Investments - Mortgage loans (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) loan item	Dec. 31, 2017 USD ($)
Mortgage Loans on Real Estate
Mortgage loans	$ 3,995	$ 3,876
Threshold percentage of mortgage loans to total carrying value	5.00%
Percentage of mortgage portfolio carrying value	100.00%	100.00%
Apartment complex
Mortgage Loans on Real Estate
Percentage of mortgage portfolio carrying value	33.50%	29.20%
Office buildings
Mortgage Loans on Real Estate
Percentage of mortgage portfolio carrying value	23.90%	23.80%
Warehouse
Mortgage Loans on Real Estate
Percentage of mortgage portfolio carrying value	15.90%	15.50%
Retail
Mortgage Loans on Real Estate
Percentage of mortgage portfolio carrying value	15.00%	19.20%
Other
Mortgage Loans on Real Estate
Percentage of mortgage portfolio carrying value	11.70%	12.30%
Texas
Mortgage Loans on Real Estate
Percentage of mortgage portfolio carrying value	15.60%	13.00%
California
Mortgage Loans on Real Estate
Percentage of mortgage portfolio carrying value	15.20%	19.60%
Illinois
Mortgage Loans on Real Estate
Percentage of mortgage portfolio carrying value	8.50%	8.20%
New Jersey
Mortgage Loans on Real Estate
Percentage of mortgage portfolio carrying value	7.20%	8.00%
Florida
Mortgage Loans on Real Estate
Percentage of mortgage portfolio carrying value	6.40%	6.70%
Mortgage loans, not in foreclosure
Mortgage Loans on Real Estate
Mortgage loans	$ 3,995
Percentage of mortgage portfolio carrying value	100.00%
Number of loans | item	263
Mortgage loans, not in foreclosure | 2019
Mortgage Loans on Real Estate
Mortgage loans	$ 108
Percentage of mortgage portfolio carrying value	2.70%
Number of loans | loan	5
Mortgage loans, not in foreclosure | 2020
Mortgage Loans on Real Estate
Mortgage loans	$ 110
Percentage of mortgage portfolio carrying value	2.80%
Number of loans | loan	13
Mortgage loans, not in foreclosure | 2021
Mortgage Loans on Real Estate
Mortgage loans	$ 428
Percentage of mortgage portfolio carrying value	10.70%
Number of loans | loan	39
Mortgage loans, not in foreclosure | 2022
Mortgage Loans on Real Estate
Mortgage loans	$ 401
Percentage of mortgage portfolio carrying value	10.00%
Number of loans | loan	28
Mortgage loans, not in foreclosure | Thereafter
Mortgage Loans on Real Estate
Mortgage loans	$ 2,948
Percentage of mortgage portfolio carrying value	73.80%
Number of loans | loan	178