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, 2021	Dec. 31, 2020
Interest Income
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Amount of gain reclassified from accumulated other comprehensive income into income	$ 0.1
Designated as Hedging Instrument | Interest Income
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Total derivatives designated as hedging instruments	0.1
Amount of gain reclassified from accumulated other comprehensive income into income as a result of a forecasted transaction is no longer probable of occurring	0.0
Interest rate contracts
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Total derivatives designated as hedging instruments	25.4	$ 12.6
Interest rate contracts | Interest Income
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Amount of gain reclassified from accumulated other comprehensive income into income	0.1
Interest rate contracts | Designated as Hedging Instrument | Interest Income
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Total derivatives designated as hedging instruments	$ 0.1