Investments - Mortgage Loans by Region and Property Type (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Mortgage Loans on Real Estate [Line Items]
Carrying Value	$ 1,960	$ 2,872
Industrial
Mortgage Loans on Real Estate [Line Items]
Carrying Value	561	743
Hotel [Member]
Mortgage Loans on Real Estate [Line Items]
Carrying Value	25	24
Multifamily
Mortgage Loans on Real Estate [Line Items]
Carrying Value	512	662
Office
Mortgage Loans on Real Estate [Line Items]
Carrying Value	423	685
Retail
Mortgage Loans on Real Estate [Line Items]
Carrying Value	291	557
Other
Mortgage Loans on Real Estate [Line Items]
Carrying Value	$ 148	$ 201
Mortgage loans
Mortgage Loans on Real Estate [Line Items]
Percent of Total	100.00%	100.00%
Mortgage loans | Industrial
Mortgage Loans on Real Estate [Line Items]
Percent of Total	28.60%	25.90%
Mortgage loans | Hotel [Member]
Mortgage Loans on Real Estate [Line Items]
Percent of Total	1.30%	0.80%
Mortgage loans | Multifamily
Mortgage Loans on Real Estate [Line Items]
Percent of Total	26.10%	23.00%
Mortgage loans | Office
Mortgage Loans on Real Estate [Line Items]
Percent of Total	21.60%	23.90%
Mortgage loans | Retail
Mortgage Loans on Real Estate [Line Items]
Percent of Total	14.80%	19.40%
Mortgage loans | Other
Mortgage Loans on Real Estate [Line Items]
Percent of Total	7.60%	7.00%
East North Central
Mortgage Loans on Real Estate [Line Items]
Carrying Value	$ 62	$ 62
East North Central | Mortgage loans
Mortgage Loans on Real Estate [Line Items]
Percent of Total	3.20%	2.10%
East South Central [Member] [Domain]
Mortgage Loans on Real Estate [Line Items]
Carrying Value	$ 19	$ 14
East South Central [Member] [Domain] | Mortgage loans
Mortgage Loans on Real Estate [Line Items]
Percent of Total	1.00%	0.50%
Middle Atlantic
Mortgage Loans on Real Estate [Line Items]
Carrying Value	$ 135	$ 291
Middle Atlantic | Mortgage loans
Mortgage Loans on Real Estate [Line Items]
Percent of Total	6.90%	10.10%
Mountain [Member]
Mortgage Loans on Real Estate [Line Items]
Carrying Value	$ 51	$ 53
Mountain [Member] | Mortgage loans
Mortgage Loans on Real Estate [Line Items]
Percent of Total	2.60%	1.90%
New England
Mortgage Loans on Real Estate [Line Items]
Carrying Value	$ 42	$ 92
New England | Mortgage loans
Mortgage Loans on Real Estate [Line Items]
Percent of Total	2.10%	3.20%
Pacific
Mortgage Loans on Real Estate [Line Items]
Carrying Value	$ 665	$ 838
Pacific | Mortgage loans
Mortgage Loans on Real Estate [Line Items]
Percent of Total	33.90%	29.20%
South Atlantic
Mortgage Loans on Real Estate [Line Items]
Carrying Value	$ 452	$ 608
South Atlantic | Mortgage loans
Mortgage Loans on Real Estate [Line Items]
Percent of Total	23.10%	21.20%
West South Central
Mortgage Loans on Real Estate [Line Items]
Carrying Value	$ 227	$ 195
West South Central | Mortgage loans
Mortgage Loans on Real Estate [Line Items]
Percent of Total	11.50%	6.80%
Other
Mortgage Loans on Real Estate [Line Items]
Carrying Value	$ 307	$ 719
Other | Mortgage loans
Mortgage Loans on Real Estate [Line Items]
Percent of Total	15.70%	25.00%