Financial Instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk - Accumulated Other Comprehensive Income Effects of Derivative Instruments (Details) - Interest Income - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Jun. 30, 2022	Jun. 30, 2022
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Amount of Loss Recognized in Other Comprehensive Income on Derivatives, net of tax	$ 6.8	$ 25.0
Amount of Gain Reclassified from Accumulated Other Comprehensive Income into Income	0.1	1.8
Interest rate contracts
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Amount of Loss Recognized in Other Comprehensive Income on Derivatives, net of tax	6.8	25.0
Amount of Gain Reclassified from Accumulated Other Comprehensive Income into Income	$ 0.1	$ 1.8