Allowances for Credit Losses and Recorded Investments in Loans and Leases by Segment (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2011	Dec. 31, 2010
Financing Receivable, Allowance for Credit Losses [Line Items]
Specifically Evaluated for Impairment	$ 247.6	$ 279.4
Evaluated for Inherent Impairment	28,908.7	28,784.5
Total Loans and Leases	29,156.3	29,063.9
Specifically Evaluated for Impairment	22.0	32.8
Evaluated for Inherent Impairment	273.5	262.0
Allowance Assigned to Loans and Leases	295.5	294.8
Allowance for Unfunded Exposures Commitments and Standby Letters of Credit	32.6	34.1
Total Allowance for Credit Losses	328.1	328.9	350.8	357.3
Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Specifically Evaluated for Impairment	109.9	121.2
Evaluated for Inherent Impairment	12,575.6	12,233.1
Total Loans and Leases	12,685.5	12,354.3
Specifically Evaluated for Impairment	12.9	22.9
Evaluated for Inherent Impairment	158.9	155.7
Allowance Assigned to Loans and Leases	171.8	178.6
Allowance for Unfunded Exposures Commitments and Standby Letters of Credit	30.2	32.4
Total Allowance for Credit Losses	202.0	211.0	247.0	256.7
Personal
Financing Receivable, Allowance for Credit Losses [Line Items]
Specifically Evaluated for Impairment	137.7	158.2
Evaluated for Inherent Impairment	16,333.1	16,551.4
Total Loans and Leases	16,470.8	16,709.6
Specifically Evaluated for Impairment	9.1	9.9
Evaluated for Inherent Impairment	114.6	106.3
Allowance Assigned to Loans and Leases	123.7	116.2
Allowance for Unfunded Exposures Commitments and Standby Letters of Credit	2.4	1.7
Total Allowance for Credit Losses	$ 126.1	$ 117.9	$ 103.8	$ 100.6