Condensed Financial Information of Parent Company Only Disclosure - Balance Sheet (Parentheticals) (Details) - USD ($) $ / shares in Units, $ in Millions	Sep. 30, 2021	Sep. 30, 2020
Preferred stock - par value (in dollar per share)	$ 0.01	$ 0.01
Preferred stock - authorized (in shares)	1,000,000	1,000,000
Preferred stock - issued (in shares)	0	0
Preferred stock - outstanding (in shares)	0	0
Common stock - par value (in dollar per share)	$ 0.01	$ 0.01
Common stock - authorized (in shares)	30,000,000	30,000,000
Common stock - issued (in shares)	22,431,233	21,798,551
Common stock - outstanding (in shares)	19,823,910	19,376,594
Treasury stock - shares (in shares)	2,607,323	2,421,957
StoneX Group
Adjustment to investment in subsidiaries for equity method accounting	$ 443.8	$ 577.2
Preferred stock - par value (in dollar per share)	$ 0.01	$ 0.01
Preferred stock - authorized (in shares)	1,000,000	1,000,000
Preferred stock - issued (in shares)	0	0
Preferred stock - outstanding (in shares)	0	0
Common stock - par value (in dollar per share)	$ 0.01	$ 0.01
Common stock - authorized (in shares)	30,000,000	30,000,000
Common stock - issued (in shares)	22,431,233	21,798,551
Common stock - outstanding (in shares)	19,823,910	19,376,594
Treasury stock - shares (in shares)	2,607,323	2,421,957