Regulatory Requirements and Subsidiary Dividend Restrictions - Regulatory Capital Requirements (Details) $ in Millions	Sep. 30, 2020 USD ($)
StoneX Financial [Member]
Capital and Other Regulatory Requirements [Line Items]
Cash and securities segregated under commodity exchange act regulation	$ 3,089.9
Secured funds	162.6
Minimum regulatory requirement	3,030.6
Secured funds required under commodity exchange act	150.1
Gain Capital Group, LLC [Member]
Capital and Other Regulatory Requirements [Line Items]
Cash and securities segregated under commodity exchange act regulation	340.4
Secured funds	4.3
Minimum regulatory requirement	319.8
Secured funds required under commodity exchange act	2.5
StoneX Financial Ltd. [Member]
Capital and Other Regulatory Requirements [Line Items]
Cash and securities segregated under commodity exchange act regulation	516.5
Minimum regulatory requirement	505.6
Gain Capital U.K. Ltd [Member]
Capital and Other Regulatory Requirements [Line Items]
Cash and securities segregated under commodity exchange act regulation	206.9
Minimum regulatory requirement	205.6
StoneX Financial Pte. Ltd. [Member]
Capital and Other Regulatory Requirements [Line Items]
Cash and securities segregated under commodity exchange act regulation	340.6
Minimum regulatory requirement	$ 338.9