Condensed Financial Information of Parent Company Only Disclosure - Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Cash flows from operating activities:
Net income	$ 116.3	$ 169.6	$ 85.1
Adjustments to reconcile net income to net cash provided by operating activities:
Amortization of operating right of use assets	12.2	9.9	0.0
Total deferred taxes	3.2	4.1	3.7
Amortization and extinguishment of debt issuance costs	3.9	6.5	1.5
Loss on extinguishment of debt		0.0
Amortization of share-based compensation expense	13.9	10.3	8.1
Gain on acquisition	(3.3)	(81.9)	(5.5)
Changes in operating assets and liabilities:
Receivable from clients, net	(68.3)	0.9	(134.3)
Notes receivable, net	(4.4)	1.2	0.9
Income taxes receivable	(9.6)	(11.1)	(4.2)
Financial instruments owned, at fair value	(1,626.9)	(552.5)	(113.3)
Other assets	(16.7)	(3.7)	(8.3)
Accounts payable and other accrued liabilities	35.7	42.7	6.7
Operating lease liabilities	(8.6)	(9.5)
Payable to clients	2,146.7	2,093.7	(46.8)
Financial instruments sold, not yet purchased, at fair value	1,085.2	(28.8)	(156.1)
Net cash provided by operating activities	2,122.7	1,950.6	195.6
Cash flows from investing activities:
Purchase of property and equipment	(62.1)	(16.6)	(11.9)
Net cash used in investing activities	(59.8)	(241.6)	(40.8)
Cash flows from financing activities:
Repayments of note payable	0.0	(0.4)	(0.8)
Proceeds from issuance of senior secured term loan	0.0	21.5	175.0
Repayments of senior secured term loan	(9.8)	(9.8)	(6.6)
Proceeds from issuance of senior secured notes	0.0	344.8	0.0
Repayment of senior secured notes	(1.6)	(92.1)	0.0
Issuance of note payable	9.0	0.0	0.0
Share repurchase	(11.7)	(7.5)	(3.8)
Debt issuance costs	0.0	(15.0)	(3.3)
Exercise of stock options	9.2	5.5	1.2
Net cash (used in)/provided by financing activities	(35.6)	312.3	9.6
Net increase in cash, segregated cash, cash equivalents, and segregated cash equivalents	2,041.1	2,017.1	163.7
Cash, segregated cash, cash equivalents, and segregated cash equivalents at beginning of period	4,468.4	2,451.3	2,287.6
Cash, segregated cash, cash equivalents, and segregated cash equivalents at end of period	6,509.5	4,468.4	2,451.3
Cash paid for interest	87.0	90.4	153.2
Income taxes paid/(received), net of cash refunds	52.0	44.0	24.6
Additional consideration payable related to acquisitions	2.6	0.9
StoneX Group
Cash flows from operating activities:
Net income	243.5	88.2	15.3
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	8.7
Amortization of operating right of use assets	6.1	4.4	0.0
Total deferred taxes	1.8	0.0	0.6
Amortization and extinguishment of debt issuance costs	3.3	6.1	1.2
Loss on extinguishment of debt	0.1		0.0
Amortization of share-based compensation expense	12.9	9.2	7.1
Dividends	(125.0)	0.0	0.0
Impairment	0.0	2.5	0.0
Gain on acquisition	(3.3)	(81.9)	(5.4)
Changes in operating assets and liabilities:
Due to/from subsidiaries	118.3	149.3	8.3
Receivable from clients, net	0.0	0.1	(0.5)
Notes receivable, net	(4.4)	1.1	(1.0)
Income taxes receivable	(17.4)	(48.4)	(0.8)
Financial instruments owned, at fair value	0.0	0.0	4.4
Other assets	(4.2)	(7.7)	(4.4)
Accounts payable and other accrued liabilities	12.7	24.0	4.6
Operating lease liabilities	(2.6)	(2.8)
Payable to clients	(0.3)	0.0	(1.4)
Financial instruments sold, not yet purchased, at fair value	(0.2)	(83.4)	25.2
Net cash provided by operating activities	250.0	67.4	58.4
Cash flows from investing activities:
Capital contribution to affiliates	(170.2)	(251.9)	(75.8)
Purchase of property and equipment	(22.0)	(10.2)	(6.2)
Net cash used in investing activities	(192.2)	(262.1)	(82.0)
Cash flows from financing activities:
Net change in lenders under loans	(23.4)	(47.0)	(138.2)
Repayments of note payable	0.0	(0.4)	(0.8)
Proceeds from issuance of senior secured term loan	0.0	21.5	175.0
Repayments of senior secured term loan	(9.8)	(9.8)	(6.6)
Proceeds from issuance of senior secured notes	0.0	344.8	0.0
Repayment of senior secured notes	(1.6)	(92.1)	0.0
Issuance of note payable	9.0	0.0	0.0
Deferred payments on acquisitions	(2.2)	(0.9)	0.0
Share repurchase	(11.7)	(7.5)	(3.8)
Debt issuance costs	0.0	(14.0)	(3.0)
Exercise of stock options	9.2	5.5	1.2
Net cash (used in)/provided by financing activities	(30.5)	200.1	23.8
Net increase in cash, segregated cash, cash equivalents, and segregated cash equivalents	27.3	5.4	0.2
Cash, segregated cash, cash equivalents, and segregated cash equivalents at beginning of period	7.4	2.0	1.8
Cash, segregated cash, cash equivalents, and segregated cash equivalents at end of period	34.7	7.4	2.0
Cash paid for interest	28.5	15.3	18.9
Income taxes paid/(received), net of cash refunds	9.8	(4.3)	(23.9)
Additional consideration payable related to acquisitions	$ 0.0	$ 21.6	$ 1.8