$ 291	$ 291
Middle Atlantic | Mortgage loans
Mortgage Loans on Real Estate [Line Items]
Percent of Total	10.20%	10.10%
New England
Mortgage Loans on Real Estate [Line Items]
Carrying Value	$ 91	$ 92
New England | Mortgage loans
Mortgage Loans on Real Estate [Line Items]
Percent of Total	3.20%	3.20%
Pacific
Mortgage Loans on Real Estate [Line Items]
Carrying Value	$ 826	$ 838
Pacific | Mortgage loans
Mortgage Loans on Real Estate [Line Items]
Percent of Total	29.10%	29.20%
South Atlantic
Mortgage Loans on Real Estate [Line Items]
Carrying Value	$ 591	$ 608
South Atlantic | Mortgage loans
Mortgage Loans on Real Estate [Line Items]
Percent of Total	20.80%	21.20%
West South Central
Mortgage Loans on Real Estate [Line Items]
Carrying Value	$ 195	$ 195
West South Central | Mortgage loans
Mortgage Loans on Real Estate [Line Items]
Percent of Total	6.90%	6.80%
Other
Mortgage Loans on Real Estate [Line Items]
Carrying Value	$ 769	$ 772
Other | Mortgage loans
Mortgage Loans on Real Estate [Line Items]
Percent of Total	27.10%	26.90%