Assets and Liabilities, at Fair Value - Narrative (Details) - USD ($) $ in Millions	Jun. 30, 2021	Sep. 30, 2020	Jun. 11, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Accounts payable and other accrued liabilities - contingent liabilities	$ 302.7	$ 272.6
Senior secured borrowings, net	508.7	515.5
Carrying value	$ 336.2	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%		8.625%
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	$ 172.5	179.5
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt, fair value	374.8
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	0.4	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	0.4	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