Mortgage Loans Sorted by Applicable Internal Credit Ratings and Loan-to-Value Ratios (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Financing Receivable, Recorded Investment
Mortgage Loans	$ 1,807.5	$ 1,815.1
Aa Credit Rating
Financing Receivable, Recorded Investment
Mortgage Loans	7.8	10.8
A Credit Rating
Financing Receivable, Recorded Investment
Mortgage Loans	693.1	683.1
Baa Credit Rating
Financing Receivable, Recorded Investment
Mortgage Loans	1,062.2	1,094.6
Ba Credit Rating
Financing Receivable, Recorded Investment
Mortgage Loans	13.2	13.5
B Credit Rating
Financing Receivable, Recorded Investment
Mortgage Loans	31.2	13.1
Loan to Value Ratio Below or Equal to 65 Percent
Financing Receivable, Recorded Investment
Mortgage Loans	871.0	777.4
Loan To Value Ratio Above 65 To 75 Percent
Financing Receivable, Recorded Investment
Mortgage Loans	806.1	867.5
Loan To Value Ratio Above 75 To 85 Percent
Financing Receivable, Recorded Investment
Mortgage Loans	76.7	107.6
Loan To Value Ratio Above 85 Percent
Financing Receivable, Recorded Investment
Mortgage Loans	$ 53.7	$ 62.6