Condensed Financial Information of Parent Company Only Disclosure - Condensed Statements of Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020
Cash flows from operating activities:
Net income	$ 207.1	$ 116.3	$ 169.6
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	44.4	36.5	19.7
Amortization of operating right of use assets	15.9	12.2	9.9
Deferred income taxes	(0.3)	3.2	4.1
Amortization and extinguishment of debt issuance costs	4.5	3.9	6.5
Loss on extinguishment of debt		0.1
Amortization of share-based compensation expense	17.8	13.9	10.3
Gain on acquisition	0.0	(3.3)	(81.9)
Changes in operating assets and liabilities:
Receivable from clients, net	(119.7)	(68.3)	0.9
Deposits with and receivables from broker-dealers, clearing organizations, and counterparties	(2,174.3)	(132.0)	(326.0)
Notes receivable, net	1.0	(4.4)	1.2
Income taxes receivable	10.4	(9.6)	(11.1)
Financial instruments owned, at fair value	187.3	(1,626.9)	(552.5)
Other assets	(9.8)	(16.7)	(3.7)
Accounts payable and other accrued liabilities	101.4	35.7	42.7
Operating lease liabilities	(16.0)	(8.6)	(9.5)
Payable to clients	2,055.1	2,146.7	2,093.7
Financial instruments sold, not yet purchased, at fair value	627.8	1,085.2	(28.8)
Net cash (used in)/provided by operating activities	(229.5)	2,122.7	1,950.6
Cash flows from investing activities:
Purchase of property and equipment and internally developed software	(49.5)	(62.1)	(16.6)
Net cash used in investing activities	(49.5)	(59.8)	(241.6)
Cash flows from financing activities:
Repayments of note payable	0.0	0.0	(0.4)
Proceeds from issuance of senior secured term loan	0.0	0.0	21.5
Repayments of senior secured term loan	(170.3)	(9.8)	(9.8)
Proceeds from issuance of senior secured notes	0.0	0.0	344.8
Repayment of senior secured notes	(0.5)	(1.6)	(92.1)
Issuance of note payable	0.0	9.0	0.0
Share repurchase	0.0	(11.7)	(7.5)
Debt issuance costs	0.0	0.0	(15.0)
Exercise of stock options	6.7	9.2	5.5
Net cash provided by/(used in) financing activities	65.8	(35.6)	312.3
Net (decrease) increase in cash, segregated cash, cash equivalents, and segregated cash equivalents	(224.4)	2,041.1	2,017.1
Cash, segregated cash, cash equivalents, and segregated cash equivalents at beginning of period	6,509.5	4,468.4	2,451.3
Cash, segregated cash, cash equivalents, and segregated cash equivalents at end of period	6,285.1	6,509.5	4,468.4
Cash paid for interest	149.2	87.0	90.4
Income taxes paid/(received), net of cash refunds	56.3	52.0	44.0
Supplemental disclosure of non-cash investing and financing activities:
Additional consideration payable related to acquisitions	3.6	2.6
StoneX Group
Cash flows from operating activities:
Net income	10.1	243.5	88.2
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	11.0	8.7	6.7
Amortization of operating right of use assets	6.1	6.1	4.4
Deferred income taxes	4.7	1.8	0.0
Amortization and extinguishment of debt issuance costs	3.2	3.3	6.1
Loss on extinguishment of debt	0.0		0.0
Amortization of share-based compensation expense	16.5	12.9	9.2
Dividends	(9.6)	(125.0)	0.0
Impairment	0.0	0.0	2.5
Gain on acquisition	0.0	(3.3)	(81.9)
Changes in operating assets and liabilities:
Payable to subsidiaries, net	113.6	118.3	149.3
Receivable from clients, net	0.2	0.0	0.1
Deposits with and receivables from broker-dealers, clearing organizations, and counterparties	(89.1)	0.0	0.0
Notes receivable, net	1.1	(4.4)	1.1
Income taxes receivable	8.2	(17.4)	(48.4)
Financial instruments owned, at fair value	(5.1)	0.0	0.0
Other assets	(7.3)	(4.2)	(7.7)
Accounts payable and other accrued liabilities	12.0	12.7	24.0
Operating lease liabilities	(1.7)	(2.6)	(2.8)
Payable to clients	0.0	(0.3)	0.0
Financial instruments sold, not yet purchased, at fair value	2.1	(0.2)	(83.4)
Net cash (used in)/provided by operating activities	76.0	250.0	67.4
Cash flows from investing activities:
Capital contribution to affiliates	(180.8)	(170.2)	(251.9)
Purchase of property and equipment and internally developed software	(17.8)	(22.0)	(10.2)
Net cash used in investing activities	(198.6)	(192.2)	(262.1)
Cash flows from financing activities:
Net change in lenders under loans	259.5	(23.4)	(47.0)
Repayments of note payable	0.0	0.0	(0.4)
Proceeds from issuance of senior secured term loan	0.0	0.0	21.5
Repayments of senior secured term loan	(170.3)	(9.8)	(9.8)
Proceeds from issuance of senior secured notes	0.0	0.0	344.8
Repayment of senior secured notes	0.0	(1.6)	(92.1)
Issuance of note payable	0.0	9.0	0.0
Deferred payments on acquisitions	(1.9)	(2.2)	(0.9)
Share repurchase	0.0	(11.7)	(7.5)
Debt issuance costs	0.0	0.0	(14.0)
Exercise of stock options	6.7	9.2	5.5
Net cash provided by/(used in) financing activities	94.0	(30.5)	200.1
Net (decrease) increase in cash, segregated cash, cash equivalents, and segregated cash equivalents	(28.6)	27.3	5.4
Cash, segregated cash, cash equivalents, and segregated cash equivalents at beginning of period	34.7	7.4	2.0
Cash, segregated cash, cash equivalents, and segregated cash equivalents at end of period	6.1	34.7	7.4
Cash paid for interest	34.9	28.5	15.3
Income taxes paid/(received), net of cash refunds	2.6	9.8	(4.3)
Supplemental disclosure of non-cash investing and financing activities:
Additional consideration payable related to acquisitions	$ 0.0	$ 0.0	$ 21.6