Financial Instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk - Income Statement Effects of Derivative Instruments (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Designated as Hedging Instrument
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total derivatives designated as hedging instruments	$ (44.7)	$ 2.4
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
Interest rate contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total derivatives designated as hedging instruments	109.0	100.4	$ 80.8
Interest rate contracts | Designated as Hedging Instrument
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total derivatives designated as hedging instruments	(47.0)	2.4
Foreign currency forwards | Designated as Hedging Instrument
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total derivatives designated as hedging instruments	$ 2.3	$ 0.0