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, 2024	Mar. 31, 2023	Mar. 31, 2024	Mar. 31, 2023
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Amount of Gain/(Loss) Recognized in Other Comprehensive Income on Derivatives, net of tax	$ (0.8)	$ 14.3	$ 19.9	$ 29.0
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	1.7	10.6	14.3	9.2
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	$ (2.5)	$ 3.7	$ 5.6	$ 19.8