Regulatory Requirements and Subsidiary Dividend Restrictions - Narrative (Details)	12 Months Ended
	Sep. 30, 2023 USD ($)
Capital and Other Regulatory Requirements [Line Items]
Adjusted net capital	$ 10,000,000
StoneX Financial Inc.
Capital and Other Regulatory Requirements [Line Items]
Adjusted net capital	$ 1,500,000
Adjusted net capital of percentage	8.00%
Broker-dealer, minimum net capital required, dividend withdrawal threshold, percent	120.00%
Net capital under commodity exchange act computation	$ 353,000,000.0
Required net capital under commodity exchange act	220,500,000
Gain Capital Group, LLC
Capital and Other Regulatory Requirements [Line Items]
Net capital under commodity exchange act computation	50,100,000
Required net capital under commodity exchange act	29,300,000
Gain Capital Group, LLC | Customer and non-customer risk maintenance margin
Capital and Other Regulatory Requirements [Line Items]
Adjusted net capital	$ 1,000,000
Adjusted net capital of percentage	8.00%
Gain Capital Group, LLC | Retail customer liabilities
Capital and Other Regulatory Requirements [Line Items]
Net capital under commodity exchange act computation	$ 20,000,000
Percentage of net capital under commodity exchange act computation	5.00%
Gain Capital Group, LLC | Liabilities owed to counterparties
Capital and Other Regulatory Requirements [Line Items]
Required net capital under commodity exchange act	$ 10,000,000
Percentage of required net capital under commodity exchange act	10.00%
StoneX Markets LLC
Capital and Other Regulatory Requirements [Line Items]
Net capital under commodity exchange act computation	$ 221,300,000
Required net capital under commodity exchange act	122,100,000
Minimum regulatory capital requirement	$ 20,000,000