Allowance for Credit Losses - Changes in Allowance for Credit Losses (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Year	$ 138.2	$ 153.8	$ 192.0
Charge-Offs	(6.5)	(10.1)	(21.5)
Recoveries	7.2	9.0	11.3
Net (Charge-Offs) Recoveries	0.7	(1.1)	(10.2)
Provision for Credit Losses	(14.5)	(14.5)	(28.0)
Balance at End of Year	124.4	138.2	153.8
Allowance for Credit Losses Assigned to:
Loans and Leases				$ 104.5	$ 112.6	$ 131.2
Undrawn Commitments and Standby Letters of Credit				19.9	25.6	22.6
Total Allowance for Credit Losses	124.4	153.8	192.0	124.4	138.2	153.8
Commercial
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Year	78.7	80.8	104.9
Charge-Offs	(3.0)	(0.9)	(11.4)
Recoveries	0.9	1.7	5.5
Net (Charge-Offs) Recoveries	(2.1)	0.8	(5.9)
Provision for Credit Losses	(2.7)	(2.9)	(18.2)
Balance at End of Year	73.9	78.7	80.8
Allowance for Credit Losses Assigned to:
Loans and Leases				58.1	57.6	63.5
Undrawn Commitments and Standby Letters of Credit				15.8	21.1	17.3
Total Allowance for Credit Losses	78.7	78.7	104.9	73.9	78.7	80.8
Personal
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Year	59.5	73.0	87.1
Charge-Offs	(3.5)	(9.2)	(10.1)
Recoveries	6.3	7.3	5.8
Net (Charge-Offs) Recoveries	2.8	(1.9)	(4.3)
Provision for Credit Losses	(11.8)	(11.6)	(9.8)
Balance at End of Year	50.5	59.5	73.0
Allowance for Credit Losses Assigned to:
Loans and Leases				46.4	55.0	67.7
Undrawn Commitments and Standby Letters of Credit				4.1	4.5	5.3
Total Allowance for Credit Losses	$ 50.5	$ 73.0	$ 73.0	$ 50.5	$ 59.5	$ 73.0