Allowance for Credit Losses - Recorded Investments in Loans and Leases (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Loans and Leases
Specifically Evaluated for Impairment	$ 139.8	$ 156.1
Evaluated for Inherent Impairment	32,452.4	33,666.0
Total Loans and Leases	32,592.2	33,822.1
Allowance for Credit Losses on Credit Exposures
Specifically Evaluated for Impairment	5.4	2.1
Evaluated for Inherent Impairment	125.8	158.9
Allowance Assigned to Loans and Leases	131.2	161.0	$ 193.8
Allowance for Undrawn Exposures
Commitments and Standby Letters of Credit	22.6	31.0	39.5
Total Allowance for Credit Losses	153.8	192.0	233.3	$ 295.9
Commercial
Loans and Leases
Specifically Evaluated for Impairment	33.9	46.9
Evaluated for Inherent Impairment	14,524.1	15,619.8
Total Loans and Leases	14,558.0	15,666.7
Allowance for Credit Losses on Credit Exposures
Specifically Evaluated for Impairment	1.1	0.0
Evaluated for Inherent Impairment	62.4	83.7
Allowance Assigned to Loans and Leases	63.5	83.7	86.3
Allowance for Undrawn Exposures
Commitments and Standby Letters of Credit	17.3	21.2	28.5
Total Allowance for Credit Losses	80.8	104.9	114.8	169.7
Personal
Loans and Leases
Specifically Evaluated for Impairment	105.9	109.2
Evaluated for Inherent Impairment	17,928.3	18,046.2
Total Loans and Leases	18,034.2	18,155.4
Allowance for Credit Losses on Credit Exposures
Specifically Evaluated for Impairment	4.3	2.1
Evaluated for Inherent Impairment	63.4	75.2
Allowance Assigned to Loans and Leases	67.7	77.3	107.5
Allowance for Undrawn Exposures
Commitments and Standby Letters of Credit	5.3	9.8	11.0
Total Allowance for Credit Losses	$ 73.0	$ 87.1	$ 118.5	$ 126.2