Changes in Allowance for Credit Losses by Segment (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at Beginning of Year	$ 233.3	$ 295.9	$ 307.9
Charge-Offs	(27.3)	(30.7)	(36.1)
Recoveries	12.1	11.2	18.1
Net (Charge-Offs) Recoveries	(15.2)	(19.5)	(18.0)
Provision for Credit Losses	(26.0)	(43.0)	6.0
Effects of Foreign Exchange Rates	(0.1)	(0.1)	0.0
Balance at End of Year	192.0	233.3	295.9
Allowance for Credit Losses Assigned to:
Loans and Leases				$ 161.0	$ 193.8	$ 267.0
Undrawn Commitments and Standby Letters of Credit				31.0	39.5	28.9
Total Allowance for Credit Losses	233.3	295.9	307.9	192.0	233.3	295.9
Commercial
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at Beginning of Year	114.8	169.7	168.0
Charge-Offs	(16.6)	(13.1)	(12.9)
Recoveries	4.8	5.5	11.1
Net (Charge-Offs) Recoveries	(11.8)	(7.6)	(1.8)
Provision for Credit Losses	2.0	(47.2)	3.5
Effects of Foreign Exchange Rates	(0.1)	(0.1)	0.0
Balance at End of Year	104.9	114.8	169.7
Allowance for Credit Losses Assigned to:
Loans and Leases				83.7	86.3	143.8
Undrawn Commitments and Standby Letters of Credit				21.2	28.5	25.9
Total Allowance for Credit Losses	114.8	169.7	168.0	104.9	114.8	169.7
Personal
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at Beginning of Year	118.5	126.2	139.9
Charge-Offs	(10.7)	(17.6)	(23.2)
Recoveries	7.3	5.7	7.0
Net (Charge-Offs) Recoveries	(3.4)	(11.9)	(16.2)
Provision for Credit Losses	(28.0)	4.2	2.5
Effects of Foreign Exchange Rates	0.0	0.0	0.0
Balance at End of Year	87.1	118.5	126.2
Allowance for Credit Losses Assigned to:
Loans and Leases				77.3	107.5	123.2
Undrawn Commitments and Standby Letters of Credit				9.8	11.0	3.0
Total Allowance for Credit Losses	$ 118.5	$ 126.2	$ 139.9	$ 87.1	$ 118.5	$ 126.2