Mortgage Loans Sorted by Applicable Internal Credit Ratings and Loan-to-Value Ratios (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Financing Receivable, Recorded Investment
Mortgage Loans	$ 1,925.9	$ 1,856.6
Aa Credit Rating
Financing Receivable, Recorded Investment
Mortgage Loans	1.1	7.7
A Credit Rating
Financing Receivable, Recorded Investment
Mortgage Loans	591.7	666.0
Baa Credit Rating
Financing Receivable, Recorded Investment
Mortgage Loans	1,322.9	1,156.7
Ba Credit Rating
Financing Receivable, Recorded Investment
Mortgage Loans	10.2	13.1
B Credit Rating
Financing Receivable, Recorded Investment
Mortgage Loans	0.0	13.1
Loan to Value Ratio Below or Equal to 65 Percent
Financing Receivable, Recorded Investment
Mortgage Loans	956.2	898.7
Loan To Value Ratio Above 65 To 75 Percent
Financing Receivable, Recorded Investment
Mortgage Loans	878.9	818.0
Loan To Value Ratio Above 75 To 85 Percent
Financing Receivable, Recorded Investment
Mortgage Loans	60.8	102.3
Loan To Value Ratio Above 85 Percent
Financing Receivable, Recorded Investment
Mortgage Loans	$ 30.0	$ 37.6