Derivative Financial Instruments (Location and Amount of Gains and Losses Recorded in Income for Non-Designated Risk Management Derivative Instruments) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Derivatives Not Designated as Hedging under GAAP
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain / (Loss) Recognized in Income	$ 58.4	$ 68.6	$ 119.7	$ 148.9
Non-Designated Risk Management Derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain / (Loss) Recognized in Income	(4.9)	(13.2)	(11.3)	(13.1)
Non-Designated Risk Management Derivatives | Foreign Exchange Contracts | Others Operating Income
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain / (Loss) Recognized in Income	(0.8)	(5.4)	0.1	(1.3)
Non-Designated Risk Management Derivatives | Other Financial Derivatives | Others Operating Income
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain / (Loss) Recognized in Income	(4.1)	(7.8)	(11.4)	(11.8)
Client-Related and Trading Derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain / (Loss) Recognized in Income	63.3	81.8	131.0	162.0
Client-Related and Trading Derivatives | Foreign Exchange Contracts | Foreign Exchange Trading Income
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain / (Loss) Recognized in Income	60.5	78.9	126.7	157.4
Client-Related and Trading Derivatives | Interest Rate Contracts | Security Commissions and Trading Income
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain / (Loss) Recognized in Income	$ 2.8	$ 2.9	$ 4.3	$ 4.6