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, 2022	Mar. 31, 2021	Mar. 31, 2022	Mar. 31, 2021
Interest Income
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Amount of gain reclassified from accumulated other comprehensive income into income	$ 1.6		$ 1.7
Designated as Hedging Instrument | Interest Income
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Total derivatives designated as hedging instruments	1.6		1.7
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	27.4	$ 30.6	52.8	$ 43.2
Interest rate contracts | Interest Income
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Amount of gain reclassified from accumulated other comprehensive income into income	1.6		1.7
Interest rate contracts | Designated as Hedging Instrument | Interest Income
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Total derivatives designated as hedging instruments	$ 1.6		$ 1.7