Loans (Delinquency Status) (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	$ 47,305.6	$ 47,553.4
NONACCRUAL	37.0	63.6
TOTAL LOANS	47,342.6	47,617.0
NONACCRUAL WITH NO ALLOWANCE	35.0	49.4
Other Real Estate Owned	0.0	1.5
Total Nonaccrual Assets	37.0	65.1
CURRENT
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	47,019.7	47,261.1
30 – 59 DAYS PAST DUE
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	226.8	239.5
60 – 89 DAYS PAST DUE
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	29.3	32.7
90 DAYS OR MORE PAST DUE
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	29.8	20.1
Commercial
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	25,403.8	25,396.5
NONACCRUAL	13.3	16.3
TOTAL LOANS	25,417.1	25,412.8
NONACCRUAL WITH NO ALLOWANCE	13.3	4.1
Commercial | Commercial and Institutional
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	10,973.0	11,539.0
NONACCRUAL	7.6	16.3
TOTAL LOANS	10,980.6	11,555.3
NONACCRUAL WITH NO ALLOWANCE	7.6	4.1
Commercial | Commercial Real Estate
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	5,327.3	5,134.2
NONACCRUAL	5.7	0.0
TOTAL LOANS	5,333.0	5,134.2
NONACCRUAL WITH NO ALLOWANCE	5.7	0.0
Commercial | Non-U.S.
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	2,372.7	2,778.5
NONACCRUAL	0.0	0.0
TOTAL LOANS	2,372.7	2,778.5
NONACCRUAL WITH NO ALLOWANCE	0.0	0.0
Commercial | Other
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	6,730.8	5,944.8
NONACCRUAL	0.0	0.0
TOTAL LOANS	6,730.8	5,944.8
NONACCRUAL WITH NO ALLOWANCE	0.0	0.0
Commercial | CURRENT
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	25,331.7	25,221.6
Commercial | CURRENT | Commercial and Institutional
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	10,963.0	11,374.6
Commercial | CURRENT | Commercial Real Estate
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	5,265.2	5,123.7
Commercial | CURRENT | Non-U.S.
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	2,372.7	2,778.5
Commercial | CURRENT | Other
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	6,730.8	5,944.8
Commercial | 30 – 59 DAYS PAST DUE
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	63.3	168.1
Commercial | 30 – 59 DAYS PAST DUE | Commercial and Institutional
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	7.3	163.7
Commercial | 30 – 59 DAYS PAST DUE | Commercial Real Estate
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	56.0	4.4
Commercial | 30 – 59 DAYS PAST DUE | Non-U.S.
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	0.0	0.0
Commercial | 30 – 59 DAYS PAST DUE | Other
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	0.0	0.0
Commercial | 60 – 89 DAYS PAST DUE
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	0.0	6.8
Commercial | 60 – 89 DAYS PAST DUE | Commercial and Institutional
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	0.0	0.7
Commercial | 60 – 89 DAYS PAST DUE | Commercial Real Estate
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	0.0	6.1
Commercial | 60 – 89 DAYS PAST DUE | Non-U.S.
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	0.0	0.0
Commercial | 60 – 89 DAYS PAST DUE | Other
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	0.0	0.0
Commercial | 90 DAYS OR MORE PAST DUE
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	8.8	0.0
Commercial | 90 DAYS OR MORE PAST DUE | Commercial and Institutional
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	2.7	0.0
Commercial | 90 DAYS OR MORE PAST DUE | Commercial Real Estate
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	6.1	0.0
Commercial | 90 DAYS OR MORE PAST DUE | Non-U.S.
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	0.0	0.0
Commercial | 90 DAYS OR MORE PAST DUE | Other
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	0.0	0.0
Personal
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	21,901.8	22,156.9
NONACCRUAL	23.7	47.3
TOTAL LOANS	21,925.5	22,204.2
NONACCRUAL WITH NO ALLOWANCE	21.7	45.3
Personal | Private Client
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	14,090.2	14,339.7
NONACCRUAL	2.0	20.3
TOTAL LOANS	14,092.2	14,360.0
NONACCRUAL WITH NO ALLOWANCE	0.0	18.3
Personal | Residential Real Estate
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	6,293.5	6,300.1
NONACCRUAL	21.7	27.0
TOTAL LOANS	6,315.2	6,327.1
NONACCRUAL WITH NO ALLOWANCE	21.7	27.0
Personal | Non-U.S.
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	387.2	428.8
NONACCRUAL	0.0	0.0
TOTAL LOANS	387.2	428.8
NONACCRUAL WITH NO ALLOWANCE	0.0	0.0
Personal | Other
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	1,130.9	1,088.3
NONACCRUAL	0.0	0.0
TOTAL LOANS	1,130.9	1,088.3
NONACCRUAL WITH NO ALLOWANCE	0.0	0.0
Personal | CURRENT
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	21,688.0	22,039.5
Personal | CURRENT | Private Client
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	13,941.1	14,240.0
Personal | CURRENT | Residential Real Estate
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	6,261.2	6,283.0
Personal | CURRENT | Non-U.S.
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	354.8	428.2
Personal | CURRENT | Other
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	1,130.9	1,088.3
Personal | 30 – 59 DAYS PAST DUE
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	163.5	71.4
Personal | 30 – 59 DAYS PAST DUE | Private Client
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	112.6	63.9
Personal | 30 – 59 DAYS PAST DUE | Residential Real Estate
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	18.5	7.5
Personal | 30 – 59 DAYS PAST DUE | Non-U.S.
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	32.4	0.0
Personal | 30 – 59 DAYS PAST DUE | Other
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	0.0	0.0
Personal | 60 – 89 DAYS PAST DUE
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	29.3	25.9
Personal | 60 – 89 DAYS PAST DUE | Private Client
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	18.5	24.8
Personal | 60 – 89 DAYS PAST DUE | Residential Real Estate
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	10.8	0.5
Personal | 60 – 89 DAYS PAST DUE | Non-U.S.
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	0.0	0.6
Personal | 60 – 89 DAYS PAST DUE | Other
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	0.0	0.0
Personal | 90 DAYS OR MORE PAST DUE
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	21.0	20.1
Personal | 90 DAYS OR MORE PAST DUE | Private Client
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	18.0	11.0
Personal | 90 DAYS OR MORE PAST DUE | Residential Real Estate
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	3.0	9.1
Personal | 90 DAYS OR MORE PAST DUE | Non-U.S.
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	0.0	0.0
Personal | 90 DAYS OR MORE PAST DUE | Other
Financing Receivable, Credit Quality Indicator [Line Items]
TOTAL ACCRUAL	$ 0.0	$ 0.0