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Financial Instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk - Schedule of Net Gains/(Losses) from Derivative Contracts (Details) - USD ($) $ in Millions | 12 Months Ended
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Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2023
Financial Instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk [Abstract] |  |  |
Derivative, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration] | Cost of sales of physical commodities, Total revenues | Cost of sales of physical commodities, Total revenues | Cost of sales of physical commodities, Total revenues
Derivative Instruments, Gain (Loss) [Line Items] |  |  |
Net gains from derivative contracts | $ 584.5 | $ 473.1 | $ 897.7
Commodities |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |
Net gains from derivative contracts | 360.5 | 383.8 | 446.5
Foreign exchange |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |
Net gains from derivative contracts | 119.1 | 124.6 | 269.2
Interest rate, equities, and indices |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |
Net gains from derivative contracts | 104.3 | 100.2 | 109.0
To be announced and forward settling securities |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |
Net gains from derivative contracts | $ 0.6 | $ (135.5) | $ 73.0

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- Definition Financial Instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk [Abstract] + References No definition available. + Details
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- Definition Amount of increase (decrease) in the fair value of derivatives recognized in the income statement. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 815 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 4A -Subparagraph (b)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480434/815-10-50-4A Reference 2: http://www.xbrl.org/2003/role/exampleRef -Name Accounting Standards Codification -Section 55 -Paragraph 10 -SubTopic 10 -Topic 825 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482881/825-10-55-10 + Details
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- Definition Indicates line item in statement of income or comprehensive income that includes gain (loss) from derivative. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 815 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 4A -Subparagraph (b)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480434/815-10-50-4A + Details
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