Financial Instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk - Income Statement Effects of Derivative Instruments (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2025	Mar. 31, 2024	Mar. 31, 2025	Mar. 31, 2024
Designated as Hedging Instrument
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Total derivatives designated as hedging instruments	$ 2.2	$ (2.8)	$ 3.0	$ (16.8)
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	7.6	15.5	49.9	42.9
Interest rate contracts \| Designated as Hedging Instrument
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Total derivatives designated as hedging instruments		(4.6)		(20.5)
Foreign currency forward contracts \| Designated as Hedging Instrument
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Total derivatives designated as hedging instruments	$ 2.2	$ 1.8	$ 3.0	$ 3.7