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Financial Instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk - Schedule of Income Statement Effects of Derivative Instruments (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
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Jun. 30, 2025 | Jun. 30, 2024 | Jun. 30, 2025 | Jun. 30, 2024
Designated as Hedging Instrument |  |  |  |
Derivative Instruments and Hedging Activities Disclosure [Line Items] |  |  |  |
Total derivatives designated as hedging instruments | $ 6.1 | $ (2.6) | $ 9.1 | $ (19.4)
Amount of gain reclassified from accumulated other comprehensive income into income as a result of a forecasted transaction that is no longer probable of occurring | 0.0 | 0.0 | 0.0 | 0.0
Interest rate contracts |  |  |  |
Derivative Instruments and Hedging Activities Disclosure [Line Items] |  |  |  |
Total derivatives designated as hedging instruments | 33.5 | 27.1 | 83.4 | 70.0
Interest rate contracts | Designated as Hedging Instrument |  |  |  |
Derivative Instruments and Hedging Activities Disclosure [Line Items] |  |  |  |
Total derivatives designated as hedging instruments | 0.0 | (4.4) | (0.2) | (24.9)
Foreign currency forward contracts | Designated as Hedging Instrument |  |  |  |
Derivative Instruments and Hedging Activities Disclosure [Line Items] |  |  |  |
Total derivatives designated as hedging instruments | $ 6.1 | $ 1.8 | $ 9.3 | $ 5.5

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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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