Credit Facilities - Credit Facilities (Details)	12 Months Ended
	Sep. 30, 2022 USD ($) facility	Jul. 28, 2022 USD ($)	Jul. 27, 2022 USD ($)	Mar. 31, 2022 USD ($)	Sep. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)
Line of Credit Facility [Line Items]
Payable to lenders under loans	$ 485,100,000				$ 248,600,000
Notes payable to bank	$ 8,100,000				8,600,000	$ 9,000,000
Debt, weighted average interest rate						2.35%
Revolving Credit Facility
Line of Credit Facility [Line Items]
Number of credit facilities | facility	4
Maximum borrowing capacity under credit facilities	$ 1,000,000,000
Number of credit facilities scheduled to expire within one year | facility	1
Revolving Credit Facility | StoneX Inc. Bank of America Syndicated Credit Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity under credit facilities	$ 475,000,000.0
Term of syndicated loan facility	3 years
Line of credit facility borrowing	$ 475,000,000.0			$ 236,100,000
Line of credit facility, unused capacity, commitment fee percentage	0.625%
Revolving Credit Facility | StoneX Inc. Bank of America Syndicated Credit Facility | Secured Overnight Financing Rate (SOFR) Rate
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate	3.00%
Line of credit facility, interest rate at period end	6.15%
Revolving Credit Facility | StoneX Inc. Bank of America Syndicated Credit Facility | Base Rate
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate	2.00%
Line of credit facility, interest rate at period end	8.25%
Revolving Credit Facility | StoneX Inc. Bank of America Syndicated Credit Facility, Extended Maturity
Line of Credit Facility [Line Items]
Line of credit facility borrowing				475,000,000
Revolving Credit Facility | 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	5.75%
Payable to lenders under loans	$ 0				0
Revolving Credit Facility | StoneX Financial Margin line of credit facility | Base Rate
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate	2.00%
Revolving Credit Facility | FMS Sub-note commodity line of credit facility
Line of Credit Facility [Line Items]
Line of credit facility borrowing	$ 400,000,000.0	$ 400,000,000	$ 325,000,000
Line of credit facility, unused capacity, commitment fee percentage	0.35%
Revolving Credit Facility | FMS Sub-note commodity line of credit facility | Secured Overnight Financing Rate (SOFR) Rate
Line of Credit Facility [Line Items]
Line of credit facility, interest rate at period end	5.41%
Revolving Credit Facility | FMS Sub-note commodity line of credit facility | Base Rate
Line of Credit Facility [Line Items]
Line of credit facility, interest rate at period end	6.25%
Revolving Credit Facility | StoneX Financial Ltd.
Line of Credit Facility [Line Items]
Line of credit facility borrowing	$ 50,000,000
Revolving Credit Facility | StoneX Financial Ltd. | Base Rate
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate	2.50%
Revolving Credit Facility | StoneX Financial Inc. Uncommitted Commodities Delivery Line
Line of Credit Facility [Line Items]
Line of credit facility borrowing	$ 45,000,000
Payable to lenders under loans	0				25,000,000
Revolving Credit Facility | 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
Revolving Credit Facility | Term Loan A Facility | StoneX Inc. Bank of America Syndicated Credit Facility
Line of Credit Facility [Line Items]
Line of credit facility borrowing				$ 165,400,000
Term loan periodic payment, percent	1.25%
Term loan periodic payment	$ 4,900,000