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		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Derivatives Not Designated as Hedging under GAAP
Derivative Instruments, Gain (Loss) [Line Items]
AMOUNT OF DERIVATIVE GAINS (LOSSES) RECOGNIZED IN INCOME	$ 67.4	$ 58.4	$ 170.3	$ 119.7
Non-Designated Risk Management Derivatives
Derivative Instruments, Gain (Loss) [Line Items]
AMOUNT OF DERIVATIVE GAINS (LOSSES) RECOGNIZED IN INCOME	(9.1)	(4.9)	(6.1)	(11.3)
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	(0.5)	(0.8)	(0.4)	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	(8.6)	(4.1)	(5.7)	(11.4)
Client-Related and Trading Derivatives
Derivative Instruments, Gain (Loss) [Line Items]
AMOUNT OF DERIVATIVE GAINS (LOSSES) RECOGNIZED IN INCOME	76.5	63.3	176.4	131.0
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	71.3	60.5	160.2	126.7
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	$ 5.2	$ 2.8	$ 16.2	$ 4.3