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Bank of America Syndicated Credit Facility Schedule of Notional Amounts of Outstanding Derivative Positions Schedule of Notional Amounts of Outstanding Derivative Positions [Table Text Block] Commodities Commodity Contract [Member] JBR Recovery Limited JBR Recovery Limited [Member] JBR Recovery Limited Precious metals sales under ASC Topic 606 Precious Metals Retail Sales [Member] Precious Metals Retail Sales Award Timing Predetermined Award Timing Predetermined [Flag] Intangible assets, gross amount Intangible Assets, Gross (Excluding Goodwill) Indefinite-lived Intangible Assets [Axis] Indefinite-Lived Intangible Assets [Axis] Entity File Number Entity File Number Additional Paid-in Capital Additional Paid-in Capital [Member] Other Expenses Other Income and Other Expense Disclosure [Text Block] Tabular List, Table Tabular List [Table Text Block] Non-contingent consideration earn-out value Business Combination, Consideration Transferred, Non-Contingent Consideration Earn-Out Business Combination, Consideration Transferred, Non-Contingent Consideration Earn-Out Securities loaned Securities loaned Securities loaned Securities loaned, net Securities Loaned OTC equity and indices derivatives OTC Equity And Indices Contract [Member] OTC Equity And Indices Contract [Member] Earnings per share: Earnings Per Share, Basic [Abstract] Intangible Asset, Acquired, Finite-Lived [Line Items] Intangible Asset, Acquired, Finite-Lived [Line Items] Pledged Status [Axis] Pledged Status [Axis] Deferred consideration Business Combination, Consideration Transferred, Liabilities Incurred Corporate and municipal bonds Debt Securities [Member] Goodwill [Line Items] Goodwill [Line Items] Shares withheld to cover taxes on vesting of equity awards Share-Based Payment Arrangement, Decrease for Tax Withholding Obligation Position [Axis] Position [Axis] Trading Arrangements, by Individual Trading Arrangements, by Individual [Table] Other expenses Other Expenses Basic (in dollars per share) Earnings Per Share, Basic Adjustment to Compensation: Adjustment to Compensation [Axis] Named Executive Officers, Footnote Named Executive Officers, Footnote [Text Block] Accounts payable and other accrued liabilities Increase (Decrease) in Accounts Payable and Accrued Liabilities Physical commodities inventory, net (including $397.0 million and $376.6 million at fair value at June 30, 2025 and September 30, 2024, respectively) Physical commodities inventory Inventory, Gross Exchange-traded commodity derivatives Exchange-traded Commodity Contracts [Member] Exchange-traded Commodity Contracts [Member] Revenue from Contracts with Clients Revenue from Contract with Customer [Text Block] Securities purchased under agreements to resell, gross Security Purchased under Agreement to Resell, Subject to Master Netting Arrangement, before Offset Denominator: Denominator Basic [Abstract] Denominator Basic Octo Finances SA Octo Finances SA [Member] Octo Finances SA Schedule of Segment Reporting Information Schedule of Segment Reporting Information, by Segment [Table Text Block] Inventory Disclosure [Abstract] Inventory Disclosure [Abstract] Impact of netting and collateral liability Derivative Liability, Subject to Master Netting Arrangement, Collateral, Right to Reclaim Cash Offset R.J. 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Non-segregated cash and other non-cash equivalent assets included within deposits and receivables from broker-dealers, clearing organizations, and counterparties Non-segregated Cash And Other Non-cash Equivalent Assets Included Within Deposits And Receivables From Broker-dealers, Clearing Organizations, And Counterparties Non-segregated Cash And Other Non-cash Equivalent Assets Included Within Deposits And Receivables From Broker-dealers, Clearing Organizations, And Counterparties Pay vs Performance Disclosure Pay vs Performance Disclosure [Table] Net cash provided by operating activities Cash Provided by (Used in) Operating Activity, Including Discontinued Operation StoneX Financial Ltd. StoneX Financial Ltd. StoneX Financial Ltd. [Member] StoneX Financial Ltd. [Member] Business combination, debt assumed Business Combination, Consideration Transferred, Liabilities Assumed, Debt Business Combination, Consideration Transferred, Liabilities Incurred Debt Equities and fixed income Equities and Fixed Income Commissions [Member] Equities and Fixed Income Commissions [Member] Preferred stock - par value (in dollars per share) Preferred Stock, Par or Stated Value Per Share Subsequent Event Subsequent Event [Member] Total Goodwill Goodwill Goodwill Equity Valuation Assumption Difference, Footnote Equity Valuation Assumption Difference, Footnote [Text Block] Exchange-traded equity index derivatives Equity Contract [Member] PEO Total Compensation Amount PEO Total Compensation Amount Long-term Debt, Type [Axis] Long-Term Debt, Type [Axis] Money market mutual funds Money Market Funds [Member] Repurchase Agreements and Similar Transactions, Maturity Periods [Axis] Repurchase Agreements and Similar Transactions, Maturity Periods [Axis] Fiscal 2025 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[Roll Forward] Increase (Decrease) in Stockholders' Equity [Roll Forward] Non-trading technology and support Information Technology and Data Processing Netting Counterparty And Cash Collateral Netting Adjustment [Member] Counterparty And Cash Collateral Netting Adjustment [Member] Schedule of Underlying Assets of Repurchase Agreements when Amount of Repurchase Agreements Exceeds 10 Percent of Assets Schedule of Underlying Assets of Repurchase Agreements when Amount of Repurchase Agreements Exceeds 10 Percent of Assets [Table Text Block] Cash flow hedges Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), after Reclassification and Tax, Parent Compensation Actually Paid vs. Total Shareholder Return Compensation Actually Paid vs. Total Shareholder Return [Text Block] Accumulated Other Comprehensive Loss, Net Comprehensive Income (Loss) Note [Text Block] Fair Value, Recurring and Nonrecurring [Table] Fair Value, Recurring and Nonrecurring [Table] Gain Capital Group, LLC Gain 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America [Member] Less than 30 Days Maturity Less than 30 Days [Member] ASSETS Assets [Abstract] Additional consideration payable related to acquisition of customer list Noncash Or Part Noncash Acquisition Additional Consideration Payable Related To Acquisition Of Customer List Noncash Or Part Noncash Acquisition Additional Consideration Payable Related To Acquisition Of Customer List Credit Facility [Axis] Credit Facility [Axis] Underlying Security Market Price Change Underlying Security Market Price Change, Percent Fiscal 2028 Finite-Lived Intangible Asset, Expected Amortization, Year Three Balance Sheet Location [Axis] Statement of Financial Position Location, Balance [Axis] Debt Instrument [Axis] Debt Instrument [Axis] Assets and Liabilities, at Fair Value Fair Value Disclosures [Text Block] Business Combination [Line Items] Business Combination [Line Items] Credit Facility [Domain] Credit Facility [Domain] Finite-lived intangible assets, gross amount Finite-Lived Intangible Assets, 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Ltd. StoneX Financial Pte. Ltd. StoneX Financial Pte Ltd [Member] StoneX Financial Pte Ltd Securities purchased under agreements to resell Securities purchased under agreements to resell, net Securities Purchased under Agreements to Resell Total liabilities Liabilities Currency [Axis] Currency [Axis] Cash and Cash Equivalents [Axis] Cash and Cash Equivalents [Axis] Agency mortgage-backed obligations Collateralized Mortgage-Backed Securities [Member] Line of credit facility borrowing Line of Credit Facility, Maximum Borrowing Capacity Client account fees Client Account Fees [Member] Client Account Fees [Member] Common stock, $0.01 par value. Authorized 200,000,000 shares; 54,895,685 issued and 49,029,208 outstanding at June 30, 2025 and 53,678,016 issued and 47,811,539 outstanding at September 30, 2024 Common Stock, Value, Outstanding Pension Adjustments Prior Service Cost Pension Adjustments Prior Service Cost [Member] Securities borrowed Securities borrowed, net Securities Borrowed Indefinite-lived Intangible Assets, Major Class Name [Domain] Indefinite-Lived Intangible Assets, Major Class Name [Domain] Material Terms of Trading Arrangement Material Terms of Trading Arrangement [Text Block] Securities borrowed, gross Security Borrowed, Subject to Master Netting Arrangement, before Offset Deferred payments on acquisitions Payment for Deferred Contingent Consideration Liability, Financing Activities Payment for Deferred Contingent Consideration Liability, Financing Activities Foreign Currency Forward Contracts Foreign Currency Forward Contracts [Member] Foreign Currency Forward Contracts Repayments of payables to lenders under loans with maturities greater than 90 days Repayments of Short-Term Debt, Maturing in More than Three Months Statement [Line Items] Statement [Line Items] Asset-backed obligations Asset-backed obligations Asset-Backed Securities [Member] Consolidation Items [Axis] Consolidation Items [Axis] Rule 10b5-1 Arrangement Adopted Rule 10b5-1 Arrangement Adopted [Flag] Bad debt expense SEC Schedule, 12-09, Valuation Allowances and Reserves, Credit Loss Expense (Reversal) Not Included in Allowance SEC Schedule, 12-09, Valuation Allowances and Reserves, Credit Loss Expense (Reversal) Not Included in Allowance Cash and cash equivalents Cash and Cash Equivalent Common Stock Common Stock [Member] Entity Incorporation, State or Country Code Entity Incorporation, State or Country Code Commission and clearing fees Total commission and clearing fees Commission and Clearing Fees [Member] Commission and Clearing Fees [Member] Non-NEOs Non-NEOs [Member] Securities received as collateral that may be repledged Fair Value of Securities Received as Collateral that Can be Resold or Repledged Net increase in cash, segregated cash, cash equivalents, and segregated cash equivalents Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Period Increase (Decrease), Including Exchange Rate Effect and Discontinued Operation Clearing fees Clearing Service [Member] Securities purchased under agreements to resell, offset Security Purchased under Agreement to Resell, Subject to Master Netting Arrangement, Liability Offset