Changes in Allowance for Credit Losses by Segment (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at Beginning of Year	$ 295.9	$ 307.9	$ 327.6
Charge-Offs	(30.7)	(36.1)	(59.3)
Recoveries	11.2	18.1	19.6
Net (Charge-Offs) Recoveries	(19.5)	(18.0)	(39.7)
Provision for Credit Losses	(43.0)	6.0	20.0
Effects of Foreign Exchange Rates	(0.1)	0.0	0.0
Balance at End of Year	233.3	295.9	307.9
Allowance for Credit Losses Assigned to:
Loans and Leases				$ 193.8	$ 267.0	$ 278.1
Undrawn Commitments and Standby Letters of Credit				39.5	28.9	29.8
Total Allowance for Credit Losses	295.9	307.9	327.6	233.3	295.9	307.9
Commercial
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at Beginning of Year	169.7	168.0	194.2
Charge-Offs	(13.1)	(12.9)	(16.7)
Recoveries	5.5	11.1	8.6
Net (Charge-Offs) Recoveries	(7.6)	(1.8)	(8.1)
Provision for Credit Losses	(47.2)	3.5	(18.1)
Effects of Foreign Exchange Rates	(0.1)	0.0	0.0
Balance at End of Year	114.8	169.7	168.0
Allowance for Credit Losses Assigned to:
Loans and Leases				86.3	143.8	140.9
Undrawn Commitments and Standby Letters of Credit				28.5	25.9	27.1
Total Allowance for Credit Losses	169.7	168.0	194.2	114.8	169.7	168.0
Personal
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at Beginning of Year	126.2	139.9	133.4
Charge-Offs	(17.6)	(23.2)	(42.6)
Recoveries	5.7	7.0	11.0
Net (Charge-Offs) Recoveries	(11.9)	(16.2)	(31.6)
Provision for Credit Losses	4.2	2.5	38.1
Effects of Foreign Exchange Rates	0.0	0.0	0.0
Balance at End of Year	118.5	126.2	139.9
Allowance for Credit Losses Assigned to:
Loans and Leases				107.5	123.2	137.2
Undrawn Commitments and Standby Letters of Credit				11.0	3.0	2.7
Total Allowance for Credit Losses	$ 126.2	$ 139.9	$ 133.4	$ 118.5	$ 126.2	$ 139.9