Basis of Presentation and Consolidation and Recently Issued Accounting Standards (Policies)	6 Months Ended
	Mar. 31, 2019
Basis of Presentation and Consolidation [Abstract]
Basis of Accounting, Policy [Policy Text Block]	The accompanying unaudited condensed consolidated balance sheet as of September 30, 2018, which has been derived from the audited consolidated balance sheet of September 30, 2018, and the unaudited interim condensed consolidated financial statements have been prepared pursuant to the rules and regulations of the Securities and Exchange Commission (“SEC”). Certain information and disclosures normally included in annual consolidated financial statements prepared in accordance with accounting principles generally accepted in the United States of America (“U.S. GAAP”) have been condensed or omitted pursuant to those rules and regulations. The Company believes that the disclosures made are adequate to make the information presented not misleading. In the opinion of management, all adjustments (consisting of normal recurring accruals) considered necessary for a fair presentation of the condensed consolidated financial statements for the interim periods presented have been reflected as required by Rule 10-01 of Regulation S-X. Operating results for interim periods are not necessarily indicative of the results that may be expected for the full year. These interim condensed consolidated financial statements should be read in conjunction with the Company’s audited consolidated financial statements and related notes contained in the Company’s Form 10-K for the fiscal year ended September 30, 2018 filed with the SEC.
Consolidation, Policy [Policy Text Block]	These condensed consolidated financial statements include the accounts of INTL FCStone Inc. and all other entities in which the Company has a controlling financial interest. All material intercompany transactions and balances have been eliminated in consolidation.
Fiscal Period, Policy [Policy Text Block]	The Company’s fiscal year end is September 30, and the fiscal quarters end on December 31, March 31, June 30 and September 30. Unless otherwise stated, all dates refer to fiscal years and fiscal interim periods.
Use of Estimates, Policy [Policy Text Block]	The preparation of condensed consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclosure of contingent liabilities as of the date of the condensed consolidated financial statements and the reported amounts of revenue and expenses during the reporting period. The most significant of these estimates and assumptions relate to fair value measurement for financial instruments and investments, revenue recognition, the provision for bad debts, valuation of inventories, valuation of goodwill and intangible assets, incomes taxes, and contingencies. Although these and other estimates and assumptions are based on the best available information, actual results could be materially different from these estimates.
Reclassification, Policy [Policy Text Block]	Reclassifications During the three and six months ended March 31, 2019, the Company reclassified certain brokerage related revenues for which the Company earns commissions on trading activity in the capacity of an agent. In performing this reclassification, the Company has made a retrospective adjustment to the condensed consolidated income statements for the three and six months ended March 31, 2018. For the three and six months ended March 31, 2018, brokerage related revenues of $9.2 million and $17.9 million, respectively, were reclassified from ‘trading gains, net’ to ‘commissions and clearing fees’. Additionally, the Company has renamed the line item ‘trading gains, net’ to ‘principal gains, net’ on the condensed consolidated income statements in order to reflect the fact that these revenue streams are earned from trading financial instruments in the capacity of a principal and in order to properly segregate revenues earned from contracts with clients in connection with the adoption of the new revenue standard as discussed below.
New Accounting Pronouncement or Change in Accounting Principle, Description	Accounting Standards Adopted On October 1, 2018, the Company adopted Financial Accounting Standards Board (“FASB”) Accounting Standards Codification (“ASC”) 606, Revenue from Contracts with Customers (“Topic 606”) using the modified retrospective transition method applied to those contracts which were not completed as of October 1, 2018. Results for reporting periods beginning after October 1, 2018, are presented under Topic 606, and amounts prior to October 1, 2018 are not adjusted and continue to be reported in accordance with historical accounting standard, FASB ASC 605, Revenue Recognition (“Topic 605”). The adoption of Topic 606 had no impact to retained earnings as of October 1, 2018, or to revenue for the three months ended December 31, 2018. The Company’s accounting for revenues within the scope of Topic 606 are materially consistent with those accounting principles and practices applied to accounting for revenues under Topic 605. The new revenue recognition model does not apply to revenues associated with financial instruments or contracts, including derivatives and interest income. For further information refer to Note 2. In August 2016, the FASB issued ASU 2016-15, Statements of Cash Flows (“Topic 230”): Classification of Certain Cash Receipts and Cash Payments, which clarifies how companies present and classify certain cash receipts and cash payments in the statement of cash flows. The Company adopted the provisions of this guidance on October 1, 2018 and the adoption had no impact on its condensed consolidated financial statements. On October 1, 2018, the Company adopted FASB Accounting Standards Update (“ASU”) 2016-18, Statements of Cash Flows (“Topic 230”): Classification and Presentation of Restricted Cash in the Statements of Cash Flows, using the retrospective transition method. In accordance with the provisions of ASU 2016-18, the Company changed its condensed consolidated statements of cash flows presentation convention to explain the changes in cash and cash equivalents, as well as cash and cash equivalents segregated for regulatory purposes. U.S. Treasury obligations with original or acquired maturities of 90 days or less held with third-party banks or pledged with exchange-clearing organizations representing investments of segregated client funds, or which are held for particular clients in lieu of cash margin, are included in segregated cash equivalents. Purchases, sales, as well as client pledges and redemptions in lieu of cash margin, of U.S. Treasury obligations with original or acquired maturities of greater than 90 days representing investments of segregated client funds are presented as operating uses and sources of cash, respectively, within the operating section of the condensed consolidated statements of cash flows. In May 2017, the FASB issued ASU No. 2017- 09, Scope of Modification Accounting (“Topic 718”), which amends the scope of modification accounting for share- based payment arrangements. ASU 2017- 09 provides guidance on the types of changes to the terms or conditions of share- based payment awards to which an entity would be required to apply modification accounting under ASC 718. Specifically, an entity would not apply modification accounting if the fair value, vesting conditions, and classification of the awards are the same immediately before and after the modification. The Company adopted ASU 2017- 09 on October 1, 2018. The adoption of the ASU had no impact on the Company's condensed consolidated financial statements and related disclosures.