Allowances for Credit Losses and Recorded Investments in Loans and Leases by Segment (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Financing Receivable, Allowance for Credit Losses [Line Items]
Specifically Evaluated for Impairment	$ 242.2	$ 293.4
Evaluated for Inherent Impairment	31,398.0	29,092.1
Total Loans and Leases	31,640.2	29,385.5
Specifically Evaluated for Impairment	6.6	10.4
Evaluated for Inherent Impairment	260.4	267.7
Allowance assigned to loans and leases	267.0	278.1	297.9
Allowance for Undrawn Exposures Commitments and Standby Letters of Credit	28.9	29.8	29.7
Total Allowance for Credit Losses	295.9	307.9	327.6	328.9
Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Specifically Evaluated for Impairment	78.9	99.5
Evaluated for Inherent Impairment	14,274.7	12,520.5
Total Loans and Leases	14,353.6	12,620.0
Specifically Evaluated for Impairment	5.8	8.1
Evaluated for Inherent Impairment	138.0	132.8
Allowance assigned to loans and leases	143.8	140.9	166.1
Allowance for Undrawn Exposures Commitments and Standby Letters of Credit	25.9	27.1	28.1
Total Allowance for Credit Losses	169.7	168.0	194.2	211.0
Personal
Financing Receivable, Allowance for Credit Losses [Line Items]
Specifically Evaluated for Impairment	163.3	193.9
Evaluated for Inherent Impairment	17,123.3	16,571.6
Total Loans and Leases	17,286.6	16,765.5
Specifically Evaluated for Impairment	0.8	2.3
Evaluated for Inherent Impairment	122.4	134.9
Allowance assigned to loans and leases	123.2	137.2	131.8
Allowance for Undrawn Exposures Commitments and Standby Letters of Credit	3.0	2.7	1.6
Total Allowance for Credit Losses	$ 126.2	$ 139.9	$ 133.4	$ 117.9