Credit Facilities - Number of Credit Facilities (Details)	12 Months Ended
	Sep. 30, 2021 USD ($) facility	Nov. 18, 2021 USD ($)	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)
Line of Credit Facility [Line Items]
Number of credit facilities | facility	4
Maximum borrowing capacity under credit facilities	$ 806,400,000
Term of syndicated loan facility	3 years
Term loan periodic principal payment percentage	1.25%
Term loan periodic payment	$ 9,800,000
Payable to lenders under loans	248,600,000			$ 268,100,000
Notes payable to bank	8,600,000		$ 9,000,000	0
Debt, weighted average interest rate			2.35%
StoneX Inc. Bank of America Syndicated Credit Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity under credit facilities	406,400,000
Line of credit facility borrowing	236,100,000			196,500,000
Debt instrument, face amount	$ 196,500,000
Line of credit facility, unused capacity, commitment fee percentage	0.625%
Long-term debt	$ 170,100,000			202,500,000
StoneX, Ltd
Line of Credit Facility [Line Items]
Line of credit facility borrowing	25,000,000
StoneX, Ltd | Subsequent Event
Line of Credit Facility [Line Items]
Line of credit facility borrowing		$ 50,000,000
StoneX Financial Margin line of credit facility
Line of Credit Facility [Line Items]
Line of credit facility borrowing	$ 75,000,000.0
Line of credit facility, unused capacity, commitment fee percentage	0.50%
Line of credit facility, interest rate at period end	5.25%
Payable to lenders under loans	$ 0			0
FMS Sub-note commodity line of credit facility
Line of Credit Facility [Line Items]
Line of credit facility borrowing	$ 300,000,000.0
Line of credit facility, unused capacity, commitment fee percentage	0.35%
StoneX Financial Inc. Uncommitted Commodities Delivery Line
Line of Credit Facility [Line Items]
Line of credit facility borrowing	$ 25,000,000
Payable to lenders under loans	25,000,000
IFFI Uncommitted Lines of Credit
Line of Credit Facility [Line Items]
Line of credit facility borrowing	100,000,000
Payable to lenders under loans	$ 0			$ 0
Base Rate | StoneX Inc. Bank of America Syndicated Credit Facility
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate	2.00%
Line of credit facility, interest rate at period end	5.25%
Base Rate | StoneX, Ltd
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate	2.50%
Base Rate | StoneX Financial Margin line of credit facility
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate	2.00%
Base Rate | FMS Sub-note commodity line of credit facility
Line of Credit Facility [Line Items]
Line of credit facility, interest rate at period end	3.25%
Eurodollar | StoneX Inc. Bank of America Syndicated Credit Facility
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate	3.00%
Line of credit facility, interest rate at period end	3.07%
Eurodollar | FMS Sub-note commodity line of credit facility
Line of Credit Facility [Line Items]
Line of credit facility, interest rate at period end	2.30%