Financial Instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk - Accumulated Other Comprehensive Income Effects of Derivative Instruments (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Mar. 31, 2022	Mar. 31, 2022
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Amount of Loss Recognized in Other Comprehensive Income on Derivatives, net of tax	$ 18.1	$ 18.2
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 | Interest Income
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Amount of Loss Recognized in Other Comprehensive Income on Derivatives, net of tax	18.1	18.2
Amount of Gain Reclassified from Accumulated Other Comprehensive Income into Income	$ 1.6	$ 1.7