Allowance for Credit Losses - Changes in Allowance for Credit Losses (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at Beginning of Year	$ 192.0	$ 233.3	$ 295.9
Charge-Offs	(21.5)	(27.3)	(30.7)
Recoveries	11.3	12.1	11.2
Net (Charge-Offs) Recoveries	(10.2)	(15.2)	(19.5)
Provision for Credit Losses	(28.0)	(26.0)	(43.0)
Effects of Foreign Exchange Rates	0.0	(0.1)	(0.1)
Balance at End of Year	153.8	192.0	233.3
Allowance for Credit Losses Assigned to:
Loans and Leases				$ 131.2	$ 161.0	$ 193.8
Undrawn Commitments and Standby Letters of Credit				22.6	31.0	39.5
Total Allowance for Credit Losses	192.0	233.3	295.9	153.8	192.0	233.3
Commercial
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at Beginning of Year	104.9	114.8	169.7
Charge-Offs	(11.4)	(16.6)	(13.1)
Recoveries	5.5	4.8	5.5
Net (Charge-Offs) Recoveries	(5.9)	(11.8)	(7.6)
Provision for Credit Losses	(18.2)	2.0	(47.2)
Effects of Foreign Exchange Rates	0.0	(0.1)	(0.1)
Balance at End of Year	80.8	104.9	114.8
Allowance for Credit Losses Assigned to:
Loans and Leases				63.5	83.7	86.3
Undrawn Commitments and Standby Letters of Credit				17.3	21.2	28.5
Total Allowance for Credit Losses	104.9	114.8	169.7	80.8	104.9	114.8
Personal
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at Beginning of Year	87.1	118.5	126.2
Charge-Offs	(10.1)	(10.7)	(17.6)
Recoveries	5.8	7.3	5.7
Net (Charge-Offs) Recoveries	(4.3)	(3.4)	(11.9)
Provision for Credit Losses	(9.8)	(28.0)	4.2
Effects of Foreign Exchange Rates	0.0	0.0	0.0
Balance at End of Year	73.0	87.1	118.5
Allowance for Credit Losses Assigned to:
Loans and Leases				67.7	77.3	107.5
Undrawn Commitments and Standby Letters of Credit				5.3	9.8	11.0
Total Allowance for Credit Losses	$ 87.1	$ 118.5	$ 126.2	$ 73.0	$ 87.1	$ 118.5