Investments - Loans by Property Type (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 5,003	$ 4,911
Loans by property type percentage of total loans	100.00%	100.00%
Retail
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 1,376	$ 1,383
Loans by property type percentage of total loans	27.50%	28.10%
Industrial
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 1,345	$ 1,326
Loans by property type percentage of total loans	26.90%	27.00%
Apartments
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 1,022	$ 948
Loans by property type percentage of total loans	20.40%	19.30%
Office
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 825	$ 829
Loans by property type percentage of total loans	16.50%	16.90%
Hotel/Motel
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 175	$ 177
Loans by property type percentage of total loans	3.50%	3.60%
Mixed Use
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 48	$ 52
Loans by property type percentage of total loans	1.00%	1.10%
Other
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 212	$ 196
Loans by property type percentage of total loans	4.20%	4.00%