Loans and Leases - Delinquency Status (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases	$ 31,409.6	$ 32,490.0
Commercial
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases	14,274.2	15,175.2
Commercial | Commercial and Institutional
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases	8,915.6	8,728.1
Commercial | Real Estate
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases	3,378.0	3,228.8
Commercial | Non-U.S.
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases	1,751.0	2,701.6
Commercial | Lease Financing, net
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases	65.6	90.7
Commercial | Other
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases	164.0	426.0
Personal
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases	17,135.4	17,314.8
Personal | Real Estate
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases	5,999.6	6,514.0
Personal | Private Client
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases	11,068.7	10,733.3
Personal | Other
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases	67.1	67.5
Performing Financing Receivable
Financing Receivable, Credit Quality Indicator [Line Items]
CURRENT	31,230.6	32,213.0
Total Loans and Leases	31,326.0	32,380.7
Performing Financing Receivable | 30 – 59 DAYS PAST DUE
Financing Receivable, Credit Quality Indicator [Line Items]
Past Due	59.6	112.5
Performing Financing Receivable | 60 – 89 DAYS PAST DUE
Financing Receivable, Credit Quality Indicator [Line Items]
Past Due	28.4	38.8
Performing Financing Receivable | 90 DAYS OR MORE PAST DUE
Financing Receivable, Credit Quality Indicator [Line Items]
Past Due	7.4	16.4
Performing Financing Receivable | Commercial
Financing Receivable, Credit Quality Indicator [Line Items]
CURRENT	14,235.9	15,087.6
Total Loans and Leases	14,262.5	15,161.1
Performing Financing Receivable | Commercial | Commercial and Institutional
Financing Receivable, Credit Quality Indicator [Line Items]
CURRENT	8,892.7	8,678.2
Total Loans and Leases	8,908.0	8,721.3
Performing Financing Receivable | Commercial | Real Estate
Financing Receivable, Credit Quality Indicator [Line Items]
CURRENT	3,363.3	3,191.5
Total Loans and Leases	3,374.4	3,221.9
Performing Financing Receivable | Commercial | Non-U.S.
Financing Receivable, Credit Quality Indicator [Line Items]
CURRENT	1,750.3	2,701.2
Total Loans and Leases	1,750.5	2,701.2
Performing Financing Receivable | Commercial | Lease Financing, net
Financing Receivable, Credit Quality Indicator [Line Items]
CURRENT	65.6	90.7
Total Loans and Leases	65.6	90.7
Performing Financing Receivable | Commercial | Other
Financing Receivable, Credit Quality Indicator [Line Items]
CURRENT	164.0	426.0
Total Loans and Leases	164.0	426.0
Performing Financing Receivable | Commercial | 30 – 59 DAYS PAST DUE
Financing Receivable, Credit Quality Indicator [Line Items]
Past Due	6.7	45.8
Performing Financing Receivable | Commercial | 30 – 59 DAYS PAST DUE | Commercial and Institutional
Financing Receivable, Credit Quality Indicator [Line Items]
Past Due	4.1	37.4
Performing Financing Receivable | Commercial | 30 – 59 DAYS PAST DUE | Real Estate
Financing Receivable, Credit Quality Indicator [Line Items]
Past Due	2.4	8.4
Performing Financing Receivable | Commercial | 30 – 59 DAYS PAST DUE | Non-U.S.
Financing Receivable, Credit Quality Indicator [Line Items]
Past Due	0.2	0.0
Performing Financing Receivable | Commercial | 30 – 59 DAYS PAST DUE | Lease Financing, net
Financing Receivable, Credit Quality Indicator [Line Items]
Past Due	0.0	0.0
Performing Financing Receivable | Commercial | 30 – 59 DAYS PAST DUE | Other
Financing Receivable, Credit Quality Indicator [Line Items]
Past Due	0.0	0.0
Performing Financing Receivable | Commercial | 60 – 89 DAYS PAST DUE
Financing Receivable, Credit Quality Indicator [Line Items]
Past Due	14.0	20.1
Performing Financing Receivable | Commercial | 60 – 89 DAYS PAST DUE | Commercial and Institutional
Financing Receivable, Credit Quality Indicator [Line Items]
Past Due	10.0	4.5
Performing Financing Receivable | Commercial | 60 – 89 DAYS PAST DUE | Real Estate
Financing Receivable, Credit Quality Indicator [Line Items]
Past Due	4.0	15.6
Performing Financing Receivable | Commercial | 60 – 89 DAYS PAST DUE | Non-U.S.
Financing Receivable, Credit Quality Indicator [Line Items]
Past Due	0.0	0.0
Performing Financing Receivable | Commercial | 60 – 89 DAYS PAST DUE | Lease Financing, net
Financing Receivable, Credit Quality Indicator [Line Items]
Past Due	0.0	0.0
Performing Financing Receivable | Commercial | 60 – 89 DAYS PAST DUE | Other
Financing Receivable, Credit Quality Indicator [Line Items]
Past Due	0.0	0.0
Performing Financing Receivable | Commercial | 90 DAYS OR MORE PAST DUE
Financing Receivable, Credit Quality Indicator [Line Items]
Past Due	5.9	7.6
Performing Financing Receivable | Commercial | 90 DAYS OR MORE PAST DUE | Commercial and Institutional
Financing Receivable, Credit Quality Indicator [Line Items]
Past Due	1.2	1.2
Performing Financing Receivable | Commercial | 90 DAYS OR MORE PAST DUE | Real Estate
Financing Receivable, Credit Quality Indicator [Line Items]
Past Due	4.7	6.4
Performing Financing Receivable | Commercial | 90 DAYS OR MORE PAST DUE | Non-U.S.
Financing Receivable, Credit Quality Indicator [Line Items]
Past Due	0.0	0.0
Performing Financing Receivable | Commercial | 90 DAYS OR MORE PAST DUE | Lease Financing, net
Financing Receivable, Credit Quality Indicator [Line Items]
Past Due	0.0	0.0
Performing Financing Receivable | Commercial | 90 DAYS OR MORE PAST DUE | Other
Financing Receivable, Credit Quality Indicator [Line Items]
Past Due	0.0	0.0
Performing Financing Receivable | Personal
Financing Receivable, Credit Quality Indicator [Line Items]
CURRENT	16,994.7	17,125.4
Total Loans and Leases	17,063.5	17,219.6
Performing Financing Receivable | Personal | Real Estate
Financing Receivable, Credit Quality Indicator [Line Items]
CURRENT	5,902.3	6,376.8
Total Loans and Leases	5,928.2	6,419.0
Performing Financing Receivable | Personal | Private Client
Financing Receivable, Credit Quality Indicator [Line Items]
CURRENT	11,025.3	10,681.1
Total Loans and Leases	11,068.2	10,733.1
Performing Financing Receivable | Personal | Other
Financing Receivable, Credit Quality Indicator [Line Items]
CURRENT	67.1	67.5
Total Loans and Leases	67.1	67.5
Performing Financing Receivable | Personal | 30 – 59 DAYS PAST DUE
Financing Receivable, Credit Quality Indicator [Line Items]
Past Due	52.9	66.7
Performing Financing Receivable | Personal | 30 – 59 DAYS PAST DUE | Real Estate
Financing Receivable, Credit Quality Indicator [Line Items]
Past Due	19.8	27.2
Performing Financing Receivable | Personal | 30 – 59 DAYS PAST DUE | Private Client
Financing Receivable, Credit Quality Indicator [Line Items]
Past Due	33.1	39.5
Performing Financing Receivable | Personal | 30 – 59 DAYS PAST DUE | Other
Financing Receivable, Credit Quality Indicator [Line Items]
Past Due	0.0	0.0
Performing Financing Receivable | Personal | 60 – 89 DAYS PAST DUE
Financing Receivable, Credit Quality Indicator [Line Items]
Past Due	14.4	18.7
Performing Financing Receivable | Personal | 60 – 89 DAYS PAST DUE | Real Estate
Financing Receivable, Credit Quality Indicator [Line Items]
Past Due	4.9	6.2
Performing Financing Receivable | Personal | 60 – 89 DAYS PAST DUE | Private Client
Financing Receivable, Credit Quality Indicator [Line Items]
Past Due	9.5	12.5
Performing Financing Receivable | Personal | 60 – 89 DAYS PAST DUE | Other
Financing Receivable, Credit Quality Indicator [Line Items]
Past Due	0.0	0.0
Performing Financing Receivable | Personal | 90 DAYS OR MORE PAST DUE
Financing Receivable, Credit Quality Indicator [Line Items]
Past Due	1.5	8.8
Performing Financing Receivable | Personal | 90 DAYS OR MORE PAST DUE | Real Estate
Financing Receivable, Credit Quality Indicator [Line Items]
Past Due	1.2	8.8
Performing Financing Receivable | Personal | 90 DAYS OR MORE PAST DUE | Private Client
Financing Receivable, Credit Quality Indicator [Line Items]
Past Due	0.3	0.0
Performing Financing Receivable | Personal | 90 DAYS OR MORE PAST DUE | Other
Financing Receivable, Credit Quality Indicator [Line Items]
Past Due	0.0	0.0
Nonperforming Financing Receivable
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases	83.6	109.3
Other Real Estate Owned	3.2	8.4
Total Nonperforming Assets	86.8	117.7
Nonperforming Financing Receivable | Commercial
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases	11.7	14.1
Nonperforming Financing Receivable | Commercial | Commercial and Institutional
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases	7.6	6.8
Nonperforming Financing Receivable | Commercial | Real Estate
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases	3.6	6.9
Nonperforming Financing Receivable | Commercial | Non-U.S.
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases	0.5	0.4
Nonperforming Financing Receivable | Commercial | Lease Financing, net
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases	0.0	0.0
Nonperforming Financing Receivable | Commercial | Other
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases	0.0	0.0
Nonperforming Financing Receivable | Personal
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases	71.9	95.2
Nonperforming Financing Receivable | Personal | Real Estate
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases	71.4	95.0
Nonperforming Financing Receivable | Personal | Private Client
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases	0.5	0.2
Nonperforming Financing Receivable | Personal | Other
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases	$ 0.0	$ 0.0