Mortgage Loans by Property Type and Geographic Region (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Mortgage Loans on Real Estate
Mortgage Loans	$ 2,560.4	$ 2,432.1
Percent of Total	100.00%	100.00%
New England
Mortgage Loans on Real Estate
Mortgage Loans	$ 54.9	$ 40.0
Percent of Total	2.10%	1.60%
Mid-Atlantic
Mortgage Loans on Real Estate
Mortgage Loans	$ 214.7	$ 202.5
Percent of Total	8.40%	8.20%
East North Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 298.4	$ 330.4
Percent of Total	11.70%	13.60%
West North Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 193.1	$ 196.1
Percent of Total	7.50%	8.10%
South Atlantic
Mortgage Loans on Real Estate
Mortgage Loans	$ 582.1	$ 512.0
Percent of Total	22.70%	21.10%
East South Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 120.7	$ 110.0
Percent of Total	4.70%	4.50%
West South Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 243.2	$ 257.4
Percent of Total	9.60%	10.60%
Mountain
Mortgage Loans on Real Estate
Mortgage Loans	$ 290.6	$ 268.8
Percent of Total	11.30%	11.10%
Pacific
Mortgage Loans on Real Estate
Mortgage Loans	$ 562.7	$ 514.9
Percent of Total	22.00%	21.20%
Apartment
Mortgage Loans on Real Estate
Mortgage Loans	$ 780.0	$ 638.0
Percent of Total	30.50%	26.20%
Industrial
Mortgage Loans on Real Estate
Mortgage Loans	$ 734.4	$ 654.0
Percent of Total	28.70%	26.90%
Office
Mortgage Loans on Real Estate
Mortgage Loans	$ 467.2	$ 517.8
Percent of Total	18.20%	21.30%
Retail
Mortgage Loans on Real Estate
Mortgage Loans	$ 533.3	$ 575.6
Percent of Total	20.80%	23.70%
Other Property
Mortgage Loans on Real Estate
Mortgage Loans	$ 45.5	$ 46.7
Percent of Total	1.80%	1.90%