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Derivatives and Hedging Activity - Summary of Amounts of Derivative Gains (Losses) Recognized In Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Accumulated Other Comprehensive Loss	$ (32.8)	$ 13.8	$ (26.5)	$ 7.1
Income (expense) related to fair value of derivative investments reclassified from accumulated other comprehensive earnings	(6.7)	(2.3)	(14.3)	(5.3)
Amount of Gain Recognized in Earnings Related to Amount Excluded from Effectiveness Testing	0.0	0.3	0.2	1.2
Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Income (expense) related to fair value of derivative investments reclassified from accumulated other comprehensive earnings	(0.3)	(0.2)	(0.9)	(0.8)
Amount of Gain Recognized in Earnings Related to Amount Excluded from Effectiveness Testing	0.0	0.0	0.0	0.0
Foreign exchange contracts
Derivative Instruments, Gain (Loss) [Line Items]
Income (expense) related to fair value of derivative investments reclassified from accumulated other comprehensive earnings	(5.9)	(1.1)	(11.1)	(1.9)
Amount of Gain Recognized in Earnings Related to Amount Excluded from Effectiveness Testing	0.0	0.0	0.0	0.0
Interest expense | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Accumulated Other Comprehensive Loss	(7.5)	(5.8)	(29.1)	(8.7)
Commission revenue | Foreign exchange contracts
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Accumulated Other Comprehensive Loss	(25.3)	19.6	2.6	15.8
Compensation expense
Derivative Instruments, Gain (Loss) [Line Items]
Income (expense) related to fair value of derivative investments reclassified from accumulated other comprehensive earnings	(0.3)	(0.6)	(1.4)	(1.5)
Amount of Gain Recognized in Earnings Related to Amount Excluded from Effectiveness Testing	0.0	0.2	0.1	0.7
Operating expense
Derivative Instruments, Gain (Loss) [Line Items]
Income (expense) related to fair value of derivative investments reclassified from accumulated other comprehensive earnings	(0.2)	(0.4)	(0.9)	(1.1)
Amount of Gain Recognized in Earnings Related to Amount Excluded from Effectiveness Testing	$ 0.0	$ 0.1	$ 0.1	$ 0.5

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Amount, before tax and reclassification, of gain (loss) from derivative instrument designated and qualifying cash flow hedge included in assessment of hedge effectiveness.

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Amount, before tax, of reclassification of gain (loss) from accumulated other comprehensive income (AOCI) for derivative instrument designated and qualifying as cash flow hedge included in assessment of hedge effectiveness.

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