Mortgage Loans by Property Type and Geographic Region (Detail) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 2,321.5	$ 2,435.4
Mortgage Loans, Percent of Total	100.00%	100.00%
New England
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 55.7	$ 52.4
Mortgage Loans, Percent of Total	2.40%	2.20%
Mid-Atlantic
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 156.8	$ 192.4
Mortgage Loans, Percent of Total	6.80%	7.90%
East North Central
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 300.2	$ 313.0
Mortgage Loans, Percent of Total	12.90%	12.90%
West North Central
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 171.2	$ 181.4
Mortgage Loans, Percent of Total	7.40%	7.40%
South Atlantic
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 553.6	$ 539.3
Mortgage Loans, Percent of Total	23.80%	22.10%
East South Central
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 98.8	$ 101.8
Mortgage Loans, Percent of Total	4.30%	4.20%
West South Central
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 202.7	$ 212.6
Mortgage Loans, Percent of Total	8.70%	8.70%
Mountain
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 284.8	$ 298.7
Mortgage Loans, Percent of Total	12.30%	12.30%
Pacific
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 497.7	$ 543.8
Mortgage Loans, Percent of Total	21.40%	22.30%
Apartment
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 674.8	$ 688.6
Mortgage Loans, Percent of Total	29.10%	28.30%
Industrial
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 715.5	$ 745.3
Mortgage Loans, Percent of Total	30.80%	30.60%
Office
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 384.1	$ 423.0
Mortgage Loans, Percent of Total	16.50%	17.40%
Retail
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 504.1	$ 534.5
Mortgage Loans, Percent of Total	21.70%	21.90%
Other Property
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 43.0	$ 44.0
Mortgage Loans, Percent of Total	1.90%	1.80%