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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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