Financial Instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk - Fair Values of Derivative Instruments Designated for Hedging Held (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
Mar. 31, 2024	Sep. 30, 2023
Derivatives, Fair Value [Line Items]
Asset Derivatives	$ 7,776.8	$ 8,200.3
Liability Derivatives	7,635.4	8,017.3
Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Asset Derivatives	7.1	1.7
Derivative assets expected to be released from Other comprehensive income into current earnings:	5.6	1.4
Liability Derivatives	5.7	27.1
Derivative liabilities expected to be released from Other comprehensive income into current earnings:	5.7	21.3
Designated as Hedging Instrument \| Foreign currency forward contracts
Derivatives, Fair Value [Line Items]
Asset Derivatives	7.1	1.7
Derivative assets expected to be released from Other comprehensive income into current earnings:	5.6	1.4
Liability Derivatives	0.0	2.5
Derivative liabilities expected to be released from Other comprehensive income into current earnings:	0.0	1.0
Designated as Hedging Instrument \| Interest rate contracts
Derivatives, Fair Value [Line Items]
Asset Derivatives	0.0	0.0
Liability Derivatives	5.7	24.6
Derivative liabilities expected to be released from Other comprehensive income into current earnings:	$ 5.7	$ 20.3

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

No definition available.

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- Definition

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

+ References

No definition available.

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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 210
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 22
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483444/210-20-55-22

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 210
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-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (a)
-Publisher FASB
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-Name Accounting Standards Codification
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-Name Accounting Standards Codification
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-Name Accounting Standards Codification
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-Paragraph 4D
-Subparagraph (c)(2)
-SubTopic 20
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-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481326/860-20-50-4D

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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.

+ References

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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.

+ References

No definition available.

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