Regulatory Requirements and Subsidiary Dividend Restrictions - Narrative (Details)	12 Months Ended
Sep. 30, 2020 USD ($)
Capital and Other Regulatory Requirements [Line Items]
Adjusted net capital	$ 5,000,000.0
Net capital under commodity exchange act computation	20,000,000.0
Required net capital under commodity exchange act	10,000,000.0
StoneX Financial [Member]
Capital and Other Regulatory Requirements [Line Items]
Adjusted net capital	1,000,000
Adjusted net capital of percentage	$ 0.08
Broker-dealer, minimum net capital required, dividend withdrawal threshold, percent	120.00%
Gain Capital Group, LLC [Member]
Capital and Other Regulatory Requirements [Line Items]
Adjusted net capital	$ 1,000,000.0
Adjusted net capital of percentage	0.08
Net capital under commodity exchange act computation	$ 71,500,000
Percentage of net capital under commodity exchange act computation	5.00%
Required net capital under commodity exchange act	$ 32,200,000
Percentage of required net capital under commodity exchange act	10.00%

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

No definition available.

Name:	intl_BrokerDealerMinimumNetCapitalRequiredParentDividendWithdrawalThresholdPercent
Namespace Prefix:	intl_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Capital and Other Regulatory Requirements [Line Items]

+ References

No definition available.

+ Details

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

- Definition

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

+ References

No definition available.

+ Details

Name:	intl_FuturesCommissionMerchantCommodityExchangeActAdjustedNetCapitalPercentOfRetailCustomerLiabilitiesOverThreshold
Namespace Prefix:	intl_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

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

+ References

No definition available.

+ Details

Name:	intl_FuturesCommissionMerchantCommodityExchangeActRequiredNetCapitalPercentOfLiabilitiesOwedToCounterparties
Namespace Prefix:	intl_
Data Type:	num:percentItemType
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

Minimum amount of net capital required to be maintained by broker-dealer parent, 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_MinimumNetCapitalRequiredForEntity1
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 (a)
-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

- Details

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

- Details

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