Regulatory Requirements and Subsidiary Dividend Restrictions - Minimum Regulatory Net Capital (Details) $ in Millions	Sep. 30, 2020 USD ($)
Capital and Other Regulatory Requirements [Line Items]
Actual	$ 20.0
Minimum Requirement	10.0
StoneX Financial Inc. [Member]
Capital and Other Regulatory Requirements [Line Items]
Actual	229.8
Minimum Requirement	137.6
StoneX Financial Ltd. [Member]
Capital and Other Regulatory Requirements [Line Items]
Actual	264.3
Minimum Requirement	145.1
Gain Capital Group, LLC [Member]
Capital and Other Regulatory Requirements [Line Items]
Actual	71.5
Minimum Requirement	32.2
Gain Capital U.K. Ltd [Member]
Capital and Other Regulatory Requirements [Line Items]
Actual	185.4
Minimum Requirement	$ 65.2