Schedule of Participating Mortgage Loans by Internal Credit Rating and Loan to Value (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 2,452.1	$ 2,397.0
Loan to Value Ratio Below or Equal to 65 Percent
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 1,244.4	$ 1,215.1
Mortgage Loans, Percent of Total	50.70%	50.70%
Loan To Value Ratio Above 65 To 75 Percent
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 1,093.5	$ 1,053.0
Mortgage Loans, Percent of Total	44.60%	43.90%
Loan To Value Ratio Above 75 To 85 Percent
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 40.5	$ 91.4
Mortgage Loans, Percent of Total	1.70%	3.80%
Loan To Value Ratio Above 85 Percent
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 73.7	$ 37.5
Mortgage Loans, Percent of Total	3.00%	1.60%
Total
Mortgage Loans on Real Estate
Mortgage Loans, Percent of Total	100.00%	100.00%
A Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 485.7	$ 485.6
Mortgage Loans, Percent of Total	19.80%	20.30%
BBB Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 1,919.6	$ 1,911.4
Mortgage Loans, Percent of Total	78.30%	79.70%
BB Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 30.0	$ 0.0
Mortgage Loans, Percent of Total	1.20%	0.00%
B Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 16.8	$ 0.0
Mortgage Loans, Percent of Total	0.70%	0.00%
Total
Mortgage Loans on Real Estate
Mortgage Loans, Percent of Total	100.00%	100.00%