Credit Facilities - Number of Credit Facilities (Details)	9 Months Ended
	Jun. 30, 2021 USD ($) facility	May 31, 2021 USD ($)	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	$ 808,800,000
Debt instrument, face amount	$ 196,500,000
Term loan periodic principal payment percentage	1.25%
Term loan periodic payment	$ 7,400,000
Lenders under loans	273,700,000			$ 268,100,000
Notes payable to bank	8,700,000		$ 9,000,000.0	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	408,800,000
Line of credit facility borrowing	236,100,000	$ 196,500,000
Long-term debt	172,500,000			202,500,000
StoneX Financial Inc.
Line of Credit Facility [Line Items]
Line of credit facility borrowing	75,000,000.0
Long-term debt	0			0
FCStone Merchants Services, LLC
Line of Credit Facility [Line Items]
Line of credit facility borrowing	300,000,000.0
StoneX Financial Ltd.
Line of Credit Facility [Line Items]
Line of credit facility borrowing	25,000,000.0
StoneX Financial Inc. Uncommitted Commodities Delivery Line
Line of Credit Facility [Line Items]
Line of credit facility borrowing	25,000,000.0
INTL FCStone Ltd
Line of Credit Facility [Line Items]
Line of credit facility borrowing	100,000,000.0
Lenders under loans	$ 0			$ 0