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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
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Dec. 31, 2020 | Dec. 31, 2019
Commodities |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Derivative instruments, gain (loss) recognized in income, net | $ 4.2 | $ 18.2
Foreign exchange |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Derivative instruments, gain (loss) recognized in income, net | 36.0 | 2.2
Interest rate, equities, and indices |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Derivative instruments, gain (loss) recognized in income, net | 12.6 | (0.4)
TBA and forward settling securities |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Derivative instruments, gain (loss) recognized in income, net | (8.5) | (0.9)
Derivatives |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Derivative instruments, gain (loss) recognized in income, net | $ 44.3 | $ 19.1

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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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