Level 4~Investments - Mortgage Loans by Region and Property Type (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Mortgage Loans on Real Estate [Line Items]
Current Weighted Average Loan to Value Ratio of Commercial Mortgage Loan	51.00%
Carrying Value	$ 3,221	$ 3,175
Industrial
Mortgage Loans on Real Estate [Line Items]
Carrying Value	830	817
Multifamily
Mortgage Loans on Real Estate [Line Items]
Carrying Value	1,043	1,006
Office
Mortgage Loans on Real Estate [Line Items]
Carrying Value	749	751
Retail
Mortgage Loans on Real Estate [Line Items]
Carrying Value	365	367
Other
Mortgage Loans on Real Estate [Line Items]
Carrying Value	234	234
East North Central
Mortgage Loans on Real Estate [Line Items]
Carrying Value	251	251
Middle Atlantic
Mortgage Loans on Real Estate [Line Items]
Carrying Value	272	272
Mountain
Mortgage Loans on Real Estate [Line Items]
Carrying Value	31	31
New England
Mortgage Loans on Real Estate [Line Items]
Carrying Value	292	293
Pacific
Mortgage Loans on Real Estate [Line Items]
Carrying Value	828	760
South Atlantic
Mortgage Loans on Real Estate [Line Items]
Carrying Value	693	710
West North Central
Mortgage Loans on Real Estate [Line Items]
Carrying Value	148	149
West South Central
Mortgage Loans on Real Estate [Line Items]
Carrying Value	277	278
Other
Mortgage Loans on Real Estate [Line Items]
Carrying Value	429	431
Allowance for Loan and Lease Losses [Member]
Mortgage Loans on Real Estate [Line Items]
Carrying Value	$ 24	$ 0
Mortgages [Member]
Mortgage Loans on Real Estate [Line Items]
Percent of Total	100.00%	100.00%
Mortgages [Member] | Industrial
Mortgage Loans on Real Estate [Line Items]
Percent of Total	25.80%	25.70%
Mortgages [Member] | Multifamily
Mortgage Loans on Real Estate [Line Items]
Percent of Total	32.40%	31.70%
Mortgages [Member] | Office
Mortgage Loans on Real Estate [Line Items]
Percent of Total	23.30%	23.70%
Mortgages [Member] | Retail
Mortgage Loans on Real Estate [Line Items]
Percent of Total	11.20%	11.50%
Mortgages [Member] | Other
Mortgage Loans on Real Estate [Line Items]
Percent of Total	7.30%	7.40%
Mortgages [Member] | East North Central
Mortgage Loans on Real Estate [Line Items]
Percent of Total	7.80%	7.90%
Mortgages [Member] | Middle Atlantic
Mortgage Loans on Real Estate [Line Items]
Percent of Total	8.40%	8.60%
Mortgages [Member] | Mountain
Mortgage Loans on Real Estate [Line Items]
Percent of Total	1.00%	1.00%
Mortgages [Member] | New England
Mortgage Loans on Real Estate [Line Items]
Percent of Total	9.10%	9.20%
Mortgages [Member] | Pacific
Mortgage Loans on Real Estate [Line Items]
Percent of Total	25.70%	23.90%
Mortgages [Member] | South Atlantic
Mortgage Loans on Real Estate [Line Items]
Percent of Total	21.50%	22.40%
Mortgages [Member] | West North Central
Mortgage Loans on Real Estate [Line Items]
Percent of Total	4.60%	4.70%
Mortgages [Member] | West South Central
Mortgage Loans on Real Estate [Line Items]
Percent of Total	8.60%	8.70%
Mortgages [Member] | Other
Mortgage Loans on Real Estate [Line Items]
Percent of Total	13.30%	13.60%