Revenue from Contracts with Clients (Revenue Disaggregation) (Details) - USD ($) $ in Millions	1 Months Ended		3 Months Ended		9 Months Ended
	Jan. 31, 2024	Nov. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Disaggregation of Revenue [Line Items]
Revenue from contract with client			$ 1,196.6	$ 1,111.9	$ 3,505.6	$ 3,271.8
Foreign Exchange Trading Income			54.1	51.8	169.5	154.9
Treasury Management Fees			8.2	7.5	26.5	23.8
Security Commissions and Trading Income			35.5	30.9	107.7	101.7
Other Operating Income			111.8	68.2	1,097.5	170.2
Investment Security Gains (Losses), net	$ (189.4)	$ (176.4)	0.0	0.0	(189.3)	6.9
Total Other Noninterest Income			209.6	158.4	1,211.9	457.5
Total Noninterest Income			1,406.2	1,270.3	4,717.5	3,729.3
Custody and Fund Administration
Disaggregation of Revenue [Line Items]
Revenue from contract with client			482.5	459.0	1,427.1	1,358.7
Investment Management and Advisory
Disaggregation of Revenue [Line Items]
Revenue from contract with client			634.8	574.1	1,838.3	1,673.4
Securities Lending
Disaggregation of Revenue [Line Items]
Revenue from contract with client			17.7	20.7	53.2	61.3
Other
Disaggregation of Revenue [Line Items]
Revenue from contract with client			$ 61.6	$ 58.1	$ 187.0	$ 178.4