Investments - Loans by U.S. Region (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 5,017	$ 4,911
Loans by region percentage of total loans	100.00%	100.00%
Pacific
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 983	$ 985
Loans by region percentage of total loans	19.60%	20.10%
South Atlantic
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 1,039	$ 982
Loans by region percentage of total loans	20.70%	20.00%
Middle Atlantic
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 1,050	$ 1,097
Loans by region percentage of total loans	20.90%	22.40%
West South Central
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 597	$ 552
Loans by region percentage of total loans	11.90%	11.20%
Mountain
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 470	$ 457
Loans by region percentage of total loans	9.40%	9.30%
East North Central
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 486	$ 468
Loans by region percentage of total loans	9.70%	9.50%
New England
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 75	$ 77
Loans by region percentage of total loans	1.50%	1.60%
West North Central
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 278	$ 243
Loans by region percentage of total loans	5.50%	4.90%
East South Central
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 39	$ 50
Loans by region percentage of total loans	0.80%	1.00%