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, 2023	Mar. 31, 2022	Mar. 31, 2023	Mar. 31, 2022
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Amount of Gain/(loss) Recognized in Other Comprehensive Income on Derivatives, net of tax	$ 14.3	$ 18.1	$ 29.0	$ 18.2
Amount of Gain/(Loss) Reclassified from Accumulated Other Comprehensive Income into Income	(10.9)	1.6	(16.5)	1.7
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	10.6	18.1	9.2	18.2
Amount of Gain/(Loss) Reclassified from Accumulated Other Comprehensive Income into Income	(11.6)	$ 1.6	(17.0)	$ 1.7
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	3.7		19.8
Amount of Gain/(Loss) Reclassified from Accumulated Other Comprehensive Income into Income	$ 0.7		$ 0.5