Investments - Mortgage loans (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) loan	Dec. 31, 2016 USD ($)
Mortgage Loans on Real Estate
Carrying value	$ 4,534	$ 4,486
Percent	100.00%	100.00%
Mortgage loans, not in foreclosure
Mortgage Loans on Real Estate
Number of loans | loan	285
Carrying value	$ 4,534
Percent	100.00%
Mortgage loans, not in foreclosure | 2018
Mortgage Loans on Real Estate
Number of loans | loan	17
Carrying value	$ 169
Percent	3.70%
Mortgage loans, not in foreclosure | 2019
Mortgage Loans on Real Estate
Number of loans | loan	10
Carrying value	$ 268
Percent	5.90%
Mortgage loans, not in foreclosure | 2020
Mortgage Loans on Real Estate
Number of loans | loan	14
Carrying value	$ 192
Percent	4.20%
Mortgage loans, not in foreclosure | 2021
Mortgage Loans on Real Estate
Number of loans | loan	43
Carrying value	$ 625
Percent	13.80%
Mortgage loans, not in foreclosure | Thereafter
Mortgage Loans on Real Estate
Number of loans | loan	201
Carrying value	$ 3,280
Percent	72.40%
Apartment complex
Mortgage Loans on Real Estate
Percent	30.90%	27.60%
Office buildings
Mortgage Loans on Real Estate
Percent	23.80%	23.90%
Retail
Mortgage Loans on Real Estate
Percent	18.00%	20.40%
Warehouse
Mortgage Loans on Real Estate
Percent	15.70%	17.00%
Other
Mortgage Loans on Real Estate
Percent	11.60%	11.10%
California
Mortgage Loans on Real Estate
Percent	19.90%	19.30%
Texas
Mortgage Loans on Real Estate
Percent	13.00%	10.50%
New Jersey
Mortgage Loans on Real Estate
Percent	7.60%	8.20%
Illinois
Mortgage Loans on Real Estate
Percent	7.10%	6.70%
Florida
Mortgage Loans on Real Estate
Percent	6.40%	5.40%