Investments - Mortgage loans (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) loan item	Dec. 31, 2016 USD ($)
Mortgage Loans on Real Estate
Mortgage loans	$ 3,876	$ 3,938
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	29.20%	26.40%
Office buildings
Mortgage Loans on Real Estate
Percentage of mortgage portfolio carrying value	23.80%	24.30%
Retail
Mortgage Loans on Real Estate
Percentage of mortgage portfolio carrying value	19.20%	21.40%
Warehouse
Mortgage Loans on Real Estate
Percentage of mortgage portfolio carrying value	15.50%	16.50%
Other
Mortgage Loans on Real Estate
Percentage of mortgage portfolio carrying value	12.30%	11.40%
California
Mortgage Loans on Real Estate
Percentage of mortgage portfolio carrying value	19.60%	19.30%
Texas
Mortgage Loans on Real Estate
Percentage of mortgage portfolio carrying value	13.00%	11.00%
Illinois
Mortgage Loans on Real Estate
Percentage of mortgage portfolio carrying value	8.20%	7.40%
New Jersey
Mortgage Loans on Real Estate
Percentage of mortgage portfolio carrying value	8.00%	8.50%
Florida
Mortgage Loans on Real Estate
Percentage of mortgage portfolio carrying value	6.70%	6.00%
Mortgage loans, not in foreclosure
Mortgage Loans on Real Estate
Mortgage loans	$ 3,876
Percentage of mortgage portfolio carrying value	100.00%
Number of loans | item	260
Mortgage loans, not in foreclosure | 2018
Mortgage Loans on Real Estate
Mortgage loans	$ 94
Percentage of mortgage portfolio carrying value	2.40%
Number of loans | loan	12
Mortgage loans, not in foreclosure | 2019
Mortgage Loans on Real Estate
Mortgage loans	$ 204
Percentage of mortgage portfolio carrying value	5.30%
Number of loans | loan	7
Mortgage loans, not in foreclosure | 2020
Mortgage Loans on Real Estate
Mortgage loans	$ 158
Percentage of mortgage portfolio carrying value	4.10%
Number of loans | loan	14
Mortgage loans, not in foreclosure | 2021
Mortgage Loans on Real Estate
Mortgage loans	$ 485
Percentage of mortgage portfolio carrying value	12.50%
Number of loans | loan	41
Mortgage loans, not in foreclosure | Thereafter
Mortgage Loans on Real Estate
Mortgage loans	$ 2,935
Percentage of mortgage portfolio carrying value	75.70%
Number of loans | loan	186