Derivative Financial Instruments (Location and Amount of Gains and Losses Recorded in Income) (Details) - Client Related and Trading - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain Recognized in Income	$ 54.3	$ 67.4	$ 103.6	$ 132.0
Foreign Exchange Contracts | Foreign Exchange Trading Income
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain Recognized in Income	49.9	64.4	98.0	124.9
Interest Rate Contracts | Security Commissions and Trading Income
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain Recognized in Income	$ 4.4	$ 3.0	$ 5.6	$ 7.1