Mortgage Loans Sorted by Applicable Internal Credit Ratings and Loan-to-Value Ratios (Detail) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Financing Receivable, Recorded Investment
Mortgage Loans	$ 1,856.5	$ 1,856.6
Aa Credit Rating
Financing Receivable, Recorded Investment
Mortgage Loans	2.8	7.7
A Credit Rating
Financing Receivable, Recorded Investment
Mortgage Loans	604.4	666.0
Baa Credit Rating
Financing Receivable, Recorded Investment
Mortgage Loans	1,223.9	1,156.7
Ba Credit Rating
Financing Receivable, Recorded Investment
Mortgage Loans	12.8	13.1
B Credit Rating
Financing Receivable, Recorded Investment
Mortgage Loans	12.6	13.1
Loan to Value Ratio Below or Equal to 65 Percent
Financing Receivable, Recorded Investment
Mortgage Loans	958.2	898.7
Loan To Value Ratio Above 65 To 75 Percent
Financing Receivable, Recorded Investment
Mortgage Loans	772.1	818.0
Loan To Value Ratio Above 75 To 85 Percent
Financing Receivable, Recorded Investment
Mortgage Loans	89.5	102.3
Loan To Value Ratio Above 85 Percent
Financing Receivable, Recorded Investment
Mortgage Loans	$ 36.7	$ 37.6