Mortgage Loans by Property Type and Geographic Region (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Mortgage Loans on Real Estate
Mortgage Loans	$ 2,318.2	$ 2,435.4
Percent of Total	100.00%	100.00%
New England
Mortgage Loans on Real Estate
Mortgage Loans	$ 55.1	$ 52.4
Percent of Total	2.40%	2.20%
Mid-Atlantic
Mortgage Loans on Real Estate
Mortgage Loans	$ 155.1	$ 192.4
Percent of Total	6.70%	7.90%
East North Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 314.4	$ 313.0
Percent of Total	13.60%	12.90%
West North Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 163.5	$ 181.4
Percent of Total	7.00%	7.40%
South Atlantic
Mortgage Loans on Real Estate
Mortgage Loans	$ 553.0	$ 539.3
Percent of Total	23.80%	22.10%
East South Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 110.7	$ 101.8
Percent of Total	4.80%	4.20%
West South Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 200.9	$ 212.6
Percent of Total	8.70%	8.70%
Mountain
Mortgage Loans on Real Estate
Mortgage Loans	$ 282.7	$ 298.7
Percent of Total	12.20%	12.30%
Pacific
Mortgage Loans on Real Estate
Mortgage Loans	$ 482.8	$ 543.8
Percent of Total	20.80%	22.30%
Apartment
Mortgage Loans on Real Estate
Mortgage Loans	$ 685.8	$ 688.6
Percent of Total	29.60%	28.30%
Industrial
Mortgage Loans on Real Estate
Mortgage Loans	$ 706.0	$ 745.3
Percent of Total	30.50%	30.60%
Office
Mortgage Loans on Real Estate
Mortgage Loans	$ 379.9	$ 423.0
Percent of Total	16.40%	17.40%
Retail
Mortgage Loans on Real Estate
Mortgage Loans	$ 503.9	$ 534.5
Percent of Total	21.70%	21.90%
Other Property
Mortgage Loans on Real Estate
Mortgage Loans	$ 42.6	$ 44.0
Percent of Total	1.80%	1.80%