Financial Instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk - TBAs and Forward Settling Securities (Details) - USD ($) $ in Millions	Sep. 30, 2021	Sep. 30, 2020
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	$ 5,760.7	$ 3,264.4
Derivative liability, fair value, gross liability	5,628.1	3,069.5
TBA Securities Purchased, Unrealized Gain \| Deposits and Receivables from Broker-dealers, Clearing Organizations and Counterparties
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	1.6	10.8
Derivative asset, notional amount	1,453.4	5,389.3
TBA Securities Purchased, Unrealized Loss \| Deposits and Receivables from Broker-dealers, Clearing Organizations and Counterparties
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	0.0	1.7
Derivative asset, notional amount	0.0	2,647.7
TBA Securities Purchased, Unrealized Loss \| Payables to Broker-dealers, Clearing Organizations and Counterparties
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	37.6	0.0
Derivative asset, notional amount	7,024.2	0.0
TBA Securities Sold, Unrealized Gain \| Deposits and Receivables from Broker-dealers, Clearing Organizations and Counterparties
Derivatives, Fair Value [Line Items]
Derivative liability, fair value, gross liability	43.0	0.0
Derivative liability, notional amount	(8,391.4)	0.0
TBA Securities Sold, Unrealized Gain \| Payables to Broker-dealers, Clearing Organizations and Counterparties
Derivatives, Fair Value [Line Items]
Derivative liability, fair value, gross liability	0.0	2.8
Derivative liability, notional amount	0.0	(2,978.7)
TBA Securities Sold, Unrealized Loss \| Payables to Broker-dealers, Clearing Organizations and Counterparties
Derivatives, Fair Value [Line Items]
Derivative liability, fair value, gross liability	2.7	13.0
Derivative liability, notional amount	(2,430.7)	(6,549.4)
Forward Settling Securities Purchased, Unrealized Gain \| Deposits and Receivables from Broker-dealers, Clearing Organizations and Counterparties
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	(0.3)	0.0
Derivative asset, notional amount	214.5	0.0
Forward Settling Securities Purchased, Unrealized Loss \| Deposits and Receivables from Broker-dealers, Clearing Organizations and Counterparties
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	0.0	7.3
Derivative asset, notional amount	0.0	2,447.1
Forward Settling Securities Purchased, Unrealized Loss \| Payables to Broker-dealers, Clearing Organizations and Counterparties
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	(11.5)	0.0
Derivative asset, notional amount	2,580.7	0.0
Forward Settling Securities Sold, Unrealized Gain \| Deposits and Receivables from Broker-dealers, Clearing Organizations and Counterparties
Derivatives, Fair Value [Line Items]
Derivative liability, fair value, gross liability	(14.1)	(17.4)
Derivative liability, notional amount	(1,867.4)	(1,946.0)
Forward Settling Securities Sold, Unrealized Loss \| Payables to Broker-dealers, Clearing Organizations and Counterparties
Derivatives, Fair Value [Line Items]
Derivative liability, fair value, gross liability	(0.4)	0.0
Derivative liability, notional amount	$ (133.1)	$ 0.0

Nominal or face amount used to calculate payments on the derivative asset.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=125515794&loc=SL5579245-113959

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=125515794&loc=SL5580258-113959

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=125515794&loc=SL5579245-113959

+ Details

Name:	us-gaap_DerivativeAssetNotionalAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Definition

Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 22
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226052-175313

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226000-175313

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 4D
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=121570589&loc=SL51823488-111719

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4B
-Subparagraph (a),(c)
-URI http://asc.fasb.org/extlink&oid=125515794&loc=SL5624163-113959

+ Details

Name:	us-gaap_DerivativeFairValueOfDerivativeAsset
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 4D
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=121570589&loc=SL51823488-111719

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4B
-Subparagraph (a),(c)
-URI http://asc.fasb.org/extlink&oid=125515794&loc=SL5624163-113959

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226000-175313

Reference 5: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 22
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226052-175313

+ Details

Name:	us-gaap_DerivativeFairValueOfDerivativeLiability
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Nominal or face amount used to calculate payments on the derivative liability.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=125515794&loc=SL5579245-113959

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=125515794&loc=SL5580258-113959

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=125515794&loc=SL5579245-113959

+ Details

Name:	us-gaap_DerivativeLiabilityNotionalAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
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_DerivativesFairValueLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=intl_TBASecuritiesPurchasedUnrealizedGainMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BalanceSheetLocationAxis=intl_DepositsAndReceivablesFromBrokerDealersClearingOrganizationsAndCounterpartiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=intl_TBASecuritiesPurchasedUnrealizedLossMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BalanceSheetLocationAxis=intl_PayablesToBrokerDealersClearingOrganizationsAndCounterpartiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=intl_TBASecuritiesSoldUnrealizedGainMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=intl_TBASecuritiesSoldUnrealizedLossMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=intl_ForwardSettlingSecuritiesPurchasedUnrealizedGainMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=intl_ForwardSettlingSecuritiesPurchasedUnrealizedLossMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=intl_ForwardSettlingSecuritiesSoldUnrealizedGainMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=intl_ForwardSettlingSecuritiesSoldUnrealizedLossMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: