Derivative Financial Instruments (Location and Amount of Gains and Losses Recorded in Income) (Details) - Client Related and Trading - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain Recognized in Income	$ 51.8	$ 56.2	$ 155.4	$ 188.2
Foreign Exchange Contracts | Foreign Exchange Trading Income
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain Recognized in Income	49.1	53.6	147.1	178.5
Interest Rate Contracts | Security Commissions and Trading Income
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain Recognized in Income	$ 2.7	$ 2.6	$ 8.3	$ 9.7