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Financial Instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk - Narrative (Details) - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Sep. 30, 2023 | Sep. 30, 2022
Derivatives, Fair Value [Line Items] |  |
Financial instruments sold, not yet purchased, at fair value | $ 3,085.6 | $ 2,469.6
Interest rate contracts |  |
Derivatives, Fair Value [Line Items] |  |
Maximum length of time hedged in cash flow hedge (in years) | 1 year |
Foreign currency forwards |  |
Derivatives, Fair Value [Line Items] |  |
Maximum length of time hedged in cash flow hedge (in years) | 2 years |
Fair Value, Measurements, Recurring | Derivatives |  |
Derivatives, Fair Value [Line Items] |  |
Financial instruments sold, not yet purchased, at fair value | $ 261.2 | $ 313.4
Fair Value, Measurements, Recurring | Not designated as hedging instrument | Derivatives |  |
Derivatives, Fair Value [Line Items] |  |
Financial instruments sold, not yet purchased, at fair value | $ 261.2 |

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- Definition Fair value of obligations incurred from short sales. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 940 -SubTopic 320 -Section 35 -Paragraph 1 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481992/940-320-35-1 + Details
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- Definition Maximum length of time over which the entity is hedging its exposure to the variability in future cash flows for forecasted transactions, excluding those forecasted transactions related to the payment of variable interest on existing financial instruments, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (d) -SubTopic 30 -Topic 815 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480870/815-30-50-1 + Details
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