Condensed Financial Information of Parent Company Only Disclosure - Balance Sheet (Details) - USD ($) $ in Millions		Sep. 30, 2020	Oct. 01, 2019	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Cash and cash equivalents		$ 952.6		$ 471.3
Income taxes receivable		16.6		5.2
Deferred income taxes, net		36.9		18.0
Property and equipment, net		62.1		43.9
Operating right of use assets		101.5	$ 33.1
Total assets		13,474.9		9,936.1	$ 7,824.7
Liabilities:
Accounts payable and other accrued liabilities		272.6		157.5
Operating lease liabilities		118.7	$ 36.2
Payable to clients		5,689.0		3,589.5
Payable to lenders under loans		268.1		202.3
Senior Secured Notes		336.0		0.0
Financial instruments sold, not yet purchased, at fair value		686.0		714.8
Total liabilities		12,707.4		9,341.9
StoneX Group Inc. (Parent Company Only) stockholders’ equity:
Common stock, $0.01 par value. Authorized 30,000,000 shares; 21,798,551 issued and 19,376,594 outstanding at September 30, 2020 and 21,297,317 issued and 19,075,360 outstanding at September 30, 2019		0.2		0.2
Common stock in treasury, at cost - 2,421,957 shares at September 30, 2020 and 2,221,957 shares at September 30, 2019		(57.6)		(50.1)
Additional paid-in capital		292.6		276.8
Retained earnings		572.4		402.1
Total equity		767.5		594.2	$ 505.3	$ 449.9
Total liabilities and stockholders' equity		13,474.9		9,936.1
StoneX Group [Member]
Cash and cash equivalents		7.4		2.0
Receivable from subsidiaries, net		0.0		17.6
Receivable from clients		0.4		0.5
Notes receivable, net		1.7		2.8
Income taxes receivable		46.2		15.7
Investment in subsidiaries		834.0		399.4
Deferred income taxes, net		4.3		8.2
Property and equipment, net		42.1		26.9
Operating right of use assets		69.0
Other assets		20.1		13.0
Total assets		1,025.2		486.1
Liabilities:
Accounts payable and other accrued liabilities		73.4		29.4
Operating lease liabilities		85.4
Payable to clients		0.3		0.3
Payable to subsidiaries, net		96.5		0.0
Payable to lenders under loans		23.0		70.4
Senior Secured Notes		515.1		167.6
Financial instruments sold, not yet purchased, at fair value		1.1		84.5
Total liabilities		794.8		352.2
StoneX Group Inc. (Parent Company Only) stockholders’ equity:
Preferred stock, $.01 par value. Authorized 1,000,000 shares; no shares issued or outstanding		0.0		0.0
Common stock, $0.01 par value. Authorized 30,000,000 shares; 21,798,551 issued and 19,376,594 outstanding at September 30, 2020 and 21,297,317 issued and 19,075,360 outstanding at September 30, 2019		0.2		0.2
Common stock in treasury, at cost - 2,421,957 shares at September 30, 2020 and 2,221,957 shares at September 30, 2019		(57.6)		(50.1)
Additional paid-in capital		292.6		276.8
Retained earnings	[1]	(4.8)		(93.0)
Total equity		230.4		133.9
Total liabilities and stockholders' equity		1,025.2		486.1
Adjustment to Investment in Subs for Equity Method Accounting		$ 577.2		$ 495.1

[1] Within the Condensed Balance Sheets and Condensed Statements of Operations of StoneX Group Inc. - Parent Company Only, the Company has accounted for its investment in wholly owned subsidiaries using the cost method of accounting. Under this method, the Company’s share of the earnings or losses of such subsidiaries are not included in the Condensed Balance Sheet or Condensed Statements of Operations. If the accounting for its investment in wholly owned subsidiaries were presented under the equity method of accounting, investment in subsidiaries and retained earnings would each increase by $577.2 million as of September 30, 2020, respectively, and $495.1 million, as of September 30, 2019, respectively.