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		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) reclassified from accumulated other comprehensive income into income	$ (12.7)	$ 0.1	$ (29.2)	$ 1.8
Designated as Hedging Instrument
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Total derivatives designated as hedging instruments	(12.7)	0.1	(29.2)	1.8
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	51.6	29.5	85.5	82.3
Amount of gain (loss) reclassified from accumulated other comprehensive income into income	(14.3)	0.1	(31.3)	1.8
Interest rate contracts | Designated as Hedging Instrument
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Total derivatives designated as hedging instruments	(14.3)	$ 0.1	(31.3)	$ 1.8
Foreign currency forward contracts
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Amount of gain (loss) reclassified from accumulated other comprehensive income into income	1.6		2.1
Foreign currency forward contracts | Designated as Hedging Instrument
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Total derivatives designated as hedging instruments	$ 1.6		$ 2.1