Impaired Mortgage Loans (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Recorded Investment	$ 17.4	$ 17.4
Unpaid Principal Balance	18.9	18.9
Related Allowance	1.5	1.5
Impaired Fin Receivables with No Related Allowance [Member]
Recorded Investment	4.3	4.3
Unpaid Principal Balance	4.3	4.3
Related Allowance	0	0
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