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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 | 12 Months Ended
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Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2018
Commodities [Member] |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |
Derivative instruments, gain (loss) recognized in income, net | $ 197.3 | $ 100.8 | $ 94.0
Foreign Exchange [Member] |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |
Derivative instruments, gain (loss) recognized in income, net | 38.2 | 8.1 | 9.2
Interest Rate, Equities, and Indices [Member] |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |
Derivative instruments, gain (loss) recognized in income, net | 20.4 | (2.6) | 1.0
TBA and Forward Settling Securities [Member] |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |
Derivative instruments, gain (loss) recognized in income, net | (49.7) | (35.3) | 14.5
Derivative [Member] |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |
Derivative instruments, gain (loss) recognized in income, net | $ 206.2 | $ 71.0 | $ 118.7

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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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