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Credit Facilities - Schedule of Debt (Details) - USD ($) | 12 Months Ended |  |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Sep. 30, 2025 | Oct. 29, 2025 | Oct. 28, 2025 | Oct. 10, 2025 | Oct. 06, 2025 | Oct. 05, 2025 | Sep. 30, 2024 | Mar. 01, 2024
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Lenders under loans | $ 782,000,000.0 |  |  |  |  |  | $ 338,800,000 |
Uncommitted Credit Facilities | 153,900,000 |  |  |  |  |  | 104,900,000 |
Notes payable to bank | 6,300,000 |  |  |  |  |  | 6,900,000 |
Total outstanding borrowings | $ 1,941,000,000 |  |  |  |  |  | 881,900,000 |
Notes payable to bank |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Basis spread on variable rate (as a percent) | 2.35% |  |  |  |  |  |  |
Senior Secured Notes due 2031 |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Senior notes | $ 544,100,000 |  |  |  |  |  | 543,100,000 |
Senior Secured Notes due 2031 | Senior notes |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Stated interest rate (as a percent) | 7.875% |  |  |  |  |  |  | 7.875%
Unamortized deferred financing costs | $ 10,100,000 |  |  |  |  |  |  |
Senior Secured Notes due 2032 |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Senior notes | $ 614,900,000 |  |  |  |  |  | 0 |
Senior Secured Notes due 2032 | Senior notes |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Stated interest rate (as a percent) | 6.875% |  |  |  |  |  |  |
Unamortized deferred financing costs | $ 5,900,000 |  |  |  |  |  |  |
Revolving Credit Facility |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Line of credit facility borrowing | 1,705,000,000 |  |  |  |  |  |  |
Lenders under loans | 621,800,000 |  |  |  |  |  | 227,000,000.0 |
Revolving Credit Facility | Senior StoneX Group Inc. Committed Credit Facility |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Line of credit facility borrowing | 650,000,000.0 |  |  |  |  |  |  |
Lenders under loans | $ 317,000,000.0 |  |  |  |  |  | 161,000,000.0 |
Revolving Credit Facility | Senior StoneX Group Inc. Committed Credit Facility | Base Rate |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Basis spread on variable rate (as a percent) | 8.25% |  |  |  |  |  |  |
Revolving Credit Facility | Senior StoneX Group Inc. Committed Credit Facility | SOFR |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Basis spread on variable rate (as a percent) | 6.23% |  |  |  |  |  |  |
Revolving Credit Facility | StoneX Financial Inc. |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Stated interest rate (as a percent) | 9.25% |  |  |  |  |  |  |
Line of credit facility borrowing | $ 325,000,000.0 |  |  |  |  |  |  |
Lenders under loans | $ 0 |  |  |  |  |  | 0 |
Revolving Credit Facility | R.J. O’Brien & Associates, LLC subordinated |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Stated interest rate (as a percent) | 8.36% |  |  |  |  |  |  |
Line of credit facility borrowing | $ 180,000,000.0 |  |  |  |  |  |  |
Lenders under loans | $ 110,800,000 |  |  |  |  |  | 0 |
Revolving Credit Facility | R.J. O’Brien Limited |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Stated interest rate (as a percent) | 7.25% |  |  |  |  |  |  |
Line of credit facility borrowing | $ 20,000,000.0 |  |  |  |  |  |  |
Lenders under loans | 0 |  |  |  |  |  | 0 |
Revolving Credit Facility | StoneX Commodity Solutions LLC |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Line of credit facility borrowing | 325,000,000.0 |  |  |  |  |  |  |
Lenders under loans | $ 104,000,000.0 |  |  |  |  |  | 66,000,000.0 |
Revolving Credit Facility | StoneX Commodity Solutions LLC | Subsequent Event |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Line of credit facility borrowing |  | $ 325,000,000 | $ 250,000,000 |  |  |  |  |
Revolving Credit Facility | StoneX Commodity Solutions LLC | Base Rate |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Basis spread on variable rate (as a percent) | 8.50% |  |  |  |  |  |  |
Revolving Credit Facility | StoneX Commodity Solutions LLC | SOFR |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Basis spread on variable rate (as a percent) | 6.64% |  |  |  |  |  |  |
Revolving Credit Facility | Right Company LLC |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Stated interest rate (as a percent) | 6.50% |  |  |  |  |  |  |
Line of credit facility borrowing | $ 15,000,000.0 |  |  |  |  |  |  |
Lenders under loans | $ 0 |  |  |  |  |  | 0 |
Revolving Credit Facility | Right Company LLC | Subsequent Event |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Line of credit facility borrowing |  |  |  | $ 15,000,000 |  |  |  |
Revolving Credit Facility | StoneX Financial Ltd |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Stated interest rate (as a percent) | 6.75% |  |  |  |  |  |  |
Line of credit facility borrowing | $ 175,000,000.0 |  |  |  |  |  |  |
Lenders under loans | $ 90,000,000.0 |  |  |  |  |  | 0 |
Revolving Credit Facility | StoneX Financial Ltd | Subsequent Event |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Line of credit facility borrowing |  |  |  |  | $ 175,000,000 | $ 115,000,000 |  |
Revolving Credit Facility | StoneX Financial Pte. Ltd. |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Stated interest rate (as a percent) | 6.74% |  |  |  |  |  |  |
Line of credit facility borrowing | $ 15,000,000.0 |  |  |  |  |  |  |
Lenders under loans | $ 0 |  |  |  |  |  | $ 0 |

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- Definition The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(22)) -SubTopic 10 -Topic 210 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 942 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(16)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147478546/942-210-S99-1 Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 944 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(16)(a)(2)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147478777/944-210-S99-1 + Details
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