Credit Facilities - Number of Credit Facilities (Details)	6 Months Ended
	Mar. 31, 2022 USD ($) facility	Sep. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)
Line of Credit Facility [Line Items]
Number of credit facilities | facility	4
Maximum borrowing capacity under credit facilities	$ 851,500,000
Term loan periodic payment	4,900,000
Lenders under loans	471,300,000	$ 248,600,000
Notes payable to bank	8,300,000	8,600,000	$ 9,000,000
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	$ 401,500,000
Term of syndicated loan facility	3 years
Line of credit facility borrowing	$ 236,100,000
Debt instrument, face amount	$ 196,500,000
Term loan periodic principal payment percentage	1.25%
StoneX Financial Inc.
Line of Credit Facility [Line Items]
Line of credit facility borrowing	$ 75,000,000
Lenders under loans	0	0
FCStone Merchants Services, LLC
Line of Credit Facility [Line Items]
Line of credit facility borrowing	325,000,000
StoneX Financial Ltd.
Line of Credit Facility [Line Items]
Line of credit facility borrowing	50,000,000
StoneX Financial Inc. Uncommitted Commodities Delivery Line
Line of Credit Facility [Line Items]
Lenders under loans	45,000,000	25,000,000
IFFI Uncommitted Lines of Credit
Line of Credit Facility [Line Items]
Line of credit facility borrowing	100,000,000
Lenders under loans	$ 0	$ 0