Allowances for Credit Losses and Recorded Investments in Loans and Leases by Segment (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
Financing Receivable, Allowance for Credit Losses [Line Items]
Specifically Evaluated for Impairment	$ 245.2	$ 242.2
Evaluated for Inherent Impairment	32,385.0	31,398.0
Total Loans and Leases	32,630.2	31,640.2
Specifically Evaluated for Impairment	8.4	6.6
Evaluated for Inherent Impairment	250.6	260.4
Allowance Assigned to Loans and Leases	259.0	267.0
Allowance for Unfunded Exposures Commitments and Standby Letters of Credit	27.7	28.9
Total Allowance for Credit Losses	286.7	295.9	309.4	307.9
Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Specifically Evaluated for Impairment	85.3	78.9
Evaluated for Inherent Impairment	15,406.0	14,274.7
Total Loans and Leases	15,491.3	14,353.6
Specifically Evaluated for Impairment	6.8	5.8
Evaluated for Inherent Impairment	132.7	138.0
Allowance Assigned to Loans and Leases	139.5	143.8
Allowance for Unfunded Exposures Commitments and Standby Letters of Credit	24.8	25.9
Total Allowance for Credit Losses	164.3	169.7	173.1	168.0
Personal
Financing Receivable, Allowance for Credit Losses [Line Items]
Specifically Evaluated for Impairment	159.9	163.3
Evaluated for Inherent Impairment	16,979.0	17,123.3
Total Loans and Leases	17,138.9	17,286.6
Specifically Evaluated for Impairment	1.6	0.8
Evaluated for Inherent Impairment	117.9	122.4
Allowance Assigned to Loans and Leases	119.5	123.2
Allowance for Unfunded Exposures Commitments and Standby Letters of Credit	2.9	3.0
Total Allowance for Credit Losses	$ 122.4	$ 126.2	$ 136.3	$ 139.9