Investments - Loans by Property Type (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 4,812	$ 4,919
Loans by property type percentage of total loans	100.00%	100.00%
Retail
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 1,308	$ 1,335
Loans by property type percentage of total loans	27.20%	27.20%
Industrial
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 1,299	$ 1,323
Loans by property type percentage of total loans	27.00%	26.90%
Apartments
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 1,088	$ 1,104
Loans by property type percentage of total loans	22.60%	22.40%
Office
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 747	$ 791
Loans by property type percentage of total loans	15.50%	16.10%
Hotel/Motel
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 118	$ 111
Loans by property type percentage of total loans	2.50%	2.30%
Mixed Use
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 45	$ 46
Loans by property type percentage of total loans	0.90%	0.90%
Other
Investment Holdings [Line Items]
Total commercial mortgage loans	$ 207	$ 209
Loans by property type percentage of total loans	4.30%	4.20%