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
	Mar. 31, 2026	Mar. 31, 2025
Derivatives Not Designated as Hedging under GAAP
Derivative Instruments, Gain (Loss) [Line Items]
AMOUNT OF DERIVATIVE GAINS (LOSSES) RECOGNIZED IN INCOME	$ 90.7	$ 53.6
Non-Designated Risk Management Derivatives
Derivative Instruments, Gain (Loss) [Line Items]
AMOUNT OF DERIVATIVE GAINS (LOSSES) RECOGNIZED IN INCOME	2.7	(5.6)
Non-Designated Risk Management Derivatives | Foreign Exchange Contracts
Derivative Instruments, Gain (Loss) [Line Items]
AMOUNT OF DERIVATIVE GAINS (LOSSES) RECOGNIZED IN INCOME	$ (0.1)	$ 0.0
Derivative, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Other Operating Income(1)	Other Operating Income(1)
Non-Designated Risk Management Derivatives | Other Financial Derivatives
Derivative Instruments, Gain (Loss) [Line Items]
AMOUNT OF DERIVATIVE GAINS (LOSSES) RECOGNIZED IN INCOME	$ 2.8	$ (5.6)
Derivative, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Other Operating Income(1)	Other Operating Income(1)
Client-Related and Trading Derivatives
Derivative Instruments, Gain (Loss) [Line Items]
AMOUNT OF DERIVATIVE GAINS (LOSSES) RECOGNIZED IN INCOME	$ 88.0	$ 59.2
Client-Related and Trading Derivatives | Foreign Exchange Contracts
Derivative Instruments, Gain (Loss) [Line Items]
AMOUNT OF DERIVATIVE GAINS (LOSSES) RECOGNIZED IN INCOME	$ 87.7	$ 58.7
Derivative, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	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	$ 0.3	$ 0.5
Derivative, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Security Commissions and Trading Income	Security Commissions and Trading Income