Regulatory Requirements and Subsidiary Dividend Restrictions - Regulatory Capital Requirements (Details)	Sep. 30, 2021 USD ($)
StoneX Financial Inc.
Capital and Other Regulatory Requirements [Line Items]
Cash and securities segregated under commodity exchange act regulation	$ 4,429,800,000
Secured funds	216,500,000
Minimum regulatory requirement	4,364,500,000
Secured funds required under commodity exchange act	203,100,000
StoneX Financial Ltd.
Capital and Other Regulatory Requirements [Line Items]
Cash and securities segregated under commodity exchange act regulation	1,071,800,000
Minimum regulatory requirement	1,040,600,000
Gain Capital Group, LLC
Capital and Other Regulatory Requirements [Line Items]
Cash and securities segregated under commodity exchange act regulation	270,700,000
Secured funds	4,000,000
Minimum regulatory requirement	255,600,000
Secured funds required under commodity exchange act	2,100,000
StoneX Financial Pte. Ltd.
Capital and Other Regulatory Requirements [Line Items]
Cash and securities segregated under commodity exchange act regulation	413,700,000
Minimum regulatory requirement	395,300,000
StoneX Markets LLC
Capital and Other Regulatory Requirements [Line Items]
Minimum regulatory capital requirement	$ 100,000,000