Condensed Financial Information of Parent Company Only Disclosure - Condensed Balance Sheets (Details) - USD ($) $ in Millions	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
ASSETS
Cash and cash equivalents	$ 1,108.5	$ 1,109.6	$ 952.6
Deposits with and receivables from subsidiary broker-dealer, net	6,842.6	5,292.9
Notes receivable, net	5.1	6.1
Income taxes receivable	16.8	26.6
Financial instruments owned	4,167.3	4,354.6
Deferred income taxes, net	52.0	35.1
Property and equipment, net	112.9	93.3
Operating right of use assets	121.8	125.3
Other assets	117.7	109.3
Total assets	19,859.6	18,839.6	13,474.9
Liabilities:
Accounts payable and other accrued liabilities	400.6	305.1
Operating lease liabilities	143.0	146.6
Lenders under loans	485.1	248.6
Senior secured borrowings, net	339.1	507.0
Financial instruments sold, not yet purchased, at fair value	2,469.6	1,771.2
Total liabilities	18,789.5	17,935.6
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; 22,911,227 issued and 20,303,904 outstanding at September 30, 2022 and 22,431,233 issued and 19,823,910 outstanding at September 30, 2021	0.2	0.2
Common stock in treasury, at cost - 2,607,323 shares at September 30, 2022 and 2021	(69.3)	(69.3)
Additional paid-in capital	340.2	315.7
Retained earnings	889.6	682.5
Accumulated other comprehensive loss, net	(90.6)	(25.1)
Total equity	1,070.1	904.0	$ 767.5	$ 594.2
Total liabilities and stockholders' equity	19,859.6	18,839.6
StoneX Group
ASSETS
Cash and cash equivalents	6.1	34.7
Receivable from clients, net	0.2	0.4
Deposits with and receivables from subsidiary broker-dealer, net	89.1	0.0
Notes receivable, net	5.0	6.1
Income taxes receivable	55.5	63.7
Investment in subsidiaries	1,286.8	1,100.4
Financial instruments owned	5.1	0.0
Deferred income taxes, net	14.7	2.4
Property and equipment, net	62.1	55.4
Operating right of use assets	69.0	66.3
Other assets	30.8	23.7
Total assets	1,624.4	1,353.1
Liabilities:
Accounts payable and other accrued liabilities	90.7	80.6
Operating lease liabilities	93.3	86.2
Payable to subsidiaries, net	293.3	185.0
Lenders under loans	268.1	8.6
Senior secured borrowings, net	339.1	506.5
Financial instruments sold, not yet purchased, at fair value	73.6	0.9
Total liabilities	1,158.1	867.8
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; 22,911,227 issued and 20,303,904 outstanding at September 30, 2022 and 22,431,233 issued and 19,823,910 outstanding at September 30, 2021	0.2	0.2
Common stock in treasury, at cost - 2,607,323 shares at September 30, 2022 and 2021	(69.3)	(69.3)
Additional paid-in capital	340.2	315.7
Retained earnings	248.8	238.7
Accumulated other comprehensive loss, net	(53.6)	0.0
Total equity	466.3	485.3
Total liabilities and stockholders' equity	1,624.4	1,353.1
Adjustment to investment in subsidiaries for equity method accounting	$ 640.8	$ 443.8