Description of Business and Significant Accounting Policies - Notes Receivable - Participation (Details)	Sep. 30, 2017	Sep. 30, 2016
Minimum [Member]
Short Term Notes Receivable Outstanding Balance Threshold, Percentage	90.00%	90.00%
Maximum [Member]
Short Term Notes Receivable Outstanding Balance Threshold, Percentage	98.00%	98.00%

Short Term Notes Receivable Outstanding Balance Threshold, Percentage

No definition available.

Name:	intl_ShortTermNotesReceivableOutstandingBalanceThresholdPercentage
Namespace Prefix:	intl_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant