Allowance for Credit Losses - Changes in Allowance for Credit Losses (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at Beginning of Year	$ 153.8	$ 192.0	$ 233.3
Charge-Offs	(10.1)	(21.5)	(27.3)
Recoveries	9.0	11.3	12.1
Net (Charge-Offs) Recoveries	(1.1)	(10.2)	(15.2)
Provision for Credit Losses	(14.5)	(28.0)	(26.0)
Effects of Foreign Exchange Rates	0.0	0.0	(0.1)
Balance at End of Year	138.2	153.8	192.0
Allowance for Credit Losses Assigned to:
Loans and Leases				$ 112.6	$ 131.2	$ 161.0
Undrawn Commitments and Standby Letters of Credit				25.6	22.6	31.0
Total Allowance for Credit Losses	153.8	192.0	233.3	138.2	153.8	192.0
Commercial
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at Beginning of Year	80.8	104.9	114.8
Charge-Offs	(0.9)	(11.4)	(16.6)
Recoveries	1.7	5.5	4.8
Net (Charge-Offs) Recoveries	0.8	(5.9)	(11.8)
Provision for Credit Losses	(2.9)	(18.2)	2.0
Effects of Foreign Exchange Rates	0.0	0.0	(0.1)
Balance at End of Year	78.7	80.8	104.9
Allowance for Credit Losses Assigned to:
Loans and Leases				57.6	63.5	83.7
Undrawn Commitments and Standby Letters of Credit				21.1	17.3	21.2
Total Allowance for Credit Losses	80.8	104.9	114.8	78.7	80.8	104.9
Personal
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at Beginning of Year	73.0	87.1	118.5
Charge-Offs	(9.2)	(10.1)	(10.7)
Recoveries	7.3	5.8	7.3
Net (Charge-Offs) Recoveries	(1.9)	(4.3)	(3.4)
Provision for Credit Losses	(11.6)	(9.8)	(28.0)
Effects of Foreign Exchange Rates	0.0	0.0	0.0
Balance at End of Year	59.5	73.0	87.1
Allowance for Credit Losses Assigned to:
Loans and Leases				55.0	67.7	77.3
Undrawn Commitments and Standby Letters of Credit				4.5	5.3	9.8
Total Allowance for Credit Losses	$ 73.0	$ 87.1	$ 118.5	$ 59.5	$ 73.0	$ 87.1