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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, 2021 | Mar. 31, 2020 | Mar. 31, 2021 | Mar. 31, 2020
Commodities |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Derivative instruments, gain (loss) recognized in income, net | $ 59.0 | $ 61.6 | $ 63.2 | $ 79.8
Foreign exchange |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Derivative instruments, gain (loss) recognized in income, net | 25.5 | 5.1 | 61.5 | 7.3
Interest rate, equities, and indices |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Derivative instruments, gain (loss) recognized in income, net | 30.6 | 0.0 | 43.2 | (0.4)
TBA and forward settling securities |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Derivative instruments, gain (loss) recognized in income, net | 8.6 | (34.4) | 0.1 | (35.3)
Derivatives |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Derivative instruments, gain (loss) recognized in income, net | $ 123.7 | $ 32.3 | $ 168.0 | $ 51.4

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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 -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4A -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5618551-113959 + 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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