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Assets and Liabilities, at Fair Value - Narrative (Details) - USD ($) $ in Millions | Dec. 31, 2020 | Sep. 30, 2020
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Accounts payable and other accrued liabilities - contingent liabilities | $ 244.5 | $ 272.6
Senior secured borrowings, net | 513.8 | 515.5
Carrying value | $ 336.5 | 336.0
Senior Secured Notes due 2025 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt instrument, interest rate, stated percentage | 8.625% |
Senior Notes due 2022 | Gain Capital Holdings Inc |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt instrument, interest rate, stated percentage | 5.00% |
StoneX Inc. Bank of America Syndicated Credit Facility |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Senior secured borrowings, net | $ 177.2 | 179.5
Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Long-term debt, fair value | 382.3 |
Fair Value, Recurring | Contingent Consideration |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Accounts payable and other accrued liabilities - contingent liabilities | 1.3 | 1.5
Fair Value, Recurring | Level 3 | Contingent Consideration |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Accounts payable and other accrued liabilities - contingent liabilities | 1.3 | 1.5
Fair Value, Recurring | Level 2 | Contingent Consideration |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Accounts payable and other accrued liabilities - contingent liabilities | $ 0.0 | $ 0.0

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