Credit Facilities - Credit Facilities and Financing Bridge Commitment (Details) - USD ($)	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	May 31, 2021	Dec. 31, 2020
Line of Credit Facility [Line Items]
Maximum borrowing capacity under credit facilities	$ 828,900,000
Loans payable	505,200,000	$ 507,000,000.0
Amount outstanding	523,700,000	385,100,000
Payable to lenders under loans	489,500,000	248,600,000
Notes payable to bank	8,500,000	8,600,000			$ 9,000,000
Senior secured notes	337,500,000	336,900,000
Lenders under loans	994,700,000	755,600,000
StoneX Inc. Bank of America Syndicated Credit Facility
Line of Credit Facility [Line Items]
Long-term debt, gross	167,800,000
Line of credit facility borrowing			$ 236,100,000	$ 196,500,000
Maximum borrowing capacity under credit facilities	403,900,000
Loans payable	167,700,000	170,100,000
Long-term line of credit, noncurrent	65,000,000.0	0
Long-term debt	232,700,000	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	300,000,000
Line of credit facility	266,000,000.0	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	80,000,000	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	$ 10,900,000