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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Derivatives Not Designated as Hedging under GAAP
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain / (Loss) Recognized in Income	$ 68.6	$ 59.3	$ 148.9	$ 107.3
Non-Designated Risk Management Derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain / (Loss) Recognized in Income	(13.2)	5.0	(13.1)	3.7
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	(5.4)	6.4	(1.3)	8.1
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	(7.8)	(1.4)	(11.8)	(4.4)
Client-Related and Trading Derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain / (Loss) Recognized in Income	81.8	54.3	162.0	103.6
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	78.9	49.9	157.4	98.0
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	$ 2.9	$ 4.4	$ 4.6	$ 5.6