Condensed Financial Information of Parent Company Only Disclosure - Balance Sheet (Details) - USD ($) $ in Millions		Sep. 30, 2021	Sep. 30, 2020	Oct. 01, 2019	Sep. 30, 2019	Sep. 30, 2018
Cash and cash equivalents		$ 1,109.6	$ 952.6		$ 471.3
Notes receivable, net		6.1	1.7
Income taxes receivable		26.6	16.6
Deferred income taxes, net		35.1	36.9
Property and equipment, net		93.3	62.1
Operating right of use assets		125.3	101.5	$ 33.1
Other assets		109.3	87.5
Total assets		18,839.6	13,474.9		9,936.1
Liabilities:
Accounts payable and other accrued liabilities		305.1	272.6
Operating lease liabilities		146.6	118.7	$ 36.2
Payable to clients		7,835.9	5,689.0
Lenders under loans		248.6	268.1
Senior secured borrowings, net		507.0	515.5
Financial instruments sold, not yet purchased, at fair value		1,771.2	686.0
Total liabilities		17,935.6	12,707.4
StoneX Group Inc. (Parent Company Only) stockholders’ equity:
Preferred stock, $0.01 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; 22,431,233 issued and 19,823,910 outstanding at September 30, 2021 and 21,798,551 issued and 19,376,594 outstanding at September 30, 2020		0.2	0.2
Common stock in treasury, at cost. 2,607,323 shares at September 30, 2021 and 2,421,957 shares at September 30, 2020		(69.3)	(57.6)
Additional paid-in capital		315.7	292.6
Retained earnings		682.5	572.4
Total equity		904.0	767.5		$ 594.2	$ 505.3
Total liabilities and stockholders' equity		18,839.6	13,474.9
StoneX Group
Cash and cash equivalents		34.7	7.4
Receivable from clients, net		0.4	0.4
Notes receivable, net		6.1	1.7
Income taxes receivable		63.7	46.2
Investment in subsidiaries		1,100.4	834.0
Deferred income taxes, net		2.4	4.3
Property and equipment, net		55.4	42.1
Operating right of use assets		66.3	69.0
Other assets		23.7	20.1
Total assets		1,353.1	1,025.2
Liabilities:
Accounts payable and other accrued liabilities		80.6	73.4
Operating lease liabilities		86.2	85.4
Payable to clients		0.0	0.3
Payable to subsidiaries, net		185.0	96.5
Lenders under loans		8.6	23.0
Senior secured borrowings, net		506.5	515.1
Financial instruments sold, not yet purchased, at fair value		0.9	1.1
Total liabilities		867.8	794.8
StoneX Group Inc. (Parent Company Only) stockholders’ equity:
Preferred stock, $0.01 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; 22,431,233 issued and 19,823,910 outstanding at September 30, 2021 and 21,798,551 issued and 19,376,594 outstanding at September 30, 2020		0.2	0.2
Common stock in treasury, at cost. 2,607,323 shares at September 30, 2021 and 2,421,957 shares at September 30, 2020		(69.3)	(57.6)
Additional paid-in capital		315.7	292.6
Retained earnings	[1]	238.7	(4.8)
Total equity		485.3	230.4
Total liabilities and stockholders' equity		$ 1,353.1	$ 1,025.2

[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 is not included in the Condensed Balance Sheet or Condensed Statements of Operations. If the accounting for its investment in wholly owned subsidiaries was presented under the equity method of accounting, investment in subsidiaries and retained earnings would each increase by $443.8 million as of September 30, 2021, respectively, and $577.2 million, as of September 30, 2020, respectively.