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Financial Instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk - Schedule of Effects of Derivative Instruments Designated for Hedging Held (Details) - USD ($) $ in Millions | 12 Months Ended
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Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2023
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |
Total derivatives designated as hedging instruments | $ 584.5 | $ 473.1 | $ 897.7
Designated as Hedging Instrument |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |
Total derivatives designated as hedging instruments | 13.9 | (17.1) |
Amount of gain reclassified from accumulated other comprehensive loss, net into income as a result of a forecasted transaction that is no longer probable of occurring | 0.0 | 0.0 |
Interest rate contracts |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |
Total derivatives designated as hedging instruments | 104.3 | 100.2 | $ 109.0
Interest rate contracts | Designated as Hedging Instrument |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |
Total derivatives designated as hedging instruments | (0.2) | (26.7) |
Foreign currency forwards | Designated as Hedging Instrument |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |
Total derivatives designated as hedging instruments | $ 14.1 | $ 9.6 |

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- Definition Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, Before Tax, Forecasted Transaction Not Probable + References No definition available. + Details
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