Credit Facilities - Credit Facilities and Financing Bridge Commitment (Details)	Jun. 30, 2021 USD ($) facility	May 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)
Line of Credit Facility [Line Items]
Maximum borrowing capacity under credit facilities	$ 808,800,000
Loans payable	508,700,000			$ 515,500,000
Amount outstanding	412,500,000			427,600,000
Notes payable to bank	8,700,000		$ 9,000,000.0	0
Senior secured notes	336,200,000			336,000,000.0
Lenders under loans	$ 782,400,000			783,600,000
Number of credit facilities set to expire within the next twelve months | facility	3
StoneX Inc. Bank of America Syndicated Credit Facility
Line of Credit Facility [Line Items]
Long-term debt, gross	$ 172,700,000
Line of credit facility borrowing	236,100,000	$ 196,500,000
Maximum borrowing capacity under credit facilities	408,800,000
Loans payable	172,500,000			179,500,000
Long-term line of credit, noncurrent	0			23,000,000.0
Long-term debt	172,500,000			202,500,000
StoneX Financial Inc.
Line of Credit Facility [Line Items]
Line of credit facility borrowing	75,000,000.0
Long-term debt	0			0
Line of credit facility	0			0
FCStone Merchants Services, LLC
Line of Credit Facility [Line Items]
Line of credit facility borrowing	300,000,000.0
Line of credit facility	240,000,000.0			200,100,000
StoneX Financial Ltd.
Line of Credit Facility [Line Items]
Line of credit facility borrowing	25,000,000.0
Line of credit facility	0			25,000,000.0
IFFI Uncommitted Lines of Credit
Line of Credit Facility [Line Items]
Short-term debt	0			0
Rabobank Uncommited Line of Credit
Line of Credit Facility [Line Items]
Short-term debt	25,000,000.0			$ 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	200,000
Senior Notes
Line of Credit Facility [Line Items]
Debt instrument, unamortized discount (premium) and debt issuance costs, net	$ 12,100,000