Credit Facilities - Number of Credit Facilities (Details)	9 Months Ended
	Jun. 30, 2022 USD ($) facility	Jul. 28, 2022 USD ($)	Jul. 27, 2022 USD ($)	Apr. 21, 2022 USD ($)	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	$ 1,000,000,000
Lenders under loans	508,200,000				$ 248,600,000
Notes payable to bank	8,200,000				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	$ 475,000,000.0
Term of syndicated loan facility	3 years
Line of credit facility borrowing	$ 475,000,000.0
Term loan periodic principal payment percentage	1.25%
Term loan periodic payment	$ 4,900,000
Revolving Credit Facility
Line of Credit Facility [Line Items]
Line of credit facility borrowing				$ 475,000,000
Revolving Credit Facility | Line of Credit
Line of Credit Facility [Line Items]
Line of credit facility borrowing				236,000,000
Revolving Credit Facility | Secured Debt
Line of Credit Facility [Line Items]
Line of credit facility borrowing				$ 165,000,000
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	400,000,000.0
FCStone Merchants Services, LLC | Subsequent Event
Line of Credit Facility [Line Items]
Line of credit facility borrowing		$ 400,000,000	$ 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