Loans and Leases - Delinquency Status (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
CURRENT	$ 32,213.0	$ 32,268.0
Total Loans and Leases	32,490.0	32,592.2
30 – 59 DAYS PAST DUE
Financing Receivable, Recorded Investment [Line Items]
Past Due	112.5	136.7
60 – 89 DAYS PAST DUE
Financing Receivable, Recorded Investment [Line Items]
Past Due	38.8	28.8
90 DAYS OR MORE PAST DUE
Financing Receivable, Recorded Investment [Line Items]
Past Due	16.4	8.0
Commercial
Financing Receivable, Recorded Investment [Line Items]
CURRENT	15,087.6	14,487.6
Total Loans and Leases	15,175.2	14,558.0
Commercial | Commercial and Institutional
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	8,728.1	9,042.2
Commercial | Real Estate
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	3,228.8	3,482.7
Commercial | Non-U.S.
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	2,701.6	1,538.5
Commercial | Lease Financing, net
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	90.7	229.2
Commercial | Other
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	426.0	265.4
Commercial | 30 – 59 DAYS PAST DUE
Financing Receivable, Recorded Investment [Line Items]
Past Due	45.8	27.6
Commercial | 60 – 89 DAYS PAST DUE
Financing Receivable, Recorded Investment [Line Items]
Past Due	20.1	7.2
Commercial | 90 DAYS OR MORE PAST DUE
Financing Receivable, Recorded Investment [Line Items]
Past Due	7.6	1.3
Personal
Financing Receivable, Recorded Investment [Line Items]
CURRENT	17,125.4	17,780.4
Total Loans and Leases	17,314.8	18,034.2
Personal | Real Estate
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	6,514.0	7,247.6
Personal | Private Client
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	10,733.3	10,753.1
Personal | Other
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	67.5	33.5
Personal | 30 – 59 DAYS PAST DUE
Financing Receivable, Recorded Investment [Line Items]
Past Due	66.7	109.1
Personal | 60 – 89 DAYS PAST DUE
Financing Receivable, Recorded Investment [Line Items]
Past Due	18.7	21.6
Personal | 90 DAYS OR MORE PAST DUE
Financing Receivable, Recorded Investment [Line Items]
Past Due	8.8	6.7
Performing Financing Receivable
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	32,380.7	32,441.5
Performing Financing Receivable | Commercial
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	15,161.1	14,523.7
Performing Financing Receivable | Commercial | Commercial and Institutional
Financing Receivable, Recorded Investment [Line Items]
CURRENT	8,678.2	8,999.4
Total Loans and Leases	8,721.3	9,016.2
Performing Financing Receivable | Commercial | Real Estate
Financing Receivable, Recorded Investment [Line Items]
CURRENT	3,191.5	3,455.3
Total Loans and Leases	3,221.9	3,474.4
Performing Financing Receivable | Commercial | Non-U.S.
Financing Receivable, Recorded Investment [Line Items]
CURRENT	2,701.2	1,538.3
Total Loans and Leases	2,701.2	1,538.5
Performing Financing Receivable | Commercial | Lease Financing, net
Financing Receivable, Recorded Investment [Line Items]
CURRENT	90.7	229.2
Total Loans and Leases	90.7	229.2
Performing Financing Receivable | Commercial | Other
Financing Receivable, Recorded Investment [Line Items]
CURRENT	426.0	265.4
Total Loans and Leases	426.0	265.4
Performing Financing Receivable | Commercial | 30 – 59 DAYS PAST DUE | Commercial and Institutional
Financing Receivable, Recorded Investment [Line Items]
Past Due	37.4	13.3
Performing Financing Receivable | Commercial | 30 – 59 DAYS PAST DUE | Real Estate
Financing Receivable, Recorded Investment [Line Items]
Past Due	8.4	14.1
Performing Financing Receivable | Commercial | 30 – 59 DAYS PAST DUE | Non-U.S.
Financing Receivable, Recorded Investment [Line Items]
Past Due	0.0	0.2
Performing Financing Receivable | Commercial | 30 – 59 DAYS PAST DUE | Lease Financing, net
Financing Receivable, Recorded Investment [Line Items]
Past Due	0.0	0.0
Performing Financing Receivable | Commercial | 30 – 59 DAYS PAST DUE | Other
Financing Receivable, Recorded Investment [Line Items]
Past Due	0.0	0.0
Performing Financing Receivable | Commercial | 60 – 89 DAYS PAST DUE | Commercial and Institutional
Financing Receivable, Recorded Investment [Line Items]
Past Due	4.5	3.1
Performing Financing Receivable | Commercial | 60 – 89 DAYS PAST DUE | Real Estate
Financing Receivable, Recorded Investment [Line Items]
Past Due	15.6	4.1
Performing Financing Receivable | Commercial | 60 – 89 DAYS PAST DUE | Non-U.S.
Financing Receivable, Recorded Investment [Line Items]
Past Due	0.0	0.0
Performing Financing Receivable | Commercial | 60 – 89 DAYS PAST DUE | Lease Financing, net
Financing Receivable, Recorded Investment [Line Items]
Past Due	0.0	0.0
Performing Financing Receivable | Commercial | 60 – 89 DAYS PAST DUE | Other
Financing Receivable, Recorded Investment [Line Items]
Past Due	0.0	0.0
Performing Financing Receivable | Commercial | 90 DAYS OR MORE PAST DUE | Commercial and Institutional
Financing Receivable, Recorded Investment [Line Items]
Past Due	1.2	0.4
Performing Financing Receivable | Commercial | 90 DAYS OR MORE PAST DUE | Real Estate
Financing Receivable, Recorded Investment [Line Items]
Past Due	6.4	0.9
Performing Financing Receivable | Commercial | 90 DAYS OR MORE PAST DUE | Non-U.S.
Financing Receivable, Recorded Investment [Line Items]
Past Due	0.0	0.0
Performing Financing Receivable | Commercial | 90 DAYS OR MORE PAST DUE | Lease Financing, net
Financing Receivable, Recorded Investment [Line Items]
Past Due	0.0	0.0
Performing Financing Receivable | Commercial | 90 DAYS OR MORE PAST DUE | Other
Financing Receivable, Recorded Investment [Line Items]
Past Due	0.0	0.0
Performing Financing Receivable | Personal
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	17,219.6	17,917.8
Performing Financing Receivable | Personal | Real Estate
Financing Receivable, Recorded Investment [Line Items]
CURRENT	6,376.8	7,059.4
Total Loans and Leases	6,419.0	7,131.2
Performing Financing Receivable | Personal | Private Client
Financing Receivable, Recorded Investment [Line Items]
CURRENT	10,681.1	10,687.5
Total Loans and Leases	10,733.1	10,753.1
Performing Financing Receivable | Personal | Other
Financing Receivable, Recorded Investment [Line Items]
CURRENT	67.5	33.5
Total Loans and Leases	67.5	33.5
Performing Financing Receivable | Personal | 30 – 59 DAYS PAST DUE | Real Estate
Financing Receivable, Recorded Investment [Line Items]
Past Due	27.2	53.8
Performing Financing Receivable | Personal | 30 – 59 DAYS PAST DUE | Private Client
Financing Receivable, Recorded Investment [Line Items]
Past Due	39.5	55.3
Performing Financing Receivable | Personal | 30 – 59 DAYS PAST DUE | Other
Financing Receivable, Recorded Investment [Line Items]
Past Due	0.0	0.0
Performing Financing Receivable | Personal | 60 – 89 DAYS PAST DUE | Real Estate
Financing Receivable, Recorded Investment [Line Items]
Past Due	6.2	11.9
Performing Financing Receivable | Personal | 60 – 89 DAYS PAST DUE | Private Client
Financing Receivable, Recorded Investment [Line Items]
Past Due	12.5	9.7
Performing Financing Receivable | Personal | 60 – 89 DAYS PAST DUE | Other
Financing Receivable, Recorded Investment [Line Items]
Past Due	0.0	0.0
Performing Financing Receivable | Personal | 90 DAYS OR MORE PAST DUE | Real Estate
Financing Receivable, Recorded Investment [Line Items]
Past Due	8.8	6.1
Performing Financing Receivable | Personal | 90 DAYS OR MORE PAST DUE | Private Client
Financing Receivable, Recorded Investment [Line Items]
Past Due	0.0	0.6
Performing Financing Receivable | Personal | 90 DAYS OR MORE PAST DUE | Other
Financing Receivable, Recorded Investment [Line Items]
Past Due	0.0	0.0
Nonperforming Financing Receivable
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	109.3	150.7
Other Real Estate Owned	8.4	4.6
Total Nonperforming Assets	117.7	155.3
Nonperforming Financing Receivable | Commercial
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	14.1	34.3
Nonperforming Financing Receivable | Commercial | Commercial and Institutional
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	6.8	26.0
Nonperforming Financing Receivable | Commercial | Real Estate
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	6.9	8.3
Nonperforming Financing Receivable | Commercial | Non-U.S.
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	0.4	0.0
Nonperforming Financing Receivable | Commercial | Lease Financing, net
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	0.0	0.0
Nonperforming Financing Receivable | Commercial | Other
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	0.0	0.0
Nonperforming Financing Receivable | Personal
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	95.2	116.4
Nonperforming Financing Receivable | Personal | Real Estate
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	95.0	116.4
Nonperforming Financing Receivable | Personal | Private Client
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	0.2	0.0
Nonperforming Financing Receivable | Personal | Other
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	$ 0.0	$ 0.0