Condensed Financial Information of Parent Company Only Disclosure - Cash Flows (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
Cash flows from operating activities:
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
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Amortization of operating right of use assets									9.9
Deferred income taxes									4.1	3.7	22.3
Amortization and extinguishment of debt issuance costs									6.5	1.5	1.0
Amortization of share-based compensation expense									10.3	8.1	6.6
Gain on acquisition	(81.8)						$ (5.4)
Changes in operating assets and liabilities:
Income taxes receivable									(11.1)	(4.2)	(1.3)
Financial instruments owned, at fair value									(552.5)	(113.3)	(308.7)
Other assets									(3.7)	(8.3)	(3.3)
Accounts payable and other accrued liabilities									42.7	6.7	18.6
Payable to clients									2,093.7	(46.8)	520.0
Financial instruments sold, not yet purchased, at fair value									(28.8)	(156.1)	153.9
Net cash provided by (used in) operating activities									1,950.6	195.6	(473.6)
Cash flows from investing activities:
Purchase of property and equipment									(16.6)	(11.9)	(12.5)
Net cash used in investing activities									(241.6)	(40.8)	(15.4)
Cash flows from financing activities:
Repayments of note payable									(0.4)	(0.8)	(0.8)
Repayments of senior secured term loan									(9.8)	(6.6)	0.0
Repayment of senior secured notes									(92.1)	0.0	0.0
Share repurchase									(7.5)	(3.8)	0.0
Debt issuance costs									(15.0)	(3.3)	(0.4)
Exercise of stock options									5.5	1.2	2.6
Withholding taxes on stock option exercises									0.0	0.0	(0.8)
Net cash provided by financing activities									312.3	9.6	120.9
Net increase (decrease) in cash, segregated cash, cash equivalents, and segregated cash equivalents									2,017.1	163.7	(372.2)
Cash, segregated cash, cash equivalents, and segregated cash equivalents at beginning of period				2,451.3				2,287.6	2,451.3	2,287.6	2,659.8
Cash, segregated cash, cash equivalents, and segregated cash equivalents at end of period	4,468.4				2,451.3				4,468.4	2,451.3	2,287.6
Cash paid for interest									90.4	153.2	78.9
Income taxes (received) paid, net of cash refunds									44.0	24.6	22.2
Additional consideration payable related to acquisitions									0.9
StoneX Group [Member]
Cash flows from operating activities:
Net income (loss)									88.2	15.3	(37.2)
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Depreciation and amortization									6.7	5.2	4.8
Amortization of operating right of use assets									4.4
Deferred income taxes									0.0	0.6	18.0
Amortization and extinguishment of debt issuance costs									6.1	1.2	0.7
Amortization of share-based compensation expense									9.2	7.1	6.1
Impairment									2.5	0.0	0.0
Gain on acquisition									(81.9)	(5.4)	0.0
Changes in operating assets and liabilities:
Due to/from subsidiaries									149.3	8.3	(69.4)
Receivables from clients, net									0.1	(0.5)	0.0
Notes receivable, net									1.1	(1.0)	2.9
Income taxes receivable									(48.4)	(0.8)	(6.6)
Financial instruments owned, at fair value									0.0	4.4	(4.4)
Other assets									(7.7)	(4.4)	(0.7)
Accounts payable and other accrued liabilities									24.0	4.6	8.6
Operating lease liabilities									(2.8)
Payable to clients									0.0	(1.4)	(0.3)
Financial instruments sold, not yet purchased, at fair value									(83.4)	25.2	34.0
Net cash provided by (used in) operating activities									67.4	58.4	(43.5)
Cash flows from investing activities:
Capital contribution in affiliates									(251.9)	(75.8)	(4.5)
Purchase of property and equipment									(10.2)	(6.2)	(5.9)
Net cash used in investing activities									(262.1)	(82.0)	(10.4)
Cash flows from financing activities:
Net change in lenders under loans									(47.0)	(138.2)	58.2
Repayments of note payable									(0.4)	(0.8)	(0.8)
Proceeds from issuance of senior secured term loan									21.5	175.0	0.0
Repayments of senior secured term loan									(9.8)	(6.6)	0.0
Proceeds from issuance of senior secured notes									344.8	0.0	0.0
Repayment of senior secured notes									(92.1)	0.0	0.0
Deferred payments on acquisitions									(0.9)	0.0	(5.5)
Share repurchase									(7.5)	(3.8)	0.0
Debt issuance costs									(14.0)	(3.0)	0.0
Exercise of stock options									5.5	1.2	2.6
Withholding taxes on stock option exercises									0.0	0.0	(0.8)
Net cash provided by financing activities									200.1	23.8	53.7
Net increase (decrease) in cash, segregated cash, cash equivalents, and segregated cash equivalents									5.4	0.2	(0.2)
Cash, segregated cash, cash equivalents, and segregated cash equivalents at beginning of period				$ 2.0				$ 1.8	2.0	1.8	2.0
Cash, segregated cash, cash equivalents, and segregated cash equivalents at end of period	$ 7.4				$ 2.0				7.4	2.0	1.8
Cash paid for interest									15.3	18.9	15.0
Income taxes (received) paid, net of cash refunds									(4.3)	(23.9)	(18.4)
Additional consideration payable related to acquisitions									$ 21.6	$ 1.8	$ 0.0