Credit Facilities - Credit Facilities and Financing Bridge Commitment (Details) - USD ($)	Mar. 31, 2022	Sep. 30, 2021	Dec. 31, 2020
Line of Credit Facility [Line Items]
Maximum borrowing capacity under credit facilities	$ 851,500,000
Loans payable	503,500,000	$ 507,000,000.0
Amount outstanding	515,300,000	385,100,000
Payable to lenders under loans	471,300,000	248,600,000
Notes payable to bank	8,300,000	8,600,000	$ 9,000,000
Senior secured notes	338,200,000	336,900,000
Lenders under loans	974,800,000	755,600,000
StoneX Inc. Bank of America Syndicated Credit Facility
Line of Credit Facility [Line Items]
Long-term debt, gross	165,400,000
Line of credit facility borrowing	236,100,000
Maximum borrowing capacity under credit facilities	401,500,000
Loans payable	165,300,000	170,100,000
Long-term line of credit, noncurrent	25,000,000.0	0
Long-term debt	190,300,000	170,100,000
StoneX Financial Inc.
Line of Credit Facility [Line Items]
Line of credit facility borrowing	75,000,000
Line of credit facility	0	0
Payable to lenders under loans	0	0
FCStone Merchants Services, LLC
Line of Credit Facility [Line Items]
Line of credit facility borrowing	325,000,000
Line of credit facility	300,000,000.0	215,000,000.0
StoneX Financial Ltd.
Line of Credit Facility [Line Items]
Line of credit facility borrowing	50,000,000
Line of credit facility	25,000,000.0	0
IFFI Uncommitted Lines of Credit
Line of Credit Facility [Line Items]
Payable to lenders under loans	68,000,000.0	0
Rabobank Uncommited Line of Credit
Line of Credit Facility [Line Items]
Short-term debt	45,000,000.0	$ 25,000,000.0
HCO Syndicated line of Credit Facility
Line of Credit Facility [Line Items]
Debt instrument, unamortized discount (premium) and debt issuance costs, net	100,000
Senior Notes
Line of Credit Facility [Line Items]
Debt instrument, unamortized discount (premium) and debt issuance costs, net	$ 10,200,000