Condensed Financial Information of Parent Company Only Disclosure - Condensed Statements of Operations (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Condensed Income Statements, Captions [Line Items]
Total revenues	$ 60,856.1	$ 66,036.0	$ 42,534.2
Interest expense	802.2	135.5	49.6
Net operating revenues	1,621.0	1,475.9	1,150.0
Non-interest expenses:
Compensation and benefits	868.6	794.8	679.1
Trade systems and market information	74.0	66.2	58.8
Occupancy and equipment rental	40.4	36.1	34.2
Selling and marketing	54.0	55.3	33.3
Professional fees	57.0	54.3	40.9
Travel and business development	24.8	16.9	4.5
Non-trading technology and support	61.6	52.4	46.0
Depreciation and amortization	51.0	44.4	36.5
Communications	9.1	8.3	9.3
Other	66.4	60.6	46.3
Total compensation and other expenses	1,323.4	1,205.1	999.3
Gain on acquisitions	25.4	6.4	3.4
Income from operations, before tax	323.0	277.2	154.1
Income tax benefit	(84.5)	(70.1)	(37.8)
Net income	238.5	207.1	116.3
Interest income
Condensed Income Statements, Captions [Line Items]
Total revenues	987.6	219.0	102.4
StoneX Group
Condensed Income Statements, Captions [Line Items]
Total revenues	615.1	233.8	426.9
Interest expense	83.1	60.9	49.6
Net operating revenues	532.0	172.9	377.3
Non-interest expenses:
Compensation and benefits	137.9	113.7	99.9
Trade systems and market information	10.2	6.5	6.0
Occupancy and equipment rental	9.3	8.4	8.7
Selling and marketing	2.2	4.9	1.0
Professional fees	12.7	11.9	6.9
Travel and business development	3.1	2.3	0.8
Non-trading technology and support	37.4	30.6	24.9
Depreciation and amortization	13.7	11.0	8.7
Communications	3.4	1.8	1.7
Impairment	0.0	0.0	0.1
Management services fees to affiliates	188.4	4.3	3.6
Other	12.1	12.4	8.7
Total compensation and other expenses	430.4	207.8	171.0
Gain on acquisitions	2.1	0.0	3.4
Income from operations, before tax	103.7	(34.9)	209.7
Income tax benefit	45.4	45.0	33.8
Net income	149.1	10.1	243.5
Investment in subsidiaries for equity method accounting, earnings (loss)	89.4	197.0	(127.2)
StoneX Group | Management fees from affiliates
Condensed Income Statements, Captions [Line Items]
Total revenues	328.7	109.9	52.5
StoneX Group | Trading gains (losses), net
Condensed Income Statements, Captions [Line Items]
Total revenues	1.8	(2.8)	(0.1)
StoneX Group | Consulting fees
Condensed Income Statements, Captions [Line Items]
Total revenues	0.2	0.1	0.3
StoneX Group | Interest income
Condensed Income Statements, Captions [Line Items]
Total revenues	3.3	2.2	1.5
StoneX Group | Dividend income from subsidiaries
Condensed Income Statements, Captions [Line Items]
Total revenues	$ 281.1	$ 124.4	$ 372.7