Investments - Loans by Property Type (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 4,892	$ 4,911
Loans by property type percentage of total loans	100.00%	100.00%
Retail
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 1,383	$ 1,383
Loans by property type percentage of total loans	28.30%	28.10%
Industrial
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 1,273	$ 1,326
Loans by property type percentage of total loans	26.10%	27.00%
Apartments
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 951	$ 948
Loans by property type percentage of total loans	19.40%	19.30%
Office
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 842	$ 829
Loans by property type percentage of total loans	17.20%	16.90%
Hotel/Motel
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 178	$ 177
Loans by property type percentage of total loans	3.60%	3.60%
Mixed Use
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 51	$ 52
Loans by property type percentage of total loans	1.00%	1.10%
Other
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 214	$ 196
Loans by property type percentage of total loans	4.40%	4.00%