Investments - Loans by Property Type (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 5,017	$ 4,911
Loans by property type percentage of total loans	100.00%	100.00%
Retail
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 1,346	$ 1,383
Loans by property type percentage of total loans	26.90%	28.10%
Industrial
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 1,326	$ 1,326
Loans by property type percentage of total loans	26.40%	27.00%
Apartments
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 1,120	$ 948
Loans by property type percentage of total loans	22.30%	19.30%
Office
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 786	$ 829
Loans by property type percentage of total loans	15.70%	16.90%
Hotel/Motel
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 181	$ 177
Loans by property type percentage of total loans	3.60%	3.60%
Mixed Use
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 47	$ 52
Loans by property type percentage of total loans	0.90%	1.10%
Other
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 211	$ 196
Loans by property type percentage of total loans	4.20%	4.00%