Allowances for Credit Losses and Recorded Investments in Loans and Leases by Segment (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Jun. 30, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Dec. 31, 2010
Financing Receivable, Allowance for Credit Losses [Line Items]
Specifically Evaluated for Impairment	$ 273.0		$ 279.4
Evaluated for Inherent Impairment	29,269.7		28,784.5
Total Loans and Leases	29,542.7		29,063.9
Specifically Evaluated for Impairment	22.1		32.8
Evaluated for Inherent Impairment	276.5		262.0
Allowance Assigned to Loans and Leases	298.6		294.8
Allowance for Unfunded Exposures Commitments and Standby Letters of Credit	29.4		34.1
Total Allowance for Credit Losses	328.0	329.9	328.9	334.6	345.8	357.3
Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Specifically Evaluated for Impairment	124.7		121.2
Evaluated for Inherent Impairment	12,874.9		12,233.1
Total Loans and Leases	12,999.6		12,354.3
Specifically Evaluated for Impairment	12.0		22.9
Evaluated for Inherent Impairment	158.6		155.7
Allowance Assigned to Loans and Leases	170.6		178.6
Allowance for Unfunded Exposures Commitments and Standby Letters of Credit	27.8		32.4
Total Allowance for Credit Losses	198.4	196.3	211.0	221.0	239.9	256.7
Personal
Financing Receivable, Allowance for Credit Losses [Line Items]
Specifically Evaluated for Impairment	148.3		158.2
Evaluated for Inherent Impairment	16,394.8		16,551.4
Total Loans and Leases	16,543.1		16,709.6
Specifically Evaluated for Impairment	10.1		9.9
Evaluated for Inherent Impairment	117.9		106.3
Allowance Assigned to Loans and Leases	128.0		116.2
Allowance for Unfunded Exposures Commitments and Standby Letters of Credit	1.6		1.7
Total Allowance for Credit Losses	$ 129.6	$ 133.6	$ 117.9	$ 113.6	$ 105.9	$ 100.6