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Financial Instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk - Narrative (Details) | 12 Months Ended |  |
--- | --- | --- | ---
Sep. 30, 2022 USD ($) | Sep. 30, 2022 GBP (£) | Sep. 30, 2021 USD ($)
Derivatives, Fair Value [Line Items] |  |  |
Financial instruments sold, not yet purchased, at fair value | $ 2,469,600,000 |  | $ 1,771,200,000
Interest rate swaps |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Derivative, notional amount | $ 1,500,000,000 |  |
Maximum length of time hedged in cash flow hedge | 2 years |  |
Derivative instruments, gain (loss) recognized in income, net | $ (9,700,000) |  |
Foreign currency forwards |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Derivative, notional amount | 207,300,000 | £ 168,000,000 |
Derivative instruments, gain (loss) recognized in income, net | (8,900,000) |  |
Fair Value, Measurements, Recurring | Derivatives |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Financial instruments sold, not yet purchased, at fair value | $ 313,400,000 |  | $ 368,500,000

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- Definition The estimated net amount of existing gains or losses on cash flow hedges at the reporting date expected to be reclassified to earnings within the next 12 months. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (c) -URI https://asc.fasb.org/extlink&oid=121549185&loc=d3e80748-113994 + Details
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- Definition Nominal or face amount used to calculate payment on derivative. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1B -URI https://asc.fasb.org/extlink&oid=125515794&loc=SL5580258-113959 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1A -Subparagraph (d) -URI https://asc.fasb.org/extlink&oid=125515794&loc=SL5579245-113959 + Details
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- Definition Fair value of obligations incurred from short sales. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 940 -SubTopic 320 -Section 35 -Paragraph 1 -Subparagraph (b) -URI https://asc.fasb.org/extlink&oid=35711043&loc=d3e42429-110968 + 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 -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (d) -URI https://asc.fasb.org/extlink&oid=121549185&loc=d3e80748-113994 + Details
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