Allowances for Credit Losses and Recorded Investments in Loans and Leases by Segment (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Jun. 30, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Dec. 31, 2012
Financing Receivable, Allowance for Credit Losses [Line Items]
Specifically Evaluated for Impairment	$ 270.4		$ 293.4
Evaluated for Inherent Impairment	30,449.5		29,092.1
Total Loans and Leases	30,719.9		29,385.5
Specifically Evaluated for Impairment	9.5		10.4
Evaluated for Inherent Impairment	259.9		267.7
Allowance Assigned to Loans and Leases	269.4		278.1
Allowance for Undrawn Exposures Commitments and Standby Letters of Credit	28.9		29.8
Total Allowance for Credit Losses	298.3	303.5	307.9	317.5	320.7	327.6
Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Specifically Evaluated for Impairment	97.9		99.5
Evaluated for Inherent Impairment	13,770.7		12,520.5
Total Loans and Leases	13,868.6		12,620.0
Specifically Evaluated for Impairment	8.6		8.1
Evaluated for Inherent Impairment	134.5		132.8
Allowance Assigned to Loans and Leases	143.1		140.9
Allowance for Undrawn Exposures Commitments and Standby Letters of Credit	26.0		27.1
Total Allowance for Credit Losses	169.1	168.7	168.0	179.6	184.1	194.2
Personal
Financing Receivable, Allowance for Credit Losses [Line Items]
Specifically Evaluated for Impairment	172.5		193.9
Evaluated for Inherent Impairment	16,678.8		16,571.6
Total Loans and Leases	16,851.3		16,765.5
Specifically Evaluated for Impairment	0.9		2.3
Evaluated for Inherent Impairment	125.4		134.9
Allowance Assigned to Loans and Leases	126.3		137.2
Allowance for Undrawn Exposures Commitments and Standby Letters of Credit	2.9		2.7
Total Allowance for Credit Losses	$ 129.2	$ 134.8	$ 139.9	$ 137.9	$ 136.6	$ 133.4