Mortgage Loans by Property Type and Geographic Region (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Mortgage Loans on Real Estate
Mortgage Loans	$ 2,432.1	$ 2,397.0
Percent of Total	100.00%	100.00%
New England
Mortgage Loans on Real Estate
Mortgage Loans	$ 40.0	$ 28.9
Percent of Total	1.60%	1.20%
Mid-Atlantic
Mortgage Loans on Real Estate
Mortgage Loans	$ 202.5	$ 184.5
Percent of Total	8.20%	7.70%
East North Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 330.4	$ 329.2
Percent of Total	13.60%	13.70%
West North Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 196.1	$ 215.4
Percent of Total	8.10%	9.00%
South Atlantic
Mortgage Loans on Real Estate
Mortgage Loans	$ 512.0	$ 509.2
Percent of Total	21.10%	21.20%
East South Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 110.0	$ 114.3
Percent of Total	4.50%	4.80%
West South Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 257.4	$ 246.6
Percent of Total	10.60%	10.30%
Mountain
Mortgage Loans on Real Estate
Mortgage Loans	$ 268.8	$ 268.2
Percent of Total	11.10%	11.20%
Pacific
Mortgage Loans on Real Estate
Mortgage Loans	$ 514.9	$ 500.7
Percent of Total	21.20%	20.90%
Apartment
Mortgage Loans on Real Estate
Mortgage Loans	$ 638.0	$ 608.8
Percent of Total	26.20%	25.40%
Industrial
Mortgage Loans on Real Estate
Mortgage Loans	$ 654.0	$ 623.6
Percent of Total	26.90%	26.00%
Office
Mortgage Loans on Real Estate
Mortgage Loans	$ 517.8	$ 549.3
Percent of Total	21.30%	22.90%
Retail
Mortgage Loans on Real Estate
Mortgage Loans	$ 575.6	$ 567.5
Percent of Total	23.70%	23.70%
Other Property
Mortgage Loans on Real Estate
Mortgage Loans	$ 46.7	$ 47.8
Percent of Total	1.90%	2.00%