Mortgage Loans by Property Type and Geographic Region (Detail) - USD ($) $ in Millions	Sep. 30, 2022	Dec. 31, 2021
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 2,476.4	$ 2,560.4
New England
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 53.1	$ 54.9
Mortgage Loans, Percent of Total	2.10%	2.10%
Mid-Atlantic
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 194.5	$ 214.7
Mortgage Loans, Percent of Total	7.90%	8.40%
East North Central
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 315.2	$ 298.4
Mortgage Loans, Percent of Total	12.70%	11.70%
West North Central
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 183.1	$ 193.1
Mortgage Loans, Percent of Total	7.40%	7.50%
South Atlantic
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 571.1	$ 582.1
Mortgage Loans, Percent of Total	23.10%	22.70%
East South Central
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 102.7	$ 120.7
Mortgage Loans, Percent of Total	4.10%	4.70%
West South Central
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 214.7	$ 243.2
Mortgage Loans, Percent of Total	8.70%	9.60%
Mountain
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 294.7	$ 290.6
Mortgage Loans, Percent of Total	11.90%	11.30%
Pacific
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 547.3	$ 562.7
Mortgage Loans, Percent of Total	22.10%	22.00%
Total
Mortgage Loans on Real Estate
Mortgage Loans, Percent of Total	100.00%	100.00%
Apartment
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 693.1	$ 780.0
Mortgage Loans, Percent of Total	28.00%	30.50%
Industrial
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 741.3	$ 734.4
Mortgage Loans, Percent of Total	29.90%	28.70%
Office
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 450.0	$ 467.2
Mortgage Loans, Percent of Total	18.20%	18.20%
Retail
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 547.7	$ 533.3
Mortgage Loans, Percent of Total	22.10%	20.80%
Other Property
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 44.3	$ 45.5
Mortgage Loans, Percent of Total	1.80%	1.80%
Total
Mortgage Loans on Real Estate
Mortgage Loans, Percent of Total	100.00%	100.00%