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
	Dec. 31, 2022	Dec. 31, 2021
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Amount of (loss)/gain reclassified from accumulated other comprehensive income into income	$ (5.6)	$ 0.1
Designated as Hedging Instrument
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Total derivatives designated as hedging instruments	(5.6)	0.1
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 Disclosure [Line Items]
Total derivatives designated as hedging instruments	8.3	25.4
Amount of (loss)/gain reclassified from accumulated other comprehensive income into income	(5.4)
Interest rate contracts | Designated as Hedging Instrument
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Total derivatives designated as hedging instruments	(5.4)	0.1
Foreign currency forward contracts
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Amount of (loss)/gain reclassified from accumulated other comprehensive income into income	(0.2)
Foreign currency forward contracts | Designated as Hedging Instrument
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Total derivatives designated as hedging instruments	$ (0.2)	$ 0.0