Credit Facilities - Number of Credit Facilities (Details)	3 Months Ended	12 Months Ended
	Dec. 31, 2020 USD ($) facility	Sep. 30, 2020 USD ($)
Line of Credit Facility [Line Items]
Number of credit facilities | facility	4
Maximum borrowing capacity under credit facilities	$ 734,100,000
Debt instrument, face amount		$ 196,500,000
Term loan periodic principal payment percentage		1.25%
Term loan periodic payment	2,400,000
Long-term debt, gross	177,600,000
Notes payable to bank	$ 9,000,000.0	$ 0
Debt, weighted average interest rate	2.35%
Lenders under loans	$ 373,600,000	268,100,000
StoneX Inc. Bank of America Syndicated Credit Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity under credit facilities	374,100,000
Line of credit facility borrowing	196,500,000
Long-term debt, gross	177,600,000
Long-term debt	298,200,000	202,500,000
FCS Margin Line of Credit Facility
Line of Credit Facility [Line Items]
Line of credit facility borrowing	75,000,000.0
FMS Sub-note Commodity Line of Credit Facility
Line of Credit Facility [Line Items]
Line of credit facility borrowing	260,000,000.0
StoneX, 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	75,000,000.0
Long-term debt	0	0
Lenders under loans		20,000,000.0
IFFI Uncommitted Lines of Credit
Line of Credit Facility [Line Items]
Short-term Debt	10,000,000.0	0
INTL FCStone Ltd
Line of Credit Facility [Line Items]
Line of credit facility borrowing		$ 100,000,000.0
Main Line of Credit Facilities
Line of Credit Facility [Line Items]
Credit facilities expiring	$ 75,000,000.0