Schedule of Participating Mortgage Loans by Internal Credit Rating and Loan to Value (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 2,499.4	$ 2,432.1
Loan to Value Ratio Below or Equal to 65 Percent
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 1,256.7	$ 1,189.4
Mortgage Loans, Percent of Total	50.30%	48.90%
Loan To Value Ratio Above 65 To 75 Percent
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 1,048.2	$ 1,000.3
Mortgage Loans, Percent of Total	41.90%	41.10%
Loan To Value Ratio Above 75 To 85 Percent
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 156.4	$ 155.8
Mortgage Loans, Percent of Total	6.30%	6.40%
Loan To Value Ratio Above 85 Percent
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 38.1	$ 86.6
Mortgage Loans, Percent of Total	1.50%	3.60%
Total
Mortgage Loans on Real Estate
Mortgage Loans, Percent of Total	100.00%	100.00%
AA Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 3.3	$ 3.5
Mortgage Loans, Percent of Total	0.10%	0.10%
A Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 487.3	$ 510.0
Mortgage Loans, Percent of Total	19.50%	21.00%
BBB Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 1,967.0	$ 1,863.0
Mortgage Loans, Percent of Total	78.70%	76.60%
BB Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 26.3	$ 39.4
Mortgage Loans, Percent of Total	1.10%	1.60%
B Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 15.5	$ 16.2
Mortgage Loans, Percent of Total	0.60%	0.70%
Total
Mortgage Loans on Real Estate
Mortgage Loans, Percent of Total	100.00%	100.00%