Condensed Financial Information of Parent Company Only Disclosure - Income Statement (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Condensed Financial Statements, Captions [Line Items]
Revenue from contract with customer	$ 2,119.5	$ 487.3	$ 452.0
Interest income	102.4	130.9	198.9
Total revenues	42,534.2	54,139.6	32,897.0
Interest expense	49.6	80.4	142.0
Net operating revenues	1,150.0	868.0	653.2
Non-interest expenses:
Compensation and benefits	679.1	518.7	393.1
Clearing and related expenses	271.7	222.5	183.5
Trading systems and market information	58.8	46.3	38.8
Occupancy and equipment rental	34.2	23.5	19.4
Professional fees	40.9	30.2	21.0
Travel and business development	4.5	8.9	16.2
Non-trading technology and support	46.0	28.4	20.1
Depreciation and amortization	36.5	19.7	14.0
Communications	9.3	7.0	6.6
Other	46.3	29.6	23.2
Total compensation and other expenses	999.3	743.2	547.7
Gain on acquisitions	3.4	81.9	5.5
Income tax benefit	(37.8)	(37.1)	(25.9)
Net income	116.3	169.6	85.1
StoneX Group
Condensed Financial Statements, Captions [Line Items]
Trading (losses)/gains, net	(0.1)	0.6	0.0
Interest income	1.5	2.4	1.5
Dividend income from subsidiaries	372.7	111.8	85.7
Total revenues	426.9	160.2	130.5
Interest expense	49.6	30.0	19.7
Net operating revenues	377.3	130.2	110.8
Non-interest expenses:
Compensation and benefits	99.9	88.0	79.7
Clearing and related expenses	0.4	0.3	0.9
Trading systems and market information	6.0	3.9	6.4
Occupancy and equipment rental	8.7	3.8	3.4
Professional fees	6.9	12.9	7.3
Travel and business development	0.8	1.7	2.9
Non-trading technology and support	24.9	19.8	12.5
Depreciation and amortization	8.7	6.7	5.2
Communications	1.7	0.7	0.8
Impairment	0.1	2.5	0.0
Management services fees to affiliates	3.6	2.3	0.5
Other	9.3	10.8	5.8
Total compensation and other expenses	171.0	153.4	125.4
Gain on acquisitions	3.4	81.9	5.3
Income before tax	209.7	58.7	(9.3)
Income tax benefit	33.8	29.5	24.6
Net income	243.5	88.2	15.3
Investment in subsidiaries for equity method accounting	(127.2)	81.4	69.8
StoneX Group | Management fees from affiliates
Condensed Financial Statements, Captions [Line Items]
Revenue from contract with customer	52.5	45.1	43.2
StoneX Group | Consulting fee
Condensed Financial Statements, Captions [Line Items]
Revenue from contract with customer	$ 0.3	$ 0.3	$ 0.1