Financial Instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk - Schedule of Fair Values of Derivative Instruments Designated for Hedging Held (Details) - USD ($) $ in Millions	Jun. 30, 2026	Sep. 30, 2025
Derivatives, Fair Value [Line Items]
Asset Derivatives	$ 8,962.5	$ 7,987.4
Liability Derivatives	9,553.6	8,461.7
Derivative contracts designated as hedging instruments
Derivatives, Fair Value [Line Items]
Asset Derivatives	1.3	9.5
Derivative assets expected to be released from Other comprehensive income into current earnings:	0.9	8.0
Liability Derivatives	21.8	1.0
Derivative liabilities expected to be released from Other comprehensive income into current earnings:	6.7	0.0
Derivative contracts designated as hedging instruments \| Interest rate contracts
Derivatives, Fair Value [Line Items]
Asset Derivatives	0.0	0.0
Liability Derivatives	20.1	0.0
Derivative liabilities expected to be released from Other comprehensive income into current earnings:	5.7	0.0
Derivative contracts designated as hedging instruments \| Foreign currency forward contracts
Derivatives, Fair Value [Line Items]
Asset Derivatives	1.3	9.5
Derivative assets expected to be released from Other comprehensive income into current earnings:	0.9	8.0
Liability Derivatives	1.7	1.0
Derivative liabilities expected to be released from Other comprehensive income into current earnings:	$ 1.0	$ 0.0

Derivative Assets, Hedge Gain (Loss) to be Reclassified within 12 Months

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- Definition

Derivative Liabilities, Hedge Gain (Loss) to be Reclassified within 12 Months

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- Definition

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.

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- Definition

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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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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