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		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Derivatives Not Designated as Hedging under GAAP
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain / (Loss) Recognized in Income	$ 62.8	$ 41.9	$ 211.7	$ 149.2
Non-Designated Risk Management Derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain / (Loss) Recognized in Income	(10.3)	(9.9)	(23.4)	(6.2)
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)	(5.9)	(2.1)	2.2
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	(9.5)	(4.0)	(21.3)	(8.4)
Client-Related and Trading Derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain / (Loss) Recognized in Income	73.1	51.8	235.1	155.4
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	71.7	49.1	229.1	147.1
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	$ 1.4	$ 2.7	$ 6.0	$ 8.3