Impaired Mortgage Loans (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Recorded Investment		$ 22.5
Unpaid Principal Balance		24.0
Related Allowance		1.5
Impaired Fin Receivables with Allowance [Member]
Recorded Investment	22.0	13.1
Unpaid Principal Balance	24.0	14.6
Related Allowance	2.0	1.5
Impaired Fin Receivables with No Related Allowance [Member]
Recorded Investment		9.4
Unpaid Principal Balance		9.4
Related Allowance		$ 0