Capital and Other Regulatory Requirements - Regulatory Capital Requirements (Details) $ in Millions	Dec. 31, 2021 USD ($)
Capital and Other Regulatory Requirements [Abstract]
Adjusted net capital	$ 10.0
StoneX Financial Inc.
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Actual	327.3
Minimum Requirement	197.4
StoneX Financial Ltd.
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Actual	389.8
Minimum Requirement	212.4
Gain Capital Group, LLC
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Actual	50.2
Minimum Requirement	29.8
StoneX Markets LLC
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Actual	159.2
Minimum Requirement	$ 100.0

Capital and Other Regulatory Requirements [Abstract]

No definition available.

Name:	intl_CapitalAndOtherRegulatoryRequirementsAbstract
Namespace Prefix:	intl_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- 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

Name:	srt_MinimumNetCapitalRequiredForBrokerDealerSubsidiary1
Namespace Prefix:	srt_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- 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 (c)
-Subparagraph (5)

+ Details

Name:	srt_NetCapitalUnderCommodityExchangeActComputation
Namespace Prefix:	srt_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

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)

+ Details

Name:	srt_RequiredNetCapitalUnderCommodityExchangeAct1
Namespace Prefix:	srt_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- 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

Name:	us-gaap_ComplianceWithRegulatoryCapitalRequirementsUnderBankingRegulationsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	dei_LegalEntityAxis=intl_StoneXFinancialIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=intl_StoneXFinancialLtdMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=intl_GainCapitalGroupLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=intl_StoneXMarketsLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: