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, 2022	Mar. 31, 2021
Derivatives Not Designated as Hedging under GAAP
Derivative Instruments, Gain (Loss) [Line Items]
AMOUNT OF DERIVATIVE GAINS (LOSSES) RECOGNIZED IN INCOME	$ 80.1	$ 78.2
Non-Designated Risk Management Derivatives
Derivative Instruments, Gain (Loss) [Line Items]
AMOUNT OF DERIVATIVE GAINS (LOSSES) RECOGNIZED IN INCOME	(1.8)	(3.5)
Non-Designated Risk Management Derivatives | Foreign Exchange Contracts | Other Operating Income
Derivative Instruments, Gain (Loss) [Line Items]
AMOUNT OF DERIVATIVE GAINS (LOSSES) RECOGNIZED IN INCOME	2.7	0.1
Non-Designated Risk Management Derivatives | Other Financial Derivatives | Other Operating Income
Derivative Instruments, Gain (Loss) [Line Items]
AMOUNT OF DERIVATIVE GAINS (LOSSES) RECOGNIZED IN INCOME	(4.5)	(3.6)
Client-Related and Trading Derivatives
Derivative Instruments, Gain (Loss) [Line Items]
AMOUNT OF DERIVATIVE GAINS (LOSSES) RECOGNIZED IN INCOME	81.9	81.7
Client-Related and Trading Derivatives | Foreign Exchange Contracts | Foreign Exchange Trading Income
Derivative Instruments, Gain (Loss) [Line Items]
AMOUNT OF DERIVATIVE GAINS (LOSSES) RECOGNIZED IN INCOME	80.9	78.7
Client-Related and Trading Derivatives | Interest Rate Contracts | Security Commissions and Trading Income
Derivative Instruments, Gain (Loss) [Line Items]
AMOUNT OF DERIVATIVE GAINS (LOSSES) RECOGNIZED IN INCOME	$ 1.0	$ 3.0