Impaired Mortgage Loans (Detail) (USD $) In Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2011	Jun. 30, 2011	Dec. 31, 2010
Financing Receivable, Recorded Investment, Past Due	$ 10.1	$ 10.1
Recorded Investment	23.2	23.2	22.9
Unpaid Principal Balance	24.7	24.7	24.4
Related Allowance	1.5	1.5	1.5
Average Recorded Investment	19.8	18.1
With an Allowance Recorded
Recorded Investment	13.1	13.1	13.1
Unpaid Principal Balance	14.6	14.6	14.6
Related Allowance	1.5	1.5	1.5
With No Related Allowance Recorded
Recorded Investment	10.1	10.1	9.8
Unpaid Principal Balance	10.1	10.1	9.8
Related Allowance	$ 0	$ 0	$ 0