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		9 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Interest Income
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Amount of gain reclassified from accumulated other comprehensive income into income	$ 0.1		$ 1.8
Designated as Hedging Instrument | Interest Income
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Total derivatives designated as hedging instruments	0.1		1.8
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
Interest rate contracts
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Total derivatives designated as hedging instruments	29.5	$ 24.3	82.3	$ 67.5
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		1.8
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		$ 1.8