Investments - Loans by U.S. Region (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 5,003	$ 4,911
Loans by region percentage of total loans	100.00%	100.00%
Pacific
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 1,007	$ 985
Loans by region percentage of total loans	20.10%	20.10%
South Atlantic
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 999	$ 982
Loans by region percentage of total loans	20.00%	20.00%
West South Central
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 580	$ 552
Loans by region percentage of total loans	11.60%	11.20%
Middle Atlantic
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 1,053	$ 1,097
Loans by region percentage of total loans	21.10%	22.40%
East North Central
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 481	$ 468
Loans by region percentage of total loans	9.60%	9.50%
Mountain
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 486	$ 457
Loans by region percentage of total loans	9.70%	9.30%
West North Central
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 279	$ 243
Loans by region percentage of total loans	5.60%	4.90%
East South Central
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 42	$ 50
Loans by region percentage of total loans	0.80%	1.00%
New England
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 76	$ 77
Loans by region percentage of total loans	1.50%	1.60%