Mortgage Loans Sorted by Applicable Internal Credit Ratings and Loan-to-Value Ratios (Detail) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Financing Receivable, Recorded Investment
Mortgage Loans	$ 1,970.4	$ 1,883.6
Aa Credit Rating
Financing Receivable, Recorded Investment
Mortgage Loans	0.8	1.1
A Credit Rating
Financing Receivable, Recorded Investment
Mortgage Loans	488.3	586.6
Baa Credit Rating
Financing Receivable, Recorded Investment
Mortgage Loans	1,445.1	1,285.8
Ba Credit Rating
Financing Receivable, Recorded Investment
Mortgage Loans	36.2	10.1
Loan to Value Ratio Below or Equal to 65 Percent
Financing Receivable, Recorded Investment
Mortgage Loans	889.1	937.2
Loan To Value Ratio Above 65 To 75 Percent
Financing Receivable, Recorded Investment
Mortgage Loans	962.6	842.5
Loan To Value Ratio Above 75 To 85 Percent
Financing Receivable, Recorded Investment
Mortgage Loans	73.1	88.4
Loan To Value Ratio Above 85 Percent
Financing Receivable, Recorded Investment
Mortgage Loans	$ 45.6	$ 15.5