Intangible Assets - Indefinite-Lived Intangible Assets (Details) - USD ($) $ in Millions	Mar. 31, 2020	Sep. 30, 2019
Indefinite-lived Intangible Assets [Line Items]
Gross Finite and Indefinite-Lived Intangible Assets	$ 38.3	$ 32.2
Finite and Indefinited-Lived Accumulated Amortization and Impairment Charges	(17.4)	(15.5)
Intangible Assets, Net (Excluding Goodwill)	$ 20.9	$ 16.7

Accumulated amount of amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life. In addition, it includes impairment recognized during the life of the intangible that results from the write-down of indefinite-lived intangible assets after comparing the implied fair value of reporting unit indefinite-lived intangible assets with the carrying amount of that indefinite-lived intangible assets, which are assessed at least annually for impairment.

No definition available.

+ Details

Name:	intl_FiniteAndIndefinitedLivedAccumulatedAmortizationAndImpairmentCharges
Namespace Prefix:	intl_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Gross amount of assets, excluding financial assets and goodwill, lacking physical substance with a finite life. In addition, it includes the carrying amount of assets (excluding financial assets) that lack physical substance, excluding goodwill, having a projected indefinite period of benefit.

+ References

No definition available.

+ Details

Name:	intl_GrossFiniteAndIndefiniteLivedIntangibleAssets
Namespace Prefix:	intl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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_IndefiniteLivedIntangibleAssetsByMajorClassLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Sum of the carrying amounts of all intangible assets, excluding goodwill, as of the balance sheet date, net of accumulated amortization and impairment charges.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph ((a)(1),(b))
-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6388964&loc=d3e16212-109274

+ Details

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