Mortgage Loans by Property Type and Geographic Region (Detail) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 2,238.9	$ 2,318.2
Mortgage Loans, Percent of Total	100.00%	100.00%
New England
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 53.2	$ 55.1
Mortgage Loans, Percent of Total	2.40%	2.40%
Mid-Atlantic
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 162.4	$ 155.1
Mortgage Loans, Percent of Total	7.30%	6.70%
East North Central
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 294.0	$ 314.4
Mortgage Loans, Percent of Total	13.10%	13.60%
West North Central
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 158.9	$ 163.5
Mortgage Loans, Percent of Total	7.10%	7.00%
South Atlantic
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 537.1	$ 553.0
Mortgage Loans, Percent of Total	24.00%	23.80%
East South Central
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 107.8	$ 110.7
Mortgage Loans, Percent of Total	4.80%	4.80%
West South Central
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 195.5	$ 200.9
Mortgage Loans, Percent of Total	8.70%	8.70%
Mountain
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 269.5	$ 282.7
Mortgage Loans, Percent of Total	12.00%	12.20%
Pacific
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 460.5	$ 482.8
Mortgage Loans, Percent of Total	20.60%	20.80%
Apartment
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 662.9	$ 685.8
Mortgage Loans, Percent of Total	29.60%	29.60%
Industrial
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 684.2	$ 706.0
Mortgage Loans, Percent of Total	30.60%	30.50%
Office
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 354.1	$ 379.9
Mortgage Loans, Percent of Total	15.80%	16.40%
Retail
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 496.1	$ 503.9
Mortgage Loans, Percent of Total	22.10%	21.70%
Other Property
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 41.6	$ 42.6
Mortgage Loans, Percent of Total	1.90%	1.80%