Allowances for Credit Losses and Recorded Investments in Loans and Leases by Segment (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012	Dec. 31, 2011
Financing Receivable, Allowance for Credit Losses [Line Items]
Specifically Evaluated for Impairment	$ 294.3	$ 269.8
Evaluated for Inherent Impairment	28,568.6	29,234.7
Total Loans and Leases	28,862.9	29,504.5
Specifically Evaluated for Impairment	20.3	18.0
Evaluated for Inherent Impairment	273.8	279.9
Allowance Assigned to Loans and Leases	294.1	297.9
Allowance for Unfunded Exposures Commitments and Standby Letters of Credit	29.7	29.7
Total Allowance for Credit Losses	323.8	327.6	328.1	328.9
Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Specifically Evaluated for Impairment	103.9	125.0
Evaluated for Inherent Impairment	12,578.1	12,772.2
Total Loans and Leases	12,682.0	12,897.2
Specifically Evaluated for Impairment	13.5	11.0
Evaluated for Inherent Impairment	146.1	155.1
Allowance Assigned to Loans and Leases	159.6	166.1
Allowance for Unfunded Exposures Commitments and Standby Letters of Credit	27.5	28.1
Total Allowance for Credit Losses	187.1	194.2	202.0	211.0
Personal
Financing Receivable, Allowance for Credit Losses [Line Items]
Specifically Evaluated for Impairment	190.4	144.8
Evaluated for Inherent Impairment	15,990.5	16,462.5
Total Loans and Leases	16,180.9	16,607.3
Specifically Evaluated for Impairment	6.8	7.0
Evaluated for Inherent Impairment	127.7	124.8
Allowance Assigned to Loans and Leases	134.5	131.8
Allowance for Unfunded Exposures Commitments and Standby Letters of Credit	2.2	1.6
Total Allowance for Credit Losses	$ 136.7	$ 133.4	$ 126.1	$ 117.9