Loans and Leases - Delinquency Status (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment [Line Items]
Current	$ 32,268.0	$ 33,511.4
Total Loans and Leases	32,592.2	33,822.1
Total Nonperforming Assets	155.3	165.4
30 – 59 DAYS PAST DUE
Financing Receivable, Recorded Investment [Line Items]
Past Due	136.7	101.6
60 – 89 DAYS PAST DUE
Financing Receivable, Recorded Investment [Line Items]
Past Due	28.8	17.9
90 DAYS OR MORE PAST DUE
Financing Receivable, Recorded Investment [Line Items]
Past Due	8.0	31.0
Commercial
Financing Receivable, Recorded Investment [Line Items]
Current	14,487.6	15,620.9
Total Loans and Leases	14,558.0	15,666.7
Commercial | Commercial and Institutional
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	9,042.2	9,287.4
Commercial | Real Estate
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	3,482.7	4,002.5
Commercial | Non-U.S.
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	1,538.5	1,877.8
Commercial | Lease Financing, net
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	229.2	293.9
Commercial | Other
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	265.4	205.1
Commercial | 30 – 59 DAYS PAST DUE
Financing Receivable, Recorded Investment [Line Items]
Past Due	27.6	16.3
Commercial | 60 – 89 DAYS PAST DUE
Financing Receivable, Recorded Investment [Line Items]
Past Due	7.2	2.9
Commercial | 90 DAYS OR MORE PAST DUE
Financing Receivable, Recorded Investment [Line Items]
Past Due	1.3	5.8
Personal
Financing Receivable, Recorded Investment [Line Items]
Current	17,780.4	17,890.5
Total Loans and Leases	18,034.2	18,155.4
Personal | Real Estate
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	7,247.6	8,077.5
Personal | Private Client
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	10,753.1	10,052.0
Personal | Other
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	33.5	25.9
Personal | 30 – 59 DAYS PAST DUE
Financing Receivable, Recorded Investment [Line Items]
Past Due	109.1	85.3
Personal | 60 – 89 DAYS PAST DUE
Financing Receivable, Recorded Investment [Line Items]
Past Due	21.6	15.0
Personal | 90 DAYS OR MORE PAST DUE
Financing Receivable, Recorded Investment [Line Items]
Past Due	6.7	25.2
Performing Financing Receivable
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	32,441.5	33,661.9
Performing Financing Receivable | Commercial
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	14,523.7	15,645.9
Performing Financing Receivable | Commercial | Commercial and Institutional
Financing Receivable, Recorded Investment [Line Items]
Current	8,999.4	9,269.8
Total Loans and Leases	9,016.2	9,278.2
Performing Financing Receivable | Commercial | Real Estate
Financing Receivable, Recorded Investment [Line Items]
Current	3,455.3	3,974.4
Total Loans and Leases	3,474.4	3,990.9
Performing Financing Receivable | Commercial | Non-U.S.
Financing Receivable, Recorded Investment [Line Items]
Current	1,538.3	1,877.7
Total Loans and Leases	1,538.5	1,877.8
Performing Financing Receivable | Commercial | Lease Financing, net
Financing Receivable, Recorded Investment [Line Items]
Current	229.2	293.9
Total Loans and Leases	229.2	293.9
Performing Financing Receivable | Commercial | Other
Financing Receivable, Recorded Investment [Line Items]
Current	265.4	205.1
Total Loans and Leases	265.4	205.1
Performing Financing Receivable | Commercial | 30 – 59 DAYS PAST DUE | Commercial and Institutional
Financing Receivable, Recorded Investment [Line Items]
Past Due	13.3	5.3
Performing Financing Receivable | Commercial | 30 – 59 DAYS PAST DUE | Real Estate
Financing Receivable, Recorded Investment [Line Items]
Past Due	14.1	10.9
Performing Financing Receivable | Commercial | 30 – 59 DAYS PAST DUE | Non-U.S.
Financing Receivable, Recorded Investment [Line Items]
Past Due	0.2	0.1
Performing Financing Receivable | Commercial | 60 – 89 DAYS PAST DUE | Commercial and Institutional
Financing Receivable, Recorded Investment [Line Items]
Past Due	3.1	1.9
Performing Financing Receivable | Commercial | 60 – 89 DAYS PAST DUE | Real Estate
Financing Receivable, Recorded Investment [Line Items]
Past Due	4.1	1.0
Performing Financing Receivable | Commercial | 90 DAYS OR MORE PAST DUE | Commercial and Institutional
Financing Receivable, Recorded Investment [Line Items]
Past Due	0.4	1.2
Performing Financing Receivable | Commercial | 90 DAYS OR MORE PAST DUE | Real Estate
Financing Receivable, Recorded Investment [Line Items]
Past Due	0.9	4.6
Performing Financing Receivable | Personal
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	17,917.8	18,016.0
Performing Financing Receivable | Personal | Real Estate
Financing Receivable, Recorded Investment [Line Items]
Current	7,059.4	7,875.9
Total Loans and Leases	7,131.2	7,938.4
Performing Financing Receivable | Personal | Private Client
Financing Receivable, Recorded Investment [Line Items]
Current	10,687.5	9,988.7
Total Loans and Leases	10,753.1	10,051.7
Performing Financing Receivable | Personal | Other
Financing Receivable, Recorded Investment [Line Items]
Current	33.5	25.9
Total Loans and Leases	33.5	25.9
Performing Financing Receivable | Personal | 30 – 59 DAYS PAST DUE | Real Estate
Financing Receivable, Recorded Investment [Line Items]
Past Due	53.8	44.5
Performing Financing Receivable | Personal | 30 – 59 DAYS PAST DUE | Private Client
Financing Receivable, Recorded Investment [Line Items]
Past Due	55.3	40.8
Performing Financing Receivable | Personal | 60 – 89 DAYS PAST DUE | Real Estate
Financing Receivable, Recorded Investment [Line Items]
Past Due	11.9	6.5
Performing Financing Receivable | Personal | 60 – 89 DAYS PAST DUE | Private Client
Financing Receivable, Recorded Investment [Line Items]
Past Due	9.7	8.5
Performing Financing Receivable | Personal | 90 DAYS OR MORE PAST DUE | Real Estate
Financing Receivable, Recorded Investment [Line Items]
Past Due	6.1	11.5
Performing Financing Receivable | Personal | 90 DAYS OR MORE PAST DUE | Private Client
Financing Receivable, Recorded Investment [Line Items]
Past Due	0.6	13.7
Nonperforming Financing Receivable
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	150.7	160.2
Other Real Estate Owned	4.6	5.2
Nonperforming Financing Receivable | Commercial
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	34.3	20.8
Nonperforming Financing Receivable | Commercial | Commercial and Institutional
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	26.0	9.2
Nonperforming Financing Receivable | Commercial | Real Estate
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	8.3	11.6
Nonperforming Financing Receivable | Personal
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	116.4	139.4
Nonperforming Financing Receivable | Personal | Real Estate
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	116.4	139.1
Nonperforming Financing Receivable | Personal | Private Client
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	$ 0.0	$ 0.3