Impaired Mortgage Loans (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Recorded Investment	$ 22.5	$ 22.9
Unpaid Principal Balance	24.0	24.4
Related Allowance	1.5	1.5
Impaired Fin Receivables with Allowance [Member]
Recorded Investment	13.1	13.1
Unpaid Principal Balance	14.6	14.6
Related Allowance	1.5	1.5
Impaired Fin Receivables with No Related Allowance [Member]
Recorded Investment	9.4	9.8
Unpaid Principal Balance	9.4	9.8
Related Allowance	$ 0	$ 0