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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Derivatives Not Designated as Hedging under GAAP
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain / (Loss) Recognized in Income	$ 56.4	$ 62.8	$ 176.1	$ 211.7
Non-Designated Risk Management Derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain / (Loss) Recognized in Income	(8.2)	(10.3)	(19.5)	(23.4)
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	(1.8)	(0.8)	(1.7)	(2.1)
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	(6.4)	(9.5)	(17.8)	(21.3)
Client-Related and Trading Derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain / (Loss) Recognized in Income	64.6	73.1	195.6	235.1
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	59.7	71.7	186.4	229.1
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	$ 4.9	$ 1.4	$ 9.2	$ 6.0