Segment Analysis (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Sep. 30, 2016
Segment Reporting Information [Line Items]
Revenues	$ 5,505.9	$ 4,868.5	$ 17,041.1	$ 11,977.3
Operating revenues	197.6	175.0	578.9	492.4
Net operating revenues	123.3	117.3	361.5	332.9
Net Segment Contribution	95.7	87.4	286.3	254.3
Segment Income	52.9	54.4	157.0	152.5
Costs not allocated to operating segments	37.9	33.0	119.3	99.0
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest	15.0	21.4	37.7	53.5
Assets	6,195.9		6,195.9		$ 5,950.3
Commercial Hedging [Member]
Segment Reporting Information [Line Items]
Revenues	57.1	71.9	177.3	182.0
Operating revenues	57.1	71.9	177.3	182.0
Net operating revenues	44.2	57.7	140.3	145.6
Net Segment Contribution	32.6	41.4	102.9	103.6
Segment Income	16.3	25.4	50.4	54.2
Assets	1,591.0		1,591.0		1,637.5
Global Payments [Member]
Segment Reporting Information [Line Items]
Revenues	22.5	18.4	67.1	54.1
Operating revenues	22.5	18.4	67.1	54.1
Net operating revenues	20.3	16.4	60.0	48.2
Net Segment Contribution	16.2	13.2	47.9	38.6
Segment Income	12.9	9.9	37.8	29.2
Assets	257.2		257.2		191.4
Securities Segment [Member]
Segment Reporting Information [Line Items]
Revenues	40.0	41.6	115.3	136.0
Operating revenues	40.0	41.6	115.3	136.0
Net operating revenues	24.6	27.7	73.8	95.5
Net Segment Contribution	20.2	21.2	58.9	75.7
Segment Income	12.9	14.4	37.5	54.5
Assets	2,300.7		2,300.7		2,130.7
Physical Commodities [Member]
Segment Reporting Information [Line Items]
Revenues	5,320.3	4,701.2	16,495.2	11,506.4
Operating revenues	12.0	7.7	33.0	21.5
Net operating revenues	10.2	6.2	27.3	17.7
Net Segment Contribution	7.6	3.6	19.9	12.3
Segment Income	4.3	1.3	11.2	4.2
Assets	304.2		304.2		258.0
Clearing and Execution Services Segment [Member]
Segment Reporting Information [Line Items]
Revenues	65.4	33.3	193.2	96.4
Operating revenues	65.4	33.3	193.2	96.4
Net operating revenues	25.6	10.0	75.1	30.8
Net Segment Contribution	19.1	8.0	56.7	24.1
Segment Income	6.5	3.4	20.1	10.4
Assets	1,590.5		1,590.5		1,617.4
Corporate [Member]
Segment Reporting Information [Line Items]
Revenues	0.6	2.1	(7.0)	2.4
Operating revenues	0.6	2.1	(7.0)	2.4
Net operating revenues	(1.6)	$ (0.7)	(15.0)	$ (4.9)
Assets	$ 152.3		$ 152.3		$ 115.3

Costs not allocated to operating segments

No definition available.

Name:	intl_Costsnotallocatedtooperatingsegments
Namespace Prefix:	intl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Net segment contribution equals revenues less cost of sales, transaction-based clearing expenses, variable bonus compensation, introducing broker commissions and interest expense.

+ References

No definition available.

+ Details

Name:	intl_NetSegmentContribution
Namespace Prefix:	intl_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Segment Income equals net contribution less non-variable direct segment costs.

+ References

No definition available.

+ Details

Name:	intl_SegmentIncome
Namespace Prefix:	intl_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(12))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(11))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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

- Definition

Aggregate revenue less cost of goods and services sold or operating expenses directly attributable to the revenue generation activity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1,2)
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

+ Details

Name:	us-gaap_GrossProfit
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of income (loss) from continuing operations before deduction of income tax expense (benefit) and income (loss) attributable to noncontrolling interest, and addition of income (loss) from equity method investments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(h)(1)(i))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

+ Details

Name:	us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1)
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

+ Details

Name:	us-gaap_Revenues
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income after deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

+ References

No definition available.

+ Details

Name:	us-gaap_RevenuesNetOfInterestExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

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

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=intl_CommodityAndRiskManagementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=intl_ForeignExchangeSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=intl_SecuritiesSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=intl_OtherSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=intl_ClearingAndExecutionServicesSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=us-gaap_CorporateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: