Credit Facilities - Credit Facilities and Financing Bridge Commitment (Details) - USD ($)	Dec. 31, 2020	Sep. 30, 2020
Line of Credit Facility [Line Items]
Long-term debt, gross	$ 177,600,000
Maximum borrowing capacity under credit facilities	734,100,000
Loans payable	513,800,000	$ 515,500,000
Amount outstanding	531,900,000	427,600,000
Notes payable to bank	9,000,000.0	0
Senior Secured Notes	336,500,000	336,000,000.0
Lenders under loans	887,400,000	783,600,000
StoneX Inc. Bank of America Syndicated Credit Facility
Line of Credit Facility [Line Items]
Long-term debt, gross	177,600,000
Line of credit facility borrowing	196,500,000
Maximum borrowing capacity under credit facilities	374,100,000
Loans payable	177,200,000	179,500,000
Long-term line of credit, noncurrent	121,000,000.0	23,000,000.0
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
Line of credit facility	0	0
FMS Sub-note Commodity Line of Credit Facility
Line of Credit Facility [Line Items]
Line of credit facility borrowing	260,000,000.0
Line of credit facility	233,700,000	200,100,000
StoneX, Ltd
Line of Credit Facility [Line Items]
Line of credit facility borrowing	25,000,000.0
Line of credit facility	0	25,000,000
IFFI Uncommitted Lines of Credit
Line of Credit Facility [Line Items]
Short-term Debt	10,000,000.0	0
Rabobank Uncommited Line of Credit
Line of Credit Facility [Line Items]
Short-term Debt	0	$ 20,000,000.0
HCO Syndicated line of Credit Facility
Line of Credit Facility [Line Items]
Debt instrument, unamortized discount (premium) and debt issuance costs, net	400,000
Senior Notes
Line of Credit Facility [Line Items]
Debt instrument, unamortized discount (premium) and debt issuance costs, net	$ 13,400,000