Derivative Financial Instruments (Location and Amount of Gains and Losses Recorded in Income for Derivatives Not Designated As Hedging Under GAAP) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Derivatives Not Designated as Hedging under GAAP
Derivative Instruments, Gain (Loss) [Line Items]
AMOUNT OF DERIVATIVE GAINS (LOSSES) RECOGNIZED IN INCOME	$ 54.6	$ 41.2	$ 160.2	$ 141.7
Non-Designated Risk Management Derivatives
Derivative Instruments, Gain (Loss) [Line Items]
AMOUNT OF DERIVATIVE GAINS (LOSSES) RECOGNIZED IN INCOME	(3.9)	(13.8)	(9.3)	(30.6)
Non-Designated Risk Management Derivatives | Foreign Exchange Contracts
Derivative Instruments, Gain (Loss) [Line Items]
AMOUNT OF DERIVATIVE GAINS (LOSSES) RECOGNIZED IN INCOME	$ 0.0	$ 0.0	$ 0.0	$ (0.2)
Derivative, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Other Operating Income	Other Operating Income	Other Operating Income	Other Operating Income
Non-Designated Risk Management Derivatives | Other Financial Derivatives
Derivative Instruments, Gain (Loss) [Line Items]
AMOUNT OF DERIVATIVE GAINS (LOSSES) RECOGNIZED IN INCOME	$ (3.9)	$ (13.8)	$ (9.3)	$ (30.4)
Derivative, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Other Operating Income	Other Operating Income	Other Operating Income	Other Operating Income
Client-Related and Trading Derivatives
Derivative Instruments, Gain (Loss) [Line Items]
AMOUNT OF DERIVATIVE GAINS (LOSSES) RECOGNIZED IN INCOME	$ 58.5	$ 55.0	$ 169.5	$ 172.3
Client-Related and Trading Derivatives | Foreign Exchange Contracts
Derivative Instruments, Gain (Loss) [Line Items]
AMOUNT OF DERIVATIVE GAINS (LOSSES) RECOGNIZED IN INCOME	$ 57.2	$ 54.1	$ 166.5	$ 169.5
Derivative, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Foreign Exchange Trading Income	Foreign Exchange Trading Income	Foreign Exchange Trading Income	Foreign Exchange Trading Income
Client-Related and Trading Derivatives | Interest Rate Contracts
Derivative Instruments, Gain (Loss) [Line Items]
AMOUNT OF DERIVATIVE GAINS (LOSSES) RECOGNIZED IN INCOME	$ 1.3	$ 0.9	$ 3.0	$ 2.8
Derivative, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Security Commissions and Trading Income	Security Commissions and Trading Income	Security Commissions and Trading Income	Security Commissions and Trading Income