Investments - Investments - Mortgage Loans by Region and Property Type (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Carrying Value	$ 3,736	$ 3,704
Percent of Total	100.00%	100.00%
Industrial
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Carrying Value	$ 1,156	$ 1,108
Percent of Total	30.90%	29.90%
Multifamily
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Carrying Value	$ 1,156	$ 1,138
Percent of Total	30.90%	30.70%
Office
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Carrying Value	$ 719	$ 708
Percent of Total	19.30%	19.10%
Retail
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Carrying Value	$ 430	$ 392
Percent of Total	11.50%	10.60%
Residential Real Estate [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Carrying Value	$ 135	$ 82
Percent of Total	3.60%	2.20%
Other
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Carrying Value	$ 140	$ 276
Percent of Total	3.80%	7.50%
East North Central
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Carrying Value	$ 274	$ 250
Middle Atlantic
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Carrying Value	306	270
Mountain
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Carrying Value	63	30
New England
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Carrying Value	345	330
Pacific
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Carrying Value	835	917
South Atlantic
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Carrying Value	743	712
West North Central
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Carrying Value	121	148
West South Central
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Carrying Value	406	420
Other
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Carrying Value	$ 643	$ 627
Mortgages [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Percent of Total	100.00%	100.00%
Mortgages [Member] | East North Central
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Percent of Total	7.30%	6.80%
Mortgages [Member] | Middle Atlantic
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Percent of Total	8.20%	7.30%
Mortgages [Member] | Mountain
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Percent of Total	1.70%	0.80%
Mortgages [Member] | New England
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Percent of Total	9.20%	8.90%
Mortgages [Member] | Pacific
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Percent of Total	22.40%	24.80%
Mortgages [Member] | South Atlantic
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Percent of Total	19.90%	19.20%
Mortgages [Member] | West North Central
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Percent of Total	3.20%	4.00%
Mortgages [Member] | West South Central
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Percent of Total	10.90%	11.30%
Mortgages [Member] | Other
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Percent of Total	17.20%	16.90%