Derivative Financial Instruments - Location and Amount of Gains and Losses Recorded in Income for Derivatives not Designated as Hedging Under GAAP (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Non-designated risk management derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Total gains/(losses) from derivatives not designated as hedging under GAAP	$ (23.3)	$ (5.1)	$ (12.8)
Client-related and trading derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Total gains/(losses) from derivatives not designated as hedging under GAAP	314.9	220.6	248.0
Total Derivatives Not Designated as Hedging under GAAP
Derivative Instruments, Gain (Loss) [Line Items]
Total gains/(losses) from derivatives not designated as hedging under GAAP	291.6	215.5	235.2
Foreign Exchange Contracts | Other Operating Income | Non-designated risk management derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Total gains/(losses) from derivatives not designated as hedging under GAAP	(4.1)	8.2	(6.7)
Foreign Exchange Contracts | Foreign Exchange Trading Income | Client-related and trading derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Total gains/(losses) from derivatives not designated as hedging under GAAP	307.2	209.9	236.6
Other Financial Derivatives | Other Operating Income | Non-designated risk management derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Total gains/(losses) from derivatives not designated as hedging under GAAP	(19.2)	(13.3)	(6.1)
Interest Rate Contracts | Security Commissions and Trading Income | Client-related and trading derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Total gains/(losses) from derivatives not designated as hedging under GAAP	$ 7.7	$ 10.7	$ 11.4