Regulatory Requirements and Subsidiary Dividend Restrictions - Narrative (Details)	12 Months Ended
	Sep. 30, 2021 USD ($)
Capital and Other Regulatory Requirements [Line Items]
Adjusted net capital	$ 10,000,000
Net capital under commodity exchange act computation	20,000,000
Required net capital under commodity exchange act	10,000,000
StoneX Financial Inc.
Capital and Other Regulatory Requirements [Line Items]
Adjusted net capital	1,500,000
Adjusted net capital of percentage	$ 0.08
Broker-dealer, minimum net capital required, dividend withdrawal threshold, percent	120.00%
Net capital under commodity exchange act computation	$ 311,200,000
Required net capital under commodity exchange act	198,900,000
Gain Capital Group, LLC
Capital and Other Regulatory Requirements [Line Items]
Adjusted net capital	1,000,000
Adjusted net capital of percentage	0.08
Net capital under commodity exchange act computation	$ 49,400,000
Percentage of net capital under commodity exchange act computation	5.00%
Required net capital under commodity exchange act	$ 30,100,000
Percentage of required net capital under commodity exchange act	10.00%