Credit Facilities - Credit Facilities and Financing Bridge Commitment (Details) - USD ($)	Jun. 30, 2022	Sep. 30, 2021	Dec. 31, 2020
Line of Credit Facility [Line Items]
Maximum borrowing capacity under credit facilities	$ 1,000,000,000
Loans payable	338,900,000	$ 507,000,000.0
Amount outstanding	442,000,000.0	385,100,000
Payable to lenders under loans	508,200,000	248,600,000
Notes payable to bank	8,200,000	8,600,000	$ 9,000,000
Senior secured notes	338,900,000	336,900,000
Lenders under loans	847,100,000	755,600,000
Debt instrument, unamortized discount (premium) and debt issuance costs, net	9,500,000
StoneX Inc. Bank of America Syndicated Credit Facility
Line of Credit Facility [Line Items]
Long-term debt, gross	0
Line of credit facility borrowing	475,000,000.0
Maximum borrowing capacity under credit facilities	475,000,000.0
Loans payable	0	170,100,000
Long-term line of credit, noncurrent	170,000,000.0	0
Long-term debt	170,000,000.0	170,100,000
StoneX Financial Inc.
Line of Credit Facility [Line Items]
Line of credit facility borrowing	75,000,000
Line of credit facility	20,000,000.0	0
Payable to lenders under loans	0	0
FCStone Merchants Services, LLC
Line of Credit Facility [Line Items]
Line of credit facility borrowing	400,000,000.0
Line of credit facility	252,000,000.0	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	0	0
IFFI Uncommitted Lines of Credit
Line of Credit Facility [Line Items]
Payable to lenders under loans	13,000,000.0	0
Rabobank Uncommited Line of Credit
Line of Credit Facility [Line Items]
Short-term debt	$ 45,000,000.0	$ 25,000,000.0