Derivative Financial Instruments (Location and Amount of Gains and Losses Recorded in Income) (Details) - Client Related and Trading - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain Recognized in Income	$ 80.2	$ 49.3
Foreign Exchange Contracts | Foreign Exchange Trading Income
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain Recognized in Income	78.5	48.1
Interest Rate Contracts | Security Commissions and Trading Income
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain Recognized in Income	$ 1.7	$ 1.2