Mortgage Loans, Sorted by Applicable Credit Quality Indicators (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Mortgage Loans on Real Estate
Mortgage Loans	$ 2,295.0	$ 2,213.2
Aa Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans	0.0	0.4
A Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans	477.5	445.7
Baa Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans	1,814.1	1,753.0
Ba Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans	3.4	14.1
Loan to Value Ratio Below or Equal to 65 Percent
Mortgage Loans on Real Estate
Mortgage Loans	1,204.8	1,101.7
Loan To Value Ratio Above 65 To 75 Percent
Mortgage Loans on Real Estate
Mortgage Loans	1,049.1	1,041.6
Loan To Value Ratio Above 75 To 85 Percent
Mortgage Loans on Real Estate
Mortgage Loans	11.8	49.3
Loan To Value Ratio Above 85 Percent
Mortgage Loans on Real Estate
Mortgage Loans	$ 29.3	$ 20.6