Mortgage Loans, Sorted by Applicable Credit Quality Indicators (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Mortgage Loans on Real Estate
Mortgage Loans	$ 2,038.9	$ 1,883.6
Aa Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans	0.7	1.1
A Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans	488.2	586.6
Baa Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans	1,506.6	1,285.8
Ba Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans	43.4	10.1
Loan to Value Ratio Below or Equal to 65 Percent
Mortgage Loans on Real Estate
Mortgage Loans	917.9	937.2
Loan To Value Ratio Above 65 To 75 Percent
Mortgage Loans on Real Estate
Mortgage Loans	1,011.5	842.5
Loan To Value Ratio Above 75 To 85 Percent
Mortgage Loans on Real Estate
Mortgage Loans	50.8	88.4
Loan To Value Ratio Above 85 Percent
Mortgage Loans on Real Estate
Mortgage Loans	$ 58.7	$ 15.5