Impaired Mortgage Loans (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Recorded Investment	$ 21.6	$ 22.5
Unpaid Principal Balance	24.4	24.0
Related Allowance	2.8	1.5
Impaired Fin Receivables with Allowance [Member]
Recorded Investment	17.3	13.1
Unpaid Principal Balance	20.1	14.6
Related Allowance	2.8	1.5
Impaired Fin Receivables with No Related Allowance [Member]
Recorded Investment	4.3	9.4
Unpaid Principal Balance	4.3	9.4
Related Allowance	$ 0	$ 0