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, 2025	Dec. 31, 2024	Dec. 31, 2023
Non-designated risk management derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Total gains from derivatives not designated as hedging under GAAP	$ (29.0)	$ (33.7)	$ (19.4)
Client-related and trading derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Total gains from derivatives not designated as hedging under GAAP	245.0	235.2	212.8
Total Derivatives Not Designated as Hedging under GAAP
Derivative Instruments, Gain (Loss) [Line Items]
Total gains from derivatives not designated as hedging under GAAP	216.0	201.5	193.4
Foreign Exchange Contracts | Non-designated risk management derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Total gains from derivatives not designated as hedging under GAAP	$ 0.0	$ (0.2)	$ 1.8
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 from derivatives not designated as hedging under GAAP	$ 240.8	$ 231.2	$ 203.9
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 from derivatives not designated as hedging under GAAP	$ (29.0)	$ (33.5)	$ (21.2)
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 from derivatives not designated as hedging under GAAP	$ 4.2	$ 4.0	$ 8.9
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