Changes in Allowance for Credit Losses by Segment (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at Beginning of Year	$ 307.9	$ 327.6	$ 328.9
Charge-Offs	(36.1)	(59.3)	(63.0)
Recoveries	18.1	19.6	36.7
Net (Charge-Offs) Recoveries	(18.0)	(39.7)	(26.3)
Provision for Credit Losses	6.0	20.0	25.0
Balance at End of Year	295.9	307.9	327.6
Loans and Leases	267.0	278.1	297.9
Undrawn Commitments and Standby Letters of Credit	28.9	29.8	29.7
Total Allowance for Credit Losses	295.9	307.9	327.6
Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at Beginning of Year	168.0	194.2	211.0
Charge-Offs	(12.9)	(16.7)	(19.9)
Recoveries	11.1	8.6	20.3
Net (Charge-Offs) Recoveries	(1.8)	(8.1)	0.4
Provision for Credit Losses	3.5	(18.1)	(17.2)
Balance at End of Year	169.7	168.0	194.2
Loans and Leases	143.8	140.9	166.1
Undrawn Commitments and Standby Letters of Credit	25.9	27.1	28.1
Total Allowance for Credit Losses	169.7	168.0	194.2
Personal
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at Beginning of Year	139.9	133.4	117.9
Charge-Offs	(23.2)	(42.6)	(43.1)
Recoveries	7.0	11.0	16.4
Net (Charge-Offs) Recoveries	(16.2)	(31.6)	(26.7)
Provision for Credit Losses	2.5	38.1	42.2
Balance at End of Year	126.2	139.9	133.4
Loans and Leases	123.2	137.2	131.8
Undrawn Commitments and Standby Letters of Credit	3.0	2.7	1.6
Total Allowance for Credit Losses	$ 126.2	$ 139.9	$ 133.4