Commitments and Contingencies	12 Months Ended
Commitments and Contingencies	Sep. 30, 2020
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies Legal and Regulatory Proceedings Certain conditions may exist as of the date the financial statements are issued, which may result in a loss to the Company but which will only be resolved when one or more future events occur or fail to occur. The Company assesses such contingent liabilities, and such assessment inherently involves an exercise of judgment. In assessing loss contingencies related to legal and regulatory proceedings that are pending against the Company or unasserted claims that may result in such proceedings, the Company’s legal counsel evaluates the perceived merits of any legal or regulatory proceedings or unasserted claims as well as the perceived merits of the amount of relief sought or expected to be sought therein. If the assessment of a contingency indicates that it is probable that a material loss had been incurred at the date of the financial statements and the amount of the liability can be estimated, then the estimated liability would be accrued in the Company’s financial statements. If the assessment indicates that a potentially material loss contingency is not probable, but is reasonably possible, or is probable but cannot be estimated, then the nature of the contingent liability, together with an estimate of the range of possible loss if determinable and material, would be disclosed. Neither accrual nor disclosure is required for loss contingencies that are deemed remote. The Company accrues legal fees related to contingent liabilities as they are incurred. From time to time and in the ordinary course of business, the Company is involved in various legal actions and proceedings, including tort claims, contractual disputes, employment matters, workers’ compensation claims and collections. The Company carries insurance that provides protection against certain types of claims, up to the limits of the respective policy. Additionally, the Company is subject to extensive regulation and supervision by U.S. federal and international governmental agencies and various self-regulatory organizations. The Company and its advisors periodically engage with such regulatory agencies and organizations, in the context of examinations or otherwise, to respond to inquiries, informational requests, and investigations. From time to time, such engagements result in regulatory complaints or other matters, the resolution of which can include fines and other remediation. As of September 30, 2020 and 2019, the consolidated balance sheets include loss contingency accruals, recorded during and prior to these fiscal years then ended, which are not material, individually or in the aggregate, to the Company’s financial position or liquidity. In the opinion of management, possible exposure from loss contingencies in excess of the amounts accrued, and in addition to the possible losses discussed below, is not material to the Company’s earnings, financial position or liquidity. The following is a summary of a significant legal matter involving the Company OptionSellers During the week ended November 16, 2018, balances in approximately 300 accounts of the FCM division of the Company’s wholly owned subsidiary, StoneX Financial Inc., declined below required maintenance margin levels, primarily as a result of significant and unexpected price fluctuations in the natural gas markets. All positions in these accounts, which were managed by OptionSellers.com Inc. (“OptionSellers”), an independent Commodity Trading Advisor (“CTA”), were liquidated in accordance with the StoneX Financial Inc.’s client agreements and obligations under market regulation standards. A CTA is registered with the U.S. Commodity Futures Trading Commission (“CFTC”) and a member of, and subject to audit by, the National Futures Association (“NFA”). OptionSellers is registered under a CFTC Rule 4.7 exemption for “qualified eligible persons,” which requires the account holders authorizing OptionSellers to act as their CTA to meet or exceed certain minimum financial requirements. OptionSellers, in its role as a CTA, had been granted by each of its clients full discretionary authority to manage the trading in the client accounts, while StoneX Financial Inc. acted solely as the clearing firm in its role as the FCM. StoneX Financial Inc.’s client agreements hold account holders liable for all losses in their accounts and obligate the account holders to reimburse StoneX Financial Inc. for any account deficits in their accounts. As of September 30, 2019, the aggregate receivable from these client accounts, net of collections and other allowable deductions, was $29.0 million, with no individual account receivable exceeding $1.4 million. StoneX Financial Inc. continues to pursue collection of these receivables and intends both to enforce and to defend its rights aggressively, and to claim interest and costs of collection where applicable. StoneX Financial Inc. has been named in arbitrations brought by clients seeking damages relating to the trading losses in these accounts. The Company believes that such cases are without merit and intend to defend them vigorously. At the same time, the Company has initiated numerous arbitration proceedings against clients to recover deficit balances in their accounts. The Company believes it has a valid claim against its clients, based on the express language of the client contracts and legal precedent, and intends to pursue collection of these claims vigorously. The Company has done an assessment of the collectability of these accounts, considered the status of arbitration proceedings, and has concluded that it does not have a sufficient basis to record an allowance against these uncollected balances. As the Company moves through the collection and arbitration processes and additional information becomes available, the Company will continue to consider the need for an allowance against the carrying value of these uncollected balances. Depending on future collections and arbitration proceedings, any provisions for bad debts and actual losses ultimately may or may not be material to the Company’s financial results. Currently, the Company does not believe that any potential losses related to this matter would impact its ability to comply with its ongoing liquidity, capital, and regulatory requirements. Contractual Commitments Purchase Commitments The Company determines an estimate of contractual purchase commitments in the ordinary course of business primarily for the purchase of precious metals and agricultural and energy commodities. Unpriced contract commitments have been estimated using September 30, 2020 fair values. The purchase commitments and other obligations as of September 30, 2020 for less than one year, one to three years, three to five years, and after five years were $5,230.9 million, $19.2 million, $17.2 million, and $3.2 million respectively. The purchase commitments for less than one year will be offset by corresponding sales commitments of $5,162.4 million. Exchange Member Guarantees The Company is a member of various exchanges that trade and clear futures and option contracts. In connection with the Sterne acquisition, the Company is also a member of and provides guarantees to securities clearinghouses and exchanges in connection with client trading activities. Associated with its memberships, the Company may be required to pay a proportionate share of the financial obligations of another member who may default on its obligations to the exchanges. While the rules governing different exchange memberships vary, in general the Company’s guarantee obligations would arise only if the exchange had previously exhausted its resources. In addition, any such guarantee obligation would be apportioned among the other non-defaulting members of the exchange. Any potential contingent liability under these arrangements is not quantifiable and could exceed the cash and securities it has posted to the clearinghouse as collateral. The Company has not recorded any contingent liability in the consolidated financial statements for these agreements and believes that any potential requirement to make payments under these agreements is remote. Self-Insurance The Company self-insures its costs related to medical and dental claims. The Company is self-insured, up to a stop loss amount, for eligible participating employees and retirees, and for qualified dependent medical and dental claims, subject to deductibles and limitations. Liabilities are recognized based on claims filed and an estimate of claims incurred but not reported. The Company has purchased stop-loss coverage to limit its exposure on a per claim basis and in aggregate in the event that aggregated actual claims would exceed 120% of actuarially estimated claims. The Company is insured for covered costs in excess of these limits. Although the ultimate outcome of these matters may exceed the amounts recorded and additional losses may be incurred, the Company does not believe that any additional potential exposure for such liabilities will have a material adverse effect on the Company’s consolidated financial position or results of operations. As of September 30, 2020 and September 30, 2019, the Company had $1.1 million and $0.8 million, respectively, accrued for self-insured medical and dental claims included in ‘accounts payable and other liabilities’ in the consolidated balance sheets.