Financial Instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk - Income Statement 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) reclassified from accumulated other comprehensive income into income	$ (10.9)	$ 1.6	$ (16.5)	$ 1.7
Designated as Hedging Instrument
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Total derivatives designated as hedging instruments	(10.9)	1.6	(16.5)	1.7
Amount of gain reclassified from accumulated other comprehensive income into income as a result of a forecasted transaction that is no longer probable of occurring	0.0	0.0	0.0	0.0
Interest rate contracts
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Total derivatives designated as hedging instruments	25.6	27.4	33.9	52.8
Amount of gain (loss) reclassified from accumulated other comprehensive income into income	(11.6)	1.6	(17.0)	1.7
Interest rate contracts | Designated as Hedging Instrument
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Total derivatives designated as hedging instruments	(11.6)	$ 1.6	(17.0)	$ 1.7
Foreign currency forward contracts
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Amount of gain (loss) reclassified from accumulated other comprehensive income into income	0.7		0.5
Foreign currency forward contracts | Designated as Hedging Instrument
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Total derivatives designated as hedging instruments	$ 0.7		$ 0.5