Capital and Other Regulatory Requirements - Regulatory Capital Requirements (Details) $ in Millions	Mar. 31, 2021 USD ($)
Capital and Other Regulatory Requirements [Abstract]
Adjusted net capital	$ 5.0
StoneX Financial Inc.
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Actual	288.4
Minimum Requirement	163.6
StoneX Financial Ltd.
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Actual	375.8
Minimum Requirement	222.4
Gain Capital Group, LLC
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Actual	69.7
Minimum Requirement	31.6
Gain Capital U.K., Ltd.
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Actual	106.6
Minimum Requirement	$ 31.5

Capital and Other Regulatory Requirements [Abstract]

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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
-Publisher SEC
-Name Rule 15c3-1
-Number 240
-Section 15c3-1

+ Details

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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
-Publisher CFTC
-Name Rule 17
-Number 1
-Section 17
-Paragraph (a)
-Subparagraph (5)

+ Details

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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
-Publisher CFTC
-Name Rule 17
-Number 1
-Section 17
-Paragraph (a)
-Subparagraph (1)(i)

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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