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Financial Instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024
Derivatives, Fair Value [Line Items]
Financial instruments sold, not yet purchased, at fair value	$ 2,919.8	$ 2,853.3
Foreign currency forwards
Derivatives, Fair Value [Line Items]
Maximum length of time hedged in cash flow hedge	2 years
Fair Value, Measurements, Recurring
Derivatives, Fair Value [Line Items]
Financial instruments sold, not yet purchased, at fair value	$ 2,919.8	2,853.3
Fair Value, Measurements, Recurring | Derivatives
Derivatives, Fair Value [Line Items]
Financial instruments sold, not yet purchased, at fair value	297.3	$ 264.8
Fair Value, Measurements, Recurring | Not Designated as Hedging Instrument | Derivatives
Derivatives, Fair Value [Line Items]
Financial instruments sold, not yet purchased, at fair value	$ 297.3

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Fair value of obligations incurred from short sales.

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