Regulatory Requirements and Subsidiary Dividend Restrictions - Minimum Regulatory Net Capital (Details)	Sep. 30, 2021 USD ($)
Capital and Other Regulatory Requirements [Abstract]
Adjusted net capital	$ 10,000,000
Capital and Other Regulatory Requirements [Line Items]
Actual	20,000,000
Minimum Requirement	10,000,000
StoneX Financial Inc.
Capital and Other Regulatory Requirements [Abstract]
Adjusted net capital	1,500,000
Capital and Other Regulatory Requirements [Line Items]
Actual	311,200,000
Minimum Requirement	198,900,000
StoneX Financial Ltd.
Capital and Other Regulatory Requirements [Line Items]
Actual	368,500,000
Minimum Requirement	218,400,000
Gain Capital Group, LLC
Capital and Other Regulatory Requirements [Abstract]
Adjusted net capital	1,000,000
Capital and Other Regulatory Requirements [Line Items]
Actual	49,400,000
Minimum Requirement	30,100,000
StoneX Financial Pte. Ltd.
Capital and Other Regulatory Requirements [Line Items]
Actual	45,400,000
Minimum Requirement	$ 15,900,000