Loans (Credit Quality Indicator At Amortized Cost By Origination Year) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Financing Receivable, Credit Quality Indicator [Line Items]
2024/2023	$ 9,388.8		$ 13,006.4
2023/2022	4,497.3		4,823.9
2022/2021	4,570.3		4,177.6
2021/2020	3,934.2		1,765.4
2020/2019	1,672.4		1,208.7
PRIOR	3,589.6		2,681.5
REVOLVING LOANS	18,959.3		19,233.5
REVOLVING LOANS CONVERTED TO TERM LOANS	730.7		720.0
TOTAL LOANS	47,342.6		47,617.0
2024/2023, Charge-offs	0.0		(2.1)
2023/2022, Charge-offs	(7.3)		(4.4)
2022/2021, Charge-offs	(3.7)		0.0
2021/2020, Charge-offs	0.0		0.0
2020/2019, Charge-offs	0.0		0.0
Prior, Charge-offs	(0.1)		(1.0)
REVOLVING LOANS, Charge-offs	0.0		0.0
REVOLVING LOANS CONVERTED TO TERM LOANS, Charge-offs	0.0		0.0
Charge-Offs	11.1	$ 4.0	7.5
Commercial
Financing Receivable, Credit Quality Indicator [Line Items]
2024/2023	8,111.7		10,601.2
2023/2022	3,180.9		3,008.8
2022/2021	2,840.7		2,212.4
2021/2020	2,144.3		536.3
2020/2019	472.9		590.8
PRIOR	1,322.2		899.0
REVOLVING LOANS	7,241.2		7,467.6
REVOLVING LOANS CONVERTED TO TERM LOANS	103.2		96.7
TOTAL LOANS	25,417.1		25,412.8
2024/2023, Charge-offs	0.0		(1.3)
2023/2022, Charge-offs	(7.3)		(4.4)
2022/2021, Charge-offs	(3.7)		0.0
2021/2020, Charge-offs	0.0		0.0
2020/2019, Charge-offs	0.0		0.0
Prior, Charge-offs	0.0		0.0
REVOLVING LOANS, Charge-offs	0.0		0.0
REVOLVING LOANS CONVERTED TO TERM LOANS, Charge-offs	0.0		0.0
Charge-Offs	11.0	4.0	5.7
Commercial | Commercial and Institutional
Financing Receivable, Credit Quality Indicator [Line Items]
2024/2023	91.3		1,259.1
2023/2022	1,173.5		1,395.0
2022/2021	1,291.0		1,458.6
2021/2020	1,400.4		228.8
2020/2019	191.4		257.8
PRIOR	759.6		590.3
REVOLVING LOANS	5,978.0		6,276.9
REVOLVING LOANS CONVERTED TO TERM LOANS	95.4		88.8
TOTAL LOANS	10,980.6		11,555.3
2024/2023, Charge-offs	0.0
2023/2022, Charge-offs	(7.3)
2022/2021, Charge-offs	(1.3)
2021/2020, Charge-offs	0.0
2020/2019, Charge-offs	0.0
Prior, Charge-offs	0.0
REVOLVING LOANS, Charge-offs	0.0
REVOLVING LOANS CONVERTED TO TERM LOANS, Charge-offs	0.0
Charge-Offs	8.6
Commercial | Commercial and Institutional | 1 to 3 Category
Financing Receivable, Credit Quality Indicator [Line Items]
2024/2023	40.2		443.9
2023/2022	411.4		534.1
2022/2021	458.1		668.3
2021/2020	715.7		78.1
2020/2019	69.9		137.2
PRIOR	539.6		409.9
REVOLVING LOANS	4,735.9		4,909.8
REVOLVING LOANS CONVERTED TO TERM LOANS	15.0		15.0
TOTAL LOANS	6,985.8		7,196.3
Commercial | Commercial and Institutional | 4 to 5 Category
Financing Receivable, Credit Quality Indicator [Line Items]
2024/2023	51.1		801.4
2023/2022	757.9		790.9
2022/2021	757.4		729.5
2021/2020	627.1		138.7
2020/2019	117.6		120.5
PRIOR	218.3		178.7
REVOLVING LOANS	1,196.2		1,332.4
REVOLVING LOANS CONVERTED TO TERM LOANS	78.6		72.0
TOTAL LOANS	3,804.2		4,164.1
Commercial | Commercial and Institutional | 6 to 9 Category
Financing Receivable, Credit Quality Indicator [Line Items]
2024/2023	0.0		13.8
2023/2022	4.2		70.0
2022/2021	75.5		60.8
2021/2020	57.6		12.0
2020/2019	3.9		0.1
PRIOR	1.7		1.7
REVOLVING LOANS	45.9		34.7
REVOLVING LOANS CONVERTED TO TERM LOANS	1.8		1.8
TOTAL LOANS	190.6		194.9
Commercial | Commercial Real Estate
Financing Receivable, Credit Quality Indicator [Line Items]
2024/2023	161.2		1,933.2
2023/2022	1,980.5		1,598.7
2022/2021	1,534.7		710.6
2021/2020	700.9		242.3
2020/2019	236.8		298.8
PRIOR	439.6		155.4
REVOLVING LOANS	271.5		187.3
REVOLVING LOANS CONVERTED TO TERM LOANS	7.8		7.9
TOTAL LOANS	5,333.0		5,134.2
2024/2023, Charge-offs	0.0		(0.7)
2023/2022, Charge-offs	0.0		(4.4)
2022/2021, Charge-offs	(2.4)		0.0
2021/2020, Charge-offs	0.0		0.0
2020/2019, Charge-offs	0.0		0.0
Prior, Charge-offs	0.0		0.0
REVOLVING LOANS, Charge-offs	0.0		0.0
REVOLVING LOANS CONVERTED TO TERM LOANS, Charge-offs	0.0		0.0
Charge-Offs	2.4		5.1
Commercial | Commercial Real Estate | 1 to 3 Category
Financing Receivable, Credit Quality Indicator [Line Items]
2024/2023	9.3		403.6
2023/2022	346.9		389.9
2022/2021	411.7		159.1
2021/2020	277.7		23.9
2020/2019	25.7		37.8
PRIOR	78.6		44.8
REVOLVING LOANS	67.0		51.0
REVOLVING LOANS CONVERTED TO TERM LOANS	0.0		0.0
TOTAL LOANS	1,216.9		1,110.1
Commercial | Commercial Real Estate | 4 to 5 Category
Financing Receivable, Credit Quality Indicator [Line Items]
2024/2023	151.9		1,513.5
2023/2022	1,617.5		1,208.8
2022/2021	1,087.7		521.0
2021/2020	392.9		218.4
2020/2019	208.6		252.8
PRIOR	340.6		96.0
REVOLVING LOANS	204.5		136.3
REVOLVING LOANS CONVERTED TO TERM LOANS	7.8		7.9
TOTAL LOANS	4,011.5		3,954.7
Commercial | Commercial Real Estate | 6 to 9 Category
Financing Receivable, Credit Quality Indicator [Line Items]
2024/2023	0.0		16.1
2023/2022	16.1		0.0
2022/2021	35.3		30.5
2021/2020	30.3		0.0
2020/2019	2.5		8.2
PRIOR	20.4		14.6
REVOLVING LOANS	0.0		0.0
REVOLVING LOANS CONVERTED TO TERM LOANS	0.0		0.0
TOTAL LOANS	104.6		69.4
Commercial | Non-U.S.
Financing Receivable, Credit Quality Indicator [Line Items]
2024/2023	1,128.4		1,464.1
2023/2022	26.9		15.1
2022/2021	15.0		43.2
2021/2020	43.0		65.2
2020/2019	44.7		34.2
PRIOR	123.0		153.3
REVOLVING LOANS	991.7		1,003.4
REVOLVING LOANS CONVERTED TO TERM LOANS	0.0		0.0
TOTAL LOANS	2,372.7		2,778.5
Commercial | Non-U.S. | 1 to 3 Category
Financing Receivable, Credit Quality Indicator [Line Items]
2024/2023	950.4		487.9
2023/2022	0.0		0.0
2022/2021	0.0		43.2
2021/2020	43.0		65.2
2020/2019	44.7		34.2
PRIOR	22.3		3.3
REVOLVING LOANS	933.5		760.0
REVOLVING LOANS CONVERTED TO TERM LOANS	0.0		0.0
TOTAL LOANS	1,993.9		1,393.8
Commercial | Non-U.S. | 4 to 5 Category
Financing Receivable, Credit Quality Indicator [Line Items]
2024/2023	176.9		974.7
2023/2022	26.9		0.8
2022/2021	0.8		0.0
2021/2020	0.0		0.0
2020/2019	0.0		0.0
PRIOR	100.7		150.0
REVOLVING LOANS	58.2		243.4
REVOLVING LOANS CONVERTED TO TERM LOANS	0.0		0.0
TOTAL LOANS	363.5		1,368.9
Commercial | Non-U.S. | 6 to 9 Category
Financing Receivable, Credit Quality Indicator [Line Items]
2024/2023	1.1		1.5
2023/2022	0.0		14.3
2022/2021	14.2		0.0
2021/2020	0.0		0.0
2020/2019	0.0		0.0
PRIOR	0.0		0.0
REVOLVING LOANS	0.0		0.0
REVOLVING LOANS CONVERTED TO TERM LOANS	0.0		0.0
TOTAL LOANS	15.3		15.8
Commercial | Other
Financing Receivable, Credit Quality Indicator [Line Items]
2024/2023	6,730.8		5,944.8
2023/2022	0.0		0.0
2022/2021	0.0		0.0
2021/2020	0.0		0.0
2020/2019	0.0		0.0
PRIOR	0.0		0.0
REVOLVING LOANS	0.0		0.0
REVOLVING LOANS CONVERTED TO TERM LOANS	0.0		0.0
TOTAL LOANS	6,730.8		5,944.8
2024/2023, Charge-offs	0.0		(0.6)
2023/2022, Charge-offs	0.0		0.0
2022/2021, Charge-offs	0.0		0.0
2021/2020, Charge-offs	0.0		0.0
2020/2019, Charge-offs	0.0		0.0
Prior, Charge-offs	0.0		0.0
REVOLVING LOANS, Charge-offs	0.0		0.0
REVOLVING LOANS CONVERTED TO TERM LOANS, Charge-offs	0.0		0.0
Charge-Offs	0.0		0.6
Commercial | Other | 1 to 3 Category
Financing Receivable, Credit Quality Indicator [Line Items]
2024/2023	3,865.8		4,313.2
2023/2022	0.0		0.0
2022/2021	0.0		0.0
2021/2020	0.0		0.0
2020/2019	0.0		0.0
PRIOR	0.0		0.0
REVOLVING LOANS	0.0		0.0
REVOLVING LOANS CONVERTED TO TERM LOANS	0.0		0.0
TOTAL LOANS	3,865.8		4,313.2
Commercial | Other | 4 to 5 Category
Financing Receivable, Credit Quality Indicator [Line Items]
2024/2023	2,865.0		1,631.6
2023/2022	0.0		0.0
2022/2021	0.0		0.0
2021/2020	0.0		0.0
2020/2019	0.0		0.0
PRIOR	0.0		0.0
REVOLVING LOANS	0.0		0.0
REVOLVING LOANS CONVERTED TO TERM LOANS	0.0		0.0
TOTAL LOANS	2,865.0		1,631.6
Personal
Financing Receivable, Credit Quality Indicator [Line Items]
2024/2023	1,277.1		2,405.2
2023/2022	1,316.4		1,815.1
2022/2021	1,729.6		1,965.2
2021/2020	1,789.9		1,229.1
2020/2019	1,199.5		617.9
PRIOR	2,267.4		1,782.5
REVOLVING LOANS	11,718.1		11,765.9
REVOLVING LOANS CONVERTED TO TERM LOANS	627.5		623.3
TOTAL LOANS	21,925.5		22,204.2
2024/2023, Charge-offs	0.0		(0.8)
2023/2022, Charge-offs	0.0		0.0
2022/2021, Charge-offs	0.0		0.0
2021/2020, Charge-offs	0.0		0.0
2020/2019, Charge-offs	0.0		0.0
Prior, Charge-offs	(0.1)		(1.0)
REVOLVING LOANS, Charge-offs	0.0		0.0
REVOLVING LOANS CONVERTED TO TERM LOANS, Charge-offs	0.0		0.0
Charge-Offs	0.1	$ 0.0	1.8
Personal | Private Client
Financing Receivable, Credit Quality Indicator [Line Items]
2024/2023	50.7		818.4
2023/2022	813.2		629.3
2022/2021	559.3		707.4
2021/2020	569.0		168.4
2020/2019	156.4		167.5
PRIOR	286.3		197.7
REVOLVING LOANS	11,037.2		11,055.4
REVOLVING LOANS CONVERTED TO TERM LOANS	620.1		615.9
TOTAL LOANS	14,092.2		14,360.0
Personal | Private Client | 1 to 3 Category
Financing Receivable, Credit Quality Indicator [Line Items]
2024/2023	28.9		504.7
2023/2022	472.7		140.8
2022/2021	109.5		52.3
2021/2020	49.5		67.5
2020/2019	60.1		8.7
PRIOR	96.0		134.7
REVOLVING LOANS	5,056.1		5,320.9
REVOLVING LOANS CONVERTED TO TERM LOANS	168.2		168.1
TOTAL LOANS	6,041.0		6,397.7
Personal | Private Client | 4 to 5 Category
Financing Receivable, Credit Quality Indicator [Line Items]
2024/2023	21.8		290.1
2023/2022	324.3		488.2
2022/2021	449.6		655.1
2021/2020	519.5		100.9
2020/2019	96.3		158.8
PRIOR	190.3		44.7
REVOLVING LOANS	5,970.2		5,721.5
REVOLVING LOANS CONVERTED TO TERM LOANS	451.9		447.8
TOTAL LOANS	8,023.9		7,907.1
Personal | Private Client | 6 to 9 Category
Financing Receivable, Credit Quality Indicator [Line Items]
2024/2023	0.0		23.6
2023/2022	16.2		0.3
2022/2021	0.2		0.0
2021/2020	0.0		0.0
2020/2019	0.0		0.0
PRIOR	0.0		18.3
REVOLVING LOANS	10.9		13.0
REVOLVING LOANS CONVERTED TO TERM LOANS	0.0		0.0
TOTAL LOANS	27.3		55.2
Personal | Residential Real Estate
Financing Receivable, Credit Quality Indicator [Line Items]
2024/2023	87.1		470.3
2023/2022	475.5		1,169.8
2022/2021	1,154.8		1,218.0
2021/2020	1,197.0		1,060.7
2020/2019	1,043.1		434.0
PRIOR	1,951.7		1,571.3
REVOLVING LOANS	406.0		403.0
REVOLVING LOANS CONVERTED TO TERM LOANS	0.0		0.0
TOTAL LOANS	6,315.2		6,327.1
2024/2023, Charge-offs	0.0		(0.8)
2023/2022, Charge-offs	0.0		0.0
2022/2021, Charge-offs	0.0		0.0
2021/2020, Charge-offs	0.0		0.0
2020/2019, Charge-offs	0.0		0.0
Prior, Charge-offs	(0.1)		(1.0)
REVOLVING LOANS, Charge-offs	0.0		0.0
REVOLVING LOANS CONVERTED TO TERM LOANS, Charge-offs	0.0		0.0
Charge-Offs	0.1		1.8
Personal | Residential Real Estate | 1 to 3 Category
Financing Receivable, Credit Quality Indicator [Line Items]
2024/2023	42.8		278.7
2023/2022	272.2		464.0
2022/2021	409.4		500.6
2021/2020	430.1		373.3
2020/2019	367.5		142.4
PRIOR	863.2		722.4
REVOLVING LOANS	230.9		219.8
REVOLVING LOANS CONVERTED TO TERM LOANS	0.0		0.0
TOTAL LOANS	2,616.1		2,701.2
Personal | Residential Real Estate | 4 to 5 Category
Financing Receivable, Credit Quality Indicator [Line Items]
2024/2023	44.3		191.6
2023/2022	203.3		694.9
2022/2021	738.1		717.4
2021/2020	766.8		686.7
2020/2019	675.2		290.0
PRIOR	1,042.9		805.3
REVOLVING LOANS	161.7		170.3
REVOLVING LOANS CONVERTED TO TERM LOANS	0.0		0.0
TOTAL LOANS	3,632.3		3,556.2
Personal | Residential Real Estate | 6 to 9 Category
Financing Receivable, Credit Quality Indicator [Line Items]
2024/2023	0.0		0.0
2023/2022	0.0		10.9
2022/2021	7.3		0.0
2021/2020	0.1		0.7
2020/2019	0.4		1.6
PRIOR	45.6		43.6
REVOLVING LOANS	13.4		12.9
REVOLVING LOANS CONVERTED TO TERM LOANS	0.0		0.0
TOTAL LOANS	66.8		69.7
Personal | Non-U.S.
Financing Receivable, Credit Quality Indicator [Line Items]
2024/2023	8.4		28.2
2023/2022	27.7		16.0
2022/2021	15.5		39.8
2021/2020	23.9		0.0
2020/2019	0.0		16.4
PRIOR	29.4		13.5
REVOLVING LOANS	274.9		307.5
REVOLVING LOANS CONVERTED TO TERM LOANS	7.4		7.4
TOTAL LOANS	387.2		428.8
Personal | Non-U.S. | 1 to 3 Category
Financing Receivable, Credit Quality Indicator [Line Items]
2024/2023	0.4		15.5
2023/2022	3.8		0.0
2022/2021	0.0		0.6
2021/2020	0.6		0.0
2020/2019	0.0		0.0
PRIOR	4.5		4.6
REVOLVING LOANS	43.9		71.4
REVOLVING LOANS CONVERTED TO TERM LOANS	0.0		0.0
TOTAL LOANS	53.2		92.1
Personal | Non-U.S. | 4 to 5 Category
Financing Receivable, Credit Quality Indicator [Line Items]
2024/2023	0.0		12.7
2023/2022	23.9		16.0
2022/2021	15.5		39.2
2021/2020	23.3		0.0
2020/2019	0.0		16.4
PRIOR	24.9		8.9
REVOLVING LOANS	231.0		236.1
REVOLVING LOANS CONVERTED TO TERM LOANS	7.4		7.4
TOTAL LOANS	326.0		336.7
Personal | Non-U.S. | 6 to 9 Category
Financing Receivable, Credit Quality Indicator [Line Items]
2024/2023	8.0		0.0
2023/2022	0.0		0.0
2022/2021	0.0		0.0
2021/2020	0.0		0.0
2020/2019	0.0		0.0
PRIOR	0.0		0.0
REVOLVING LOANS	0.0		0.0
REVOLVING LOANS CONVERTED TO TERM LOANS	0.0		0.0
TOTAL LOANS	8.0		0.0
Personal | Other
Financing Receivable, Credit Quality Indicator [Line Items]
2024/2023	1,130.9		1,088.3
2023/2022	0.0		0.0
2022/2021	0.0		0.0
2021/2020	0.0		0.0
2020/2019	0.0		0.0
PRIOR	0.0		0.0
REVOLVING LOANS	0.0		0.0
REVOLVING LOANS CONVERTED TO TERM LOANS	0.0		0.0
TOTAL LOANS	1,130.9		1,088.3
Personal | Other | 1 to 3 Category
Financing Receivable, Credit Quality Indicator [Line Items]
2024/2023	636.3		461.7
2023/2022	0.0		0.0
2022/2021	0.0		0.0
2021/2020	0.0		0.0
2020/2019	0.0		0.0
PRIOR	0.0		0.0
REVOLVING LOANS	0.0		0.0
REVOLVING LOANS CONVERTED TO TERM LOANS	0.0		0.0
TOTAL LOANS	636.3		461.7
Personal | Other | 4 to 5 Category
Financing Receivable, Credit Quality Indicator [Line Items]
2024/2023	494.6		626.6
2023/2022	0.0		0.0
2022/2021	0.0		0.0
2021/2020	0.0		0.0
2020/2019	0.0		0.0
PRIOR	0.0		0.0
REVOLVING LOANS	0.0		0.0
REVOLVING LOANS CONVERTED TO TERM LOANS	0.0		0.0
TOTAL LOANS	$ 494.6		$ 626.6