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, 2021	Dec. 31, 2020	Dec. 31, 2019
Non-designated risk management derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Total gains (losses) from derivatives not designated as hedging under GAAP	$ (20.1)	$ (11.9)	$ (21.6)
Client-related and trading derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Total gains (losses) from derivatives not designated as hedging under GAAP	308.3	312.8	263.8
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	288.2	300.9	242.2
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	1.2	6.4	(1.6)
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	292.6	290.4	250.9
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	(21.3)	(18.3)	(20.0)
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	$ 15.7	$ 22.4	$ 12.9