Financial Instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk - Accumulated Other Comprehensive Income Effects of Derivative Instruments (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Gain (loss) Recognized in Other Comprehensive Income on Derivatives, net of tax	$ 35.1	$ 53.5
Amount of Loss Reclassified from Accumulated Other Comprehensive Income into Income	(44.7)	2.4
Interest rate contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Gain (loss) Recognized in Other Comprehensive Income on Derivatives, net of tax	18.4	37.0
Amount of Loss Reclassified from Accumulated Other Comprehensive Income into Income	(47.0)	2.4
Foreign currency forwards
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Gain (loss) Recognized in Other Comprehensive Income on Derivatives, net of tax	16.7	16.5
Amount of Loss Reclassified from Accumulated Other Comprehensive Income into Income	$ 2.3	$ 0.0