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, 2023	Dec. 31, 2022	Dec. 31, 2021
Non-designated risk management derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Total gains (losses) from derivatives not designated as hedging under GAAP	$ (19.4)	$ (25.3)	$ (20.1)
Client-related and trading derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Total gains (losses) from derivatives not designated as hedging under GAAP	212.8	298.2	308.3
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	193.4	272.9	288.2
Foreign Exchange Contracts | Non-designated risk management derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Total gains (losses) from derivatives not designated as hedging under GAAP	$ 1.8	$ (3.1)	$ 1.2
Derivative, gain (loss), statement of income or comprehensive income [Extensible Enumeration]	Other Operating Income	Other Operating Income	Other Operating Income
Foreign Exchange Contracts | Client-related and trading derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Total gains (losses) from derivatives not designated as hedging under GAAP	$ 203.9	$ 288.6	$ 292.6
Derivative, gain (loss), statement of income or comprehensive income [Extensible Enumeration]	Foreign Exchange Trading Income	Foreign Exchange Trading Income	Foreign Exchange Trading Income
Other Financial Derivatives | Non-designated risk management derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Total gains (losses) from derivatives not designated as hedging under GAAP	$ (21.2)	$ (22.2)	$ (21.3)
Derivative, gain (loss), statement of income or comprehensive income [Extensible Enumeration]	Other Operating Income	Other Operating Income	Other Operating Income
Interest Rate Contracts | Client-related and trading derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Total gains (losses) from derivatives not designated as hedging under GAAP	$ 8.9	$ 9.6	$ 15.7
Derivative, gain (loss), statement of income or comprehensive income [Extensible Enumeration]	Security Commissions and Trading Income	Security Commissions and Trading Income	Security Commissions and Trading Income