Credit Facilities - Number of Credit Facilities (Details)	12 Months Ended
	Sep. 30, 2020 USD ($) facility	Jun. 11, 2020 USD ($)	Sep. 30, 2019 USD ($)	Apr. 30, 2015 USD ($)
Line of Credit Facility [Line Items]
Number of credit facilities | facility	4
Maximum borrowing capacity under credit facilities	$ 736,600,000
Term of syndicated loan facility	3 years
Maximum available under credit facility	$ 393,000,000.0
Line of credit facility borrowing	$ 20,000,000.0
Debt instrument, face amount		$ 350,000,000
Term loan periodic principal payment percentage	1.25%
Term loan periodic payment	$ 9,800,000
Long-term debt, gross	180,100,000
Line of credit facility	0		$ 0
Notes payable to bank	$ 0		400,000	$ 4,000,000.0
Debt, weighted average interest rate	2.00%
Rabobank Uncommited Line of Credit [Domain]
Line of Credit Facility [Line Items]
Short-term Debt	$ 20,000,000.0		0
StoneX Inc. Bank of America Syndicated Credit Facility [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity under credit facilities	376,600,000
Line of credit facility borrowing	196,500,000
Debt instrument, face amount	196,500,000
Long-term debt, gross	$ 180,100,000
Line of credit facility, unused capacity, commitment fee percentage	0.625%
Long-term debt	$ 202,500,000		237,600,000
StoneX, Ltd [Member]
Line of Credit Facility [Line Items]
Line of credit facility borrowing	25,000,000.0
Line of credit facility	25,000,000.0		0
StoneX Financial Margin line of credit facility [Member]
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	5.25%
Long-term debt	$ 0		0
Line of credit facility	0		0
FMS Sub-note commodity line of credit facility [Member]
Line of Credit Facility [Line Items]
Line of credit facility borrowing	$ 260,000,000.0
Line of credit facility, unused capacity, commitment fee percentage	0.35%
Line of credit facility	$ 200,100,000		128,500,000
StoneX Financial Inc. Uncommitted Commodities Delivery Line [Domain]
Line of Credit Facility [Line Items]
Line of credit facility borrowing	20,000,000.0
IFL Uncommitted Line of Credit [Member]
Line of Credit Facility [Line Items]
Short-term Debt	$ 0		$ 3,400,000
Base Rate [Member] | StoneX Inc. Bank of America Syndicated Credit Facility [Member]
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate	2.00%
Line of credit facility, interest rate at period end	5.25%
Base Rate [Member] | StoneX, Ltd [Member]
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate	2.50%
Base Rate [Member] | StoneX Financial Margin line of credit facility [Member]
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate	2.00%
Base Rate [Member] | FMS Sub-note commodity line of credit facility [Member]
Line of Credit Facility [Line Items]
Line of credit facility, interest rate at period end	3.25%
Eurodollar [Member] | StoneX Inc. Bank of America Syndicated Credit Facility [Member]
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate	3.00%
Line of credit facility, interest rate at period end	3.10%
Eurodollar [Member] | FMS Sub-note commodity line of credit facility [Member]
Line of Credit Facility [Line Items]
Line of credit facility, interest rate at period end	2.40%