Cover - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Dec. 10, 2020	Mar. 31, 2020
Cover [Abstract]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Sep. 30, 2020
Document Transition Report	false
Entity File Number	000-23554
Entity Registrant Name	StoneX Group Inc.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	59-2921318
Entity Address, Address Line One	155 East 44th Street
Entity Address, Address Line Two	Suite 900
Entity Address, City or Town	New York
Entity Address, State or Province	NY
Entity Address, Postal Zip Code	10017
City Area Code	212
Local Phone Number	485-3500
Title of 12(b) Security	Common Stock, $0.01 par value
Trading Symbol	SNEX
Security Exchange Name	NASDAQ
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Public Float			$ 429.6
Entity Common Stock, Shares Outstanding		19,434,929
Documents Incorporated by Reference	Certain portions of the definitive Proxy Statement for the Registrant’s Annual Meeting of Stockholders to be held on February 24, 2021 are incorporated by reference into Part III of this Annual Report on Form 10-K.
Entity Central Index Key	0000913760
Current Fiscal Year End Date	--09-30
Document Fiscal Year Focus	2020
Amendment Flag	false
Document Fiscal Period Focus	FY