Financial Instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk - Schedule of Fair Value of Derivative Instruments Designated for Hedging (Details) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2025	Sep. 30, 2024
Derivatives, Fair Value [Line Items]
Asset Derivatives	$ 7,987.4	$ 7,235.9
Liability Derivatives	8,461.7	7,144.1
Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Asset Derivatives	9.5	9.6
Derivative assets, net expected to be released from Other comprehensive income into earnings within the next 12 months:	8.0	9.2
Liability Derivatives	1.0	0.2
Derivative liabilities, net expected to be released from Other comprehensive income into earnings within the next 12 months:	0.0	0.2
Designated as Hedging Instrument \| Foreign currency forwards
Derivatives, Fair Value [Line Items]
Asset Derivatives	9.5	9.6
Derivative assets, net expected to be released from Other comprehensive income into earnings within the next 12 months:	8.0	9.2
Liability Derivatives	1.0	0.0
Designated as Hedging Instrument \| Interest rate contracts
Derivatives, Fair Value [Line Items]
Liability Derivatives	0.0	0.2
Derivative liabilities, net expected to be released from Other comprehensive income into earnings within the next 12 months:	$ 0.0	$ 0.2

Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

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