Condensed Financial Information of Parent Company Only Disclosure - Condensed Balance Sheets (Details) - USD ($) $ in Millions	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020
ASSETS
Cash and cash equivalents	$ 1,108.3	$ 1,108.5	$ 1,109.6
Deposits with and receivables from subsidiary broker-dealer, net	7,443.8	6,842.6
Notes receivable, net	5.2	5.1
Income taxes receivable	25.1	16.8
Financial instruments owned	5,044.8	4,167.3
Deferred tax assets	45.4	52.0
Property and equipment, net	123.5	112.9
Operating right of use assets	122.1	121.8
Other assets	182.8	117.7
Total assets	21,938.7	19,859.6	18,839.6
Liabilities:
Accounts payable and other accrued liabilities	533.0	400.6
Operating lease liabilities	149.3	143.0
Lenders under loans	341.0	485.1
Financial instruments sold, not yet purchased, at fair value	3,085.6	2,469.6
Total liabilities	20,559.6	18,789.5
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 200,000,000 shares; 23,403,960 issued and 20,796,637 outstanding at September 30, 2023 and 22,911,227 issued and 20,303,904 outstanding at September 30, 2022	0.2	0.2
Common stock in treasury, at cost - 2,607,323 shares at September 30, 2023 and 2022	(69.3)	(69.3)
Additional paid-in capital	371.9	340.2
Retained earnings	1,128.1	889.6
Accumulated other comprehensive loss, net	(51.8)	(90.6)
Total equity	1,379.1	1,070.1	$ 904.0	$ 767.5
Total liabilities and stockholders' equity	21,938.7	19,859.6
StoneX Group
ASSETS
Cash and cash equivalents	2.1	6.1
Receivable from clients, net	0.0	0.2
Deposits with and receivables from subsidiary broker-dealer, net	77.0	89.1
Notes receivable, net	5.0	5.0
Income taxes receivable	132.1	55.5
Investment in subsidiaries	1,325.3	1,286.8
Financial instruments owned	7.6	5.1
Deferred tax assets	7.2	14.7
Property and equipment, net	66.3	62.1
Operating right of use assets	65.5	69.0
Other assets	35.6	30.8
Total assets	1,723.7	1,624.4
Liabilities:
Accounts payable and other accrued liabilities	135.7	90.7
Operating lease liabilities	91.1	93.3
Payable to subsidiaries, net	288.3	293.3
Lenders under loans	157.5	268.1
Senior secured borrowings, net	342.1	339.1
Financial instruments sold, not yet purchased, at fair value	27.0	73.6
Total liabilities	1,041.7	1,158.1
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 200,000,000 shares; 23,403,960 issued and 20,796,637 outstanding at September 30, 2023 and 22,911,227 issued and 20,303,904 outstanding at September 30, 2022	0.2	0.2
Common stock in treasury, at cost - 2,607,323 shares at September 30, 2023 and 2022	(69.3)	(69.3)
Additional paid-in capital	371.9	340.2
Retained earnings	397.9	248.8
Accumulated other comprehensive loss, net	(18.7)	(53.6)
Total equity	682.0	466.3
Total liabilities and stockholders' equity	1,723.7	1,624.4
Adjustment to investment in subsidiaries for equity method accounting	$ 730.2	$ 640.8