Credit Facilities - Credit Facilities and Financing Bridge Commitment (Details)	Jun. 30, 2023 USD ($) lender	Mar. 31, 2023 USD ($)	Sep. 30, 2022 USD ($)	Dec. 31, 2020 USD ($)
Line of Credit Facility [Line Items]
Maximum borrowing capacity under credit facilities	$ 1,155,000,000
Amount outstanding	392,100,000		$ 477,000,000.0
Uncommitted Credit Facilities	22,900,000		0
Note Payable to Bank	7,600,000		8,100,000	$ 9,000,000
Senior Secured Notes	341,300,000		339,100,000
Total outstanding borrowings	763,900,000		824,200,000
Debt instrument, unamortized discount (premium) and debt issuance costs, net	6,600,000		8,800,000
Senior StoneX Group Inc. Committed Credit Facility - Revolving Line of Credit
Line of Credit Facility [Line Items]
Line of credit facility borrowing	500,000,000	$ 475,000,000
Long-term line of credit, noncurrent	230,000,000.0		260,000,000.0
Senior StoneX Group Inc. Committed Credit Facility - Revolving Line of Credit | Secured Debt
Line of Credit Facility [Line Items]
Line of credit facility borrowing	$ 42,500,000
Number of lender | lender	1
StoneX Financial Inc.
Line of Credit Facility [Line Items]
Line of credit facility borrowing	$ 180,000,000		75,000,000
Line of credit facility	0		0
StoneX Commodity Solutions LLC
Line of Credit Facility [Line Items]
Line of credit facility borrowing	400,000,000.0
Line of credit facility	162,100,000		217,000,000.0
StoneX Financial Ltd.
Line of Credit Facility [Line Items]
Line of credit facility borrowing	75,000,000
Line of credit facility	$ 0		$ 0