Condensed Financial Information of Parent Company Only Disclosure - Income Statement (Details) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Condensed Financial Statements, Captions [Line Items]
Interest income										$ 130.9	$ 198.9	$ 123.3
Operating revenues		$ 342.1	$ 322.6	$ 366.8	$ 276.8	$ 286.9	$ 283.4	$ 271.1	$ 264.7	1,308.3	1,106.1	975.8
Interest expense		10.0	11.5	27.8	31.1	37.2	39.4	35.2	30.2	80.4	142.0	70.5
Net operating revenues		226.2	227.9	243.4	170.5	173.4	165.6	165.2	149.0	868.0	653.2	581.6
Non-interest expenses:
Compensation and benefits		145.5	132.5	136.7	104.0	105.2	100.9	97.9	89.1	518.7	393.1	337.7
Clearing and related expenses		57.1	55.3	63.8	46.3	45.0	45.7	42.7	50.1	222.5	183.5	179.7
Occupancy and equipment rental										23.5	19.4	16.5
Professional fees										30.2	21.0	18.1
Travel and business development										8.9	16.2	13.8
Depreciation and amortization										19.7	14.0	11.6
Communications										7.0	6.6	5.4
Other										29.6	23.2	20.1
Total compensation and other expenses		228.1	178.9	187.3	148.9	139.4	144.0	139.7	124.6	743.2	547.7	482.1
Gain on acquisitions		81.8	0.0	0.0	0.1	0.1	0.0	5.4	0.0	81.9	5.5	2.0
Income before tax		79.9	49.0	56.1	21.7	34.1	21.6	30.9	24.4	206.7	111.0	101.5
Income tax expense		2.5	12.4	16.8	5.4	6.9	5.3	7.5	6.2	37.1	25.9	46.0
Net income (loss)		$ 77.4	$ 36.6	$ 39.3	$ 16.3	$ 27.2	$ 16.3	$ 23.4	$ 18.2	169.6	85.1	55.5
StoneX Group [Member]
Condensed Financial Statements, Captions [Line Items]
Management fees from affiliates										45.1	43.2	40.4
Trading losses, net										0.6	0.0	0.0
Consulting fees										0.3	0.1	0.0
Interest income										2.4	1.5	2.3
Dividend income from subsidiaries	[1]									111.8	85.7	41.9
Operating revenues										160.2	130.5	84.6
Interest expense										30.0	19.7	15.7
Net operating revenues										130.2	110.8	68.9
Non-interest expenses:
Compensation and benefits										88.0	79.7	73.0
Clearing and related expenses										0.3	0.9	1.1
Trading systems and market information										3.9	6.4	5.8
Occupancy and equipment rental										3.8	3.4	2.6
Professional fees										12.9	7.3	6.7
Travel and business development										1.7	2.9	2.6
Non-trading technology and support										19.8	12.5	9.1
Depreciation and amortization										6.7
Communications										0.7	0.8	0.9
Impairment										2.5	0.0	0.0
Management services fees to affiliates										2.3	0.5	0.0
Other										10.8	5.8	6.9
Total compensation and other expenses										153.4	125.4	113.5
Gain on acquisitions										81.9	5.3	0.0
Income before tax										58.7	(9.3)	(44.6)
Income tax expense										29.5	24.6	7.4
Net income (loss)										88.2	15.3	(37.2)
Investment in subsidiaries for equity method accounting										$ 81.4	$ 69.8	$ 92.7

[1] Within the Condensed Balance Sheets and Condensed Statements of Operations of StoneX Group Inc. - Parent Company Only, the Company has accounted for its investment in wholly owned subsidiaries using the cost method of accounting. Under this method, the Company’s share of the earnings or losses of such subsidiaries are not included in the Condensed Balance Sheet or Condensed Statements of Operations. If the accounting for its investment in wholly owned subsidiaries were presented under the equity method of accounting, revenues would also include income from investment in subsidiaries of $81.4 million, $69.8 million, and $92.7 million for the years ended September 30, 2020, 2019, and 2018, respectively.