Acquisitions - Cotton Distributors Inc. (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
	Oct. 31, 2022	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Business Acquisition [Line Items]
Net operating revenues		$ 435.9	$ 374.1	$ 1,217.3	$ 1,088.2
Net income		69.5	$ 49.1	187.8	154.8
Identifiable intangible assets acquired
Gain on acquisition				23.5	$ 0.0
Cotton Distributors Inc.
Business Acquisition [Line Items]
Consideration transferred	$ 42.7
Net operating revenues		11.7		24.0
Net income		$ 7.2		$ 12.3
Identifiable intangible assets acquired
Cash and cash equivalents	8.2
Deposits with and receivables from broker-dealers, clearing organizations, and counterparties	7.7
Receivables from clients, net	51.9
Financial instruments owned, at fair value	45.7
Deferred income taxes, net	(3.3)
Property and equipment, net	0.1
Physical commodities inventory, net	22.5
Other assets	6.7
Total fair value of tangible assets acquired	139.5
Accounts payable and other accrued liabilities	40.0
Financial instruments sold, not yet purchased, at fair value	28.3
Payables to lenders under loans	10.1
Payable to broker-dealers, clearing organizations, and counterparties	0.4
Payable to clients	2.6
Income taxes payable	0.8
Total fair value of tangible liabilities assumed	82.2
Fair value of tangible net assets acquired	57.3
Total fair value of intangible assets acquired	8.9
Fair value of identifiable net assets acquired	66.2
Total merger consideration	42.7
Gain on acquisition	23.5
Cotton Distributors Inc. | Client relationships
Identifiable intangible assets acquired
Total fair value of intangible assets acquired	4.7
Cotton Distributors Inc. | Supplier relationships
Identifiable intangible assets acquired
Total fair value of intangible assets acquired	3.7
Cotton Distributors Inc. | Trade name
Identifiable intangible assets acquired
Total fair value of intangible assets acquired	0.4
Cotton Distributors Inc. | Non-compete
Identifiable intangible assets acquired
Total fair value of intangible assets acquired	$ 0.1