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Financial Instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk - Realized Gains/Losses on Derivative Contracts (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
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Mar. 31, 2025 | Mar. 31, 2024 | Mar. 31, 2025 | Mar. 31, 2024
Commodities |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Derivative instruments, gain (loss) recognized in income, net | $ 131.2 | $ 30.4 | $ 186.0 | $ 117.5
Foreign exchange |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Derivative instruments, gain (loss) recognized in income, net | 21.7 | 32.6 | 67.1 | 57.8
Interest rate, equities, and indices |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Derivative instruments, gain (loss) recognized in income, net | 7.6 | 15.5 | 49.9 | 42.9
To be announced and forward settling securities |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Derivative instruments, gain (loss) recognized in income, net | (73.2) | 25.5 | 18.8 | (67.7)
Derivatives |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Derivative instruments, gain (loss) recognized in income, net | $ 87.3 | $ 104.0 | $ 321.8 | $ 150.5

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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 + 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 Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 815 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 4E -Publisher FASB -URI https://asc.fasb.org/1943274/2147480434/815-10-50-4E + Details
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