Mortgage Loans by Property Type and Geographic Region (Detail) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 2,390.0	$ 2,435.4
Mortgage Loans, Percent of Total	100.00%	100.00%
New England
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 56.8	$ 52.4
Mortgage Loans, Percent of Total	2.40%	2.20%
Mid-Atlantic
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 190.3	$ 192.4
Mortgage Loans, Percent of Total	8.00%	7.90%
East North Central
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 303.8	$ 313.0
Mortgage Loans, Percent of Total	12.70%	12.90%
West North Central
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 179.5	$ 181.4
Mortgage Loans, Percent of Total	7.50%	7.40%
South Atlantic
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 547.5	$ 539.3
Mortgage Loans, Percent of Total	22.90%	22.10%
East South Central
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 100.9	$ 101.8
Mortgage Loans, Percent of Total	4.20%	4.20%
West South Central
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 210.5	$ 212.6
Mortgage Loans, Percent of Total	8.80%	8.70%
Mountain
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 296.6	$ 298.7
Mortgage Loans, Percent of Total	12.40%	12.30%
Pacific
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 504.1	$ 543.8
Mortgage Loans, Percent of Total	21.10%	22.30%
Apartment
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 683.8	$ 688.6
Mortgage Loans, Percent of Total	28.60%	28.30%
Industrial
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 709.7	$ 745.3
Mortgage Loans, Percent of Total	29.70%	30.60%
Office
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 423.3	$ 423.0
Mortgage Loans, Percent of Total	17.70%	17.40%
Retail
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 529.6	$ 534.5
Mortgage Loans, Percent of Total	22.20%	21.90%
Other Property
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 43.6	$ 44.0
Mortgage Loans, Percent of Total	1.80%	1.80%