Condensed Financial Information of Parent Company Only Disclosure - Condensed Statements of Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Cash flows from operating activities:
Net income	$ 238.5	$ 207.1	$ 116.3
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	51.0	44.4	36.5
Amortization of operating right of use assets	14.0	15.9	12.2
Deferred income taxes	(2.4)	(0.3)	3.2
Amortization and extinguishment of debt issuance costs	5.8	4.5	3.9
Loss on extinguishment of debt		0.0
Amortization of share-based compensation expense	28.0	17.8	13.9
Gain on acquisition	(23.5)	0.0	(3.3)
Changes in operating assets and liabilities:
Receivable from clients, net	(80.5)	(119.7)	(68.3)
Deposits with and receivables from broker-dealers, clearing organizations, and counterparties, net	(595.6)	(2,174.3)	(132.0)
Notes receivable, net	(0.1)	1.0	(4.4)
Income taxes receivable	(13.4)	10.4	(9.6)
Financial instruments owned, at fair value	(857.9)	187.3	(1,626.9)
Other assets	(60.4)	(9.8)	(16.7)
Accounts payable and other accrued liabilities	82.5	101.4	35.7
Operating lease liabilities	(8.0)	(16.0)	(8.6)
Payable to clients	81.4	2,055.1	2,146.7
Financial instruments sold, not yet purchased, at fair value	658.4	627.8	1,085.2
Net cash (used in)/provided by operating activities	(23.7)	(229.5)	2,122.7
Cash flows from investing activities:
Purchase of property and equipment and internally developed software	(46.9)	(49.5)	(62.1)
Net cash used in investing activities	(53.0)	(49.5)	(59.8)
Cash flows from financing activities:
Repayments of senior secured term loan	0.0	(170.3)	(9.8)
Repayment of senior secured notes	0.0	(0.5)	(1.6)
Issuance of note payable	0.0	0.0	9.0
Deferred payments on acquisitions	(18.7)	(3.0)	(2.2)
Share repurchase	0.0	0.0	(11.7)
Exercise of stock options	3.7	6.7	9.2
Net cash (used in)/provided by financing activities	(169.3)	65.8	(35.6)
Net (decrease)/increase in cash, segregated cash, cash equivalents, and segregated cash equivalents	(243.4)	(224.4)	2,041.1
Cash, segregated cash, cash equivalents, and segregated cash equivalents at beginning of period	6,285.1	6,509.5	4,468.4
Cash, segregated cash, cash equivalents, and segregated cash equivalents at end of period	6,041.7	6,285.1	6,509.5
Supplemental disclosure of cash flow information:
Cash paid for interest	786.5	149.2	87.0
Income taxes paid, net of cash refunds	71.0	56.3	52.0
StoneX Group
Cash flows from operating activities:
Net income	149.1	10.1	243.5
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	13.7	11.0	8.7
Amortization of operating right of use assets	5.8	6.1	6.1
Deferred income taxes	(3.6)	4.7	1.8
Amortization and extinguishment of debt issuance costs	3.9	3.2	3.3
Loss on extinguishment of debt	0.0		0.1
Amortization of share-based compensation expense	26.2	16.5	12.9
Dividends	(12.7)	(9.6)	(125.0)
Gain on acquisition	0.0	0.0	(3.3)
Changes in operating assets and liabilities:
Payable to subsidiaries, net	11.0	113.6	118.3
Receivable from clients, net	0.2	0.2	0.0
Deposits with and receivables from broker-dealers, clearing organizations, and counterparties, net	12.1	(89.1)	0.0
Notes receivable, net	0.0	1.1	(4.4)
Income taxes receivable	(76.6)	8.2	(17.4)
Financial instruments owned, at fair value	(0.8)	(5.1)	0.0
Other assets	(5.7)	(7.3)	(4.2)
Accounts payable and other accrued liabilities	45.8	12.0	12.7
Operating lease liabilities	(4.5)	(1.7)	(2.6)
Payable to clients	0.0	0.0	(0.3)
Financial instruments sold, not yet purchased, at fair value	(3.1)	2.1	(0.2)
Net cash (used in)/provided by operating activities	160.8	76.0	250.0
Cash flows from investing activities:
Capital contribution to affiliates	(40.0)	(180.8)	(170.2)
Purchase of property and equipment and internally developed software	(17.9)	(17.8)	(22.0)
Net cash used in investing activities	(57.9)	(198.6)	(192.2)
Cash flows from financing activities:
Net change in lenders under loans	(110.6)	259.5	(23.4)
Repayments of senior secured term loan	0.0	(170.3)	(9.8)
Repayment of senior secured notes	0.0	0.0	(1.6)
Issuance of note payable	0.0	0.0	9.0
Deferred payments on acquisitions	0.0	(1.9)	(2.2)
Share repurchase	0.0	0.0	(11.7)
Exercise of stock options	3.7	6.7	9.2
Net cash (used in)/provided by financing activities	(106.9)	94.0	(30.5)
Net (decrease)/increase in cash, segregated cash, cash equivalents, and segregated cash equivalents	(4.0)	(28.6)	27.3
Cash, segregated cash, cash equivalents, and segregated cash equivalents at beginning of period	6.1	34.7	7.4
Cash, segregated cash, cash equivalents, and segregated cash equivalents at end of period	2.1	6.1	34.7
Supplemental disclosure of cash flow information:
Cash paid for interest	15.3	34.9	28.5
Income taxes paid, net of cash refunds	$ 34.9	$ 2.6	$ 9.8