Allowance for Credit Losses - Recorded Investments in Loans and Leases (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Loans and Leases
Specifically Evaluated for Impairment	$ 116.2	$ 139.8
Evaluated for Inherent Impairment	32,373.8	32,452.4
Total Loans and Leases	32,490.0	32,592.2
Allowance for Credit Losses on Credit Exposures
Specifically Evaluated for Impairment	7.2	5.4
Evaluated for Inherent Impairment	105.4	125.8
Allowance Assigned to Loans and Leases	112.6	131.2	$ 161.0
Allowance for Undrawn Exposures
Commitments and Standby Letters of Credit	25.6	22.6	31.0
Total Allowance for Credit Losses	138.2	153.8	192.0	$ 233.3
Commercial
Loans and Leases
Specifically Evaluated for Impairment	15.0	33.9
Evaluated for Inherent Impairment	15,160.2	14,524.1
Total Loans and Leases	15,175.2	14,558.0
Allowance for Credit Losses on Credit Exposures
Specifically Evaluated for Impairment	4.1	1.1
Evaluated for Inherent Impairment	53.5	62.4
Allowance Assigned to Loans and Leases	57.6	63.5	83.7
Allowance for Undrawn Exposures
Commitments and Standby Letters of Credit	21.1	17.3	21.2
Total Allowance for Credit Losses	78.7	80.8	104.9	114.8
Personal
Loans and Leases
Specifically Evaluated for Impairment	101.2	105.9
Evaluated for Inherent Impairment	17,213.6	17,928.3
Total Loans and Leases	17,314.8	18,034.2
Allowance for Credit Losses on Credit Exposures
Specifically Evaluated for Impairment	3.1	4.3
Evaluated for Inherent Impairment	51.9	63.4
Allowance Assigned to Loans and Leases	55.0	67.7	77.3
Allowance for Undrawn Exposures
Commitments and Standby Letters of Credit	4.5	5.3	9.8
Total Allowance for Credit Losses	$ 59.5	$ 73.0	$ 87.1	$ 118.5