Revenue from Contracts with Clients (Revenue Disaggregated by Major Source) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Disaggregation of Revenue [Line Items]
Trust, Investment and Other Servicing Fees			$ 4,361.8	$ 4,432.6	$ 4,361.1
Foreign Exchange Trading Income			203.9	288.6	292.6
Treasury Management Fees			31.6	39.3	44.3
Security Commissions and Trading Income			135.0	136.2	140.2
Other Operating Income			228.7	191.3	243.9
Investment Security Gains (Losses), net	$ (176.4)	$ 6.9	(169.5)	(214.0)	(0.3)
Total Other Noninterest Income			429.7	441.4	720.7
Total Noninterest Income			4,791.5	4,874.0	5,081.8
Custody and Fund Administration
Disaggregation of Revenue [Line Items]
Trust, Investment and Other Servicing Fees			1,805.3	1,816.0	1,917.5
Investment Management and Advisory
Disaggregation of Revenue [Line Items]
Trust, Investment and Other Servicing Fees			2,232.3	2,299.1	2,140.0
Securities Lending
Disaggregation of Revenue [Line Items]
Trust, Investment and Other Servicing Fees			83.9	81.6	76.9
Other
Disaggregation of Revenue [Line Items]
Trust, Investment and Other Servicing Fees			$ 240.3	$ 235.9	$ 226.7