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		9 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Amount of Gain/(loss) Recognized in Other Comprehensive Income on Derivatives, net of tax	$ 2.6	$ 6.8	$ 31.6	$ 25.0
Amount of Gain/(Loss) Reclassified from Accumulated Other Comprehensive Income into Income	(12.7)	0.1	(29.2)	1.8
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	(0.7)	6.8	8.5	25.0
Amount of Gain/(Loss) Reclassified from Accumulated Other Comprehensive Income into Income	(14.3)	$ 0.1	(31.3)	$ 1.8
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	3.3		23.1
Amount of Gain/(Loss) Reclassified from Accumulated Other Comprehensive Income into Income	$ 1.6		$ 2.1