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, 2021	Mar. 31, 2020
Derivatives Not Designated as Hedging under GAAP
Derivative Instruments, Gain (Loss) [Line Items]
AMOUNT OF DERIVATIVE GAINS (LOSSES) RECOGNIZED IN INCOME	$ 78.2	$ 102.9
Non-Designated Risk Management Derivatives
Derivative Instruments, Gain (Loss) [Line Items]
AMOUNT OF DERIVATIVE GAINS (LOSSES) RECOGNIZED IN INCOME	(3.5)	3.0
Non-Designated Risk Management Derivatives | Foreign Exchange Contracts | Others Operating Income
Derivative Instruments, Gain (Loss) [Line Items]
AMOUNT OF DERIVATIVE GAINS (LOSSES) RECOGNIZED IN INCOME	0.1	0.1
Non-Designated Risk Management Derivatives | Other Financial Derivatives | Others Operating Income
Derivative Instruments, Gain (Loss) [Line Items]
AMOUNT OF DERIVATIVE GAINS (LOSSES) RECOGNIZED IN INCOME	(3.6)	2.9
Client-Related and Trading Derivatives
Derivative Instruments, Gain (Loss) [Line Items]
AMOUNT OF DERIVATIVE GAINS (LOSSES) RECOGNIZED IN INCOME	81.7	99.9
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	78.7	88.9
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	$ 3.0	$ 11.0