Regulatory Requirements and Subsidiary Dividend Restrictions - Narrative (Details)	12 Months Ended
Sep. 30, 2025 USD ($)
Capital and Other Regulatory Requirements [Line Items]
Adjusted net capital	$ 10,000,000
StoneX Financial Inc.
Capital and Other Regulatory Requirements [Line Items]
Adjusted net capital	$ 1,500,000
Adjusted net capital of percentage (as a percent)	8.00%
Broker-dealer, minimum net capital required, dividend withdrawal threshold, percent (as a percent)	120.00%
Net capital under commodity exchange act computation	$ 519,100,000
Required net capital under commodity exchange act	362,800,000
GAIN Capital Group, LLC
Capital and Other Regulatory Requirements [Line Items]
Net capital under commodity exchange act computation	65,800,000
Required net capital under commodity exchange act	30,400,000
GAIN Capital Group, LLC \| Customer and Non-Customer Risk Maintenance Margin
Capital and Other Regulatory Requirements [Line Items]
Adjusted net capital	$ 1,000,000
Adjusted net capital of percentage (as a percent)	8.00%
GAIN Capital Group, LLC \| Retail Customer Liabilities
Capital and Other Regulatory Requirements [Line Items]
Net capital under commodity exchange act computation	$ 20,000,000
Percentage of net capital under commodity exchange act computation (as a percent)	5.00%
GAIN Capital Group, LLC \| Liabilities Owed to Counterparties
Capital and Other Regulatory Requirements [Line Items]
Required net capital under commodity exchange act	$ 10,000,000
Percentage of required net capital under commodity exchange act (as a percent)	10.00%
R.J. O’Brien & Associates LLC
Capital and Other Regulatory Requirements [Line Items]
Net capital under commodity exchange act computation	$ 367,400,000
Required net capital under commodity exchange act	275,800,000
R.J. O’Brien & Associates LLC \| Customer and Non-Customer Risk Maintenance Margin
Capital and Other Regulatory Requirements [Line Items]
Adjusted net capital	$ 20,000,000
R.J. O’Brien & Associates LLC \| Customer Risk Maintenance Margin
Capital and Other Regulatory Requirements [Line Items]
Adjusted net capital of percentage (as a percent)	8.00%
R.J. O’Brien & Associates LLC \| Non-customer Risk Maintenance Margin
Capital and Other Regulatory Requirements [Line Items]
Adjusted net capital of percentage (as a percent)	8.00%
StoneX Markets LLC
Capital and Other Regulatory Requirements [Line Items]
Net capital under commodity exchange act computation	$ 252,500,000
Required net capital under commodity exchange act	154,400,000
Minimum regulatory capital requirement	$ 20,000,000

Broker-Dealer, Minimum Net Capital Required, Broker-Dealer Subsidiary, Aggregate Indebtedness Standard, Percentage

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Broker-Dealer, Minimum Net Capital Required, Parent, Dividend Withdrawal Threshold, Percent

+ References

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Capital and Other Regulatory Requirements [Line Items]

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- Definition

Futures Commission Merchant, Commodity Exchange Act, Adjusted Net Capital, Percent Of Retail Customer Liabilities Over Threshold

+ References

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Futures Commission Merchant, Commodity Exchange Act, Required Net Capital, Percent Of Liabilities Owed to Counterparties

+ References

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- Definition

Minimum amount of net capital required to be maintained by broker-dealer subsidiary to engage in security transaction, calculated under Aggregate Indebtedness Standard.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name Rule 15c3-1
-Number 240
-Section 15c3-1
-Publisher SEC

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- Definition

Amount of adjusted net capital of futures commission merchant (FCM) per Commodity Futures Trading Commission's (CFTC) requirement.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name CFTC Regulation
-Number 1
-Section 17
-Paragraph c
-Subparagraph (5)
-Publisher CFTC

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Amount of net capital required to be maintained by futures commission merchant (FCM) per Commodity Futures Trading Commission's (CFTC) requirement.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name CFTC Regulation
-Number 1
-Section 17
-Paragraph a
-Subparagraph (1)(i)
-Publisher CFTC

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Amount of minimum total risk-based capital required for capital adequacy as defined by regulatory framework.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 942
-SubTopic 505
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (c)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477270/942-505-50-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 942
-SubTopic 505
-Name Accounting Standards Codification
-Section 50
-Paragraph 1B
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477270/942-505-50-1B

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