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
	Dec. 31, 2022	Dec. 31, 2021
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Amount of Gain/(loss) Recognized in Other Comprehensive Income on Derivatives, net of tax	$ 14.7	$ 0.1
Amount Reclassified from Accumulated Other Comprehensive Income into Income	(5.6)	0.1
Interest Income [Member]
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Amount Reclassified from Accumulated Other Comprehensive Income into Income		0.1
Interest rate contracts
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Amount of Gain/(loss) Recognized in Other Comprehensive Income on Derivatives, net of tax	(1.4)	0.1
Amount Reclassified from Accumulated Other Comprehensive Income into Income	(5.4)
Interest rate contracts | Interest Income [Member]
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Amount Reclassified from Accumulated Other Comprehensive Income into Income		$ 0.1
Foreign currency forward contracts
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Amount of Gain/(loss) Recognized in Other Comprehensive Income on Derivatives, net of tax	16.1
Amount Reclassified from Accumulated Other Comprehensive Income into Income	$ (0.2)