Mortgage Loans, Sorted by Applicable Credit Quality Indicators (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Mortgage Loans on Real Estate
Mortgage Loans	$ 1,883.6	$ 1,856.6
Loan to Value Ratio Below or Equal to 65 Percent
Mortgage Loans on Real Estate
Mortgage Loans	937.2	898.7
Loan To Value Ratio Above 65 To 75 Percent
Mortgage Loans on Real Estate
Mortgage Loans	842.5	818.0
Loan To Value Ratio Above 75 To 85 Percent
Mortgage Loans on Real Estate
Mortgage Loans	88.4	102.3
Loan To Value Ratio Above 85 Percent
Mortgage Loans on Real Estate
Mortgage Loans	15.5	37.6
Aa Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans	1.1	7.7
A Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans	586.6	666.0
Baa Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans	1,285.8	1,156.7
Ba Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans	10.1	13.1
B Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans	$ 0.0	$ 13.1