Receivables from customers, net and notes receivable, net - Allowance for Notes Receivable (Details) - USD ($) $ in Millions	Sep. 30, 2017	Sep. 30, 2016
Receivables from customers and notes receivable, net [Abstract]
Allowance Notes Receivable	$ 0.0	$ 0.2

A valuation allowance relating to a written agreement to receive money (at a specified future date(s), consisting of principal as well as any accrued interest) for the portion that is expected to be uncollectible.

No definition available.

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