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Credit Facilities - Number of Credit Facilities (Details) | 12 Months Ended |  |  |
--- | --- | --- | --- | ---
Sep. 30, 2020 USD ($) facility | Jun. 11, 2020 USD ($) | Sep. 30, 2019 USD ($) | Apr. 30, 2015 USD ($)
Line of Credit Facility [Line Items] |  |  |  |
Number of credit facilities | facility | 4 |  |  |
Maximum borrowing capacity under credit facilities | $ 736,600,000 |  |  |
Term of syndicated loan facility | 3 years |  |  |
Maximum available under credit facility | $ 393,000,000.0 |  |  |
Line of credit facility borrowing | $ 20,000,000.0 |  |  |
Debt instrument, face amount |  | $ 350,000,000 |  |
Term loan periodic principal payment percentage | 1.25% |  |  |
Term loan periodic payment | $ 9,800,000 |  |  |
Long-term debt, gross | 180,100,000 |  |  |
Line of credit facility | 0 |  | $ 0 |
Notes payable to bank | $ 0 |  | 400,000 | $ 4,000,000.0
Debt, weighted average interest rate | 2.00% |  |  |
Rabobank Uncommited Line of Credit [Domain] |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |
Short-term Debt | $ 20,000,000.0 |  | 0 |
StoneX Inc. Bank of America Syndicated Credit Facility [Member] |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |
Maximum borrowing capacity under credit facilities | 376,600,000 |  |  |
Line of credit facility borrowing | 196,500,000 |  |  |
Debt instrument, face amount | 196,500,000 |  |  |
Long-term debt, gross | $ 180,100,000 |  |  |
Line of credit facility, unused capacity, commitment fee percentage | 0.625% |  |  |
Long-term debt | $ 202,500,000 |  | 237,600,000 |
StoneX, Ltd [Member] |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |
Line of credit facility borrowing | 25,000,000.0 |  |  |
Line of credit facility | 25,000,000.0 |  | 0 |
StoneX Financial Margin line of credit facility [Member] |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |
Line of credit facility borrowing | $ 75,000,000.0 |  |  |
Line of credit facility, unused capacity, commitment fee percentage | 0.50% |  |  |
Line of credit facility, interest rate at period end | 5.25% |  |  |
Long-term debt | $ 0 |  | 0 |
Line of credit facility | 0 |  | 0 |
FMS Sub-note commodity line of credit facility [Member] |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |
Line of credit facility borrowing | $ 260,000,000.0 |  |  |
Line of credit facility, unused capacity, commitment fee percentage | 0.35% |  |  |
Line of credit facility | $ 200,100,000 |  | 128,500,000 |
StoneX Financial Inc. Uncommitted Commodities Delivery Line [Domain] |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |
Line of credit facility borrowing | 20,000,000.0 |  |  |
IFL Uncommitted Line of Credit [Member] |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |
Short-term Debt | $ 0 |  | $ 3,400,000 |
Base Rate [Member] | StoneX Inc. Bank of America Syndicated Credit Facility [Member] |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |
Debt instrument, basis spread on variable rate | 2.00% |  |  |
Line of credit facility, interest rate at period end | 5.25% |  |  |
Base Rate [Member] | StoneX, Ltd [Member] |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |
Debt instrument, basis spread on variable rate | 2.50% |  |  |
Base Rate [Member] | StoneX Financial Margin line of credit facility [Member] |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |
Debt instrument, basis spread on variable rate | 2.00% |  |  |
Base Rate [Member] | FMS Sub-note commodity line of credit facility [Member] |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |
Line of credit facility, interest rate at period end | 3.25% |  |  |
Eurodollar [Member] | StoneX Inc. Bank of America Syndicated Credit Facility [Member] |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |
Debt instrument, basis spread on variable rate | 3.00% |  |  |
Line of credit facility, interest rate at period end | 3.10% |  |  |
Eurodollar [Member] | FMS Sub-note commodity line of credit facility [Member] |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |
Line of credit facility, interest rate at period end | 2.40% |  |  |

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- Definition Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=120520924&loc=SL6031897-161870 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(16)) -URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(22)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(16)) -URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878 + Details
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- Definition Period remaining on line of credit facility before it terminates, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
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- Definition Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
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- Definition The carrying value as of the balance sheet date of the current portion 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 -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(13)) -URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(19)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
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