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Financial Instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk - Narrative (Details) - USD ($) $ in Millions | Dec. 31, 2020 | Sep. 30, 2020
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Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items] |  |
Financial instruments sold, not yet purchased | $ 827.4 | $ 686.0
Fair Value, Recurring | Derivatives |  |
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items] |  |
Financial instruments sold, not yet purchased | $ 239.8 | $ 176.8

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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 http://asc.fasb.org/extlink&oid=35711043&loc=d3e42429-110968 + Details
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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. + Details
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- Details
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- Details
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