Condensed Financial Information of Parent Company Only Disclosure - Condensed Statements of Operations (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020
Condensed Income Statements, Captions [Line Items]
Revenue from contract with customer	$ 3,607.5	$ 2,119.5	$ 487.3
Interest income	219.0	102.4	130.9
Total revenues	66,036.0	42,534.2	54,139.6
Interest expense	135.5	49.6	80.4
Net operating revenues	1,475.9	1,150.0	868.0
Non-interest expenses:
Compensation and benefits	794.8	679.1	518.7
Trade systems and market information	66.2	58.8	46.3
Occupancy and equipment rental	36.1	34.2	23.5
Selling and marketing	55.3	33.3	12.2
Professional fees	54.3	40.9	30.2
Travel and business development	16.9	4.5	8.9
Non-trading technology and support	52.4	46.0	28.4
Depreciation and amortization	44.4	36.5	19.7
Communications	8.3	9.3	7.0
Other	60.6	46.3	29.6
Total compensation and other expenses	1,205.1	999.3	743.2
Gain on acquisitions	6.4	3.4	81.9
Income from operations, before tax	277.2	154.1	206.7
Income tax benefit	(70.1)	(37.8)	(37.1)
Net income	207.1	116.3	169.6
StoneX Group
Condensed Income Statements, Captions [Line Items]
Trading (losses) gains, net	(2.8)	(0.1)	0.6
Interest income	2.2	1.5	2.4
Dividend income from subsidiaries	124.4	372.7	111.8
Total revenues	233.8	426.9	160.2
Interest expense	60.9	49.6	30.0
Net operating revenues	172.9	377.3	130.2
Non-interest expenses:
Compensation and benefits	113.7	99.9	88.0
Trade systems and market information	6.5	6.0	3.9
Occupancy and equipment rental	8.4	8.7	3.8
Selling and marketing	4.9	1.0	3.8
Professional fees	11.9	6.9	12.9
Travel and business development	2.3	0.8	1.7
Non-trading technology and support	30.6	24.9	19.8
Depreciation and amortization	11.0	8.7	6.7
Communications	1.8	1.7	0.7
Impairment	0.0	0.1	2.5
Management services fees to affiliates	4.3	3.6	2.3
Other	12.4	8.7	7.3
Total compensation and other expenses	207.8	171.0	153.4
Gain on acquisitions	0.0	3.4	81.9
Income from operations, before tax	(34.9)	209.7	58.7
Income tax benefit	45.0	33.8	29.5
Net income	10.1	243.5	88.2
Investment in subsidiaries for equity method accounting	197.0	(127.2)	81.4
StoneX Group | Management fees from affiliates
Condensed Income Statements, Captions [Line Items]
Revenue from contract with customer	109.9	52.5	45.1
StoneX Group | Consulting fees
Condensed Income Statements, Captions [Line Items]
Revenue from contract with customer	$ 0.1	$ 0.3	$ 0.3