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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
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