Acquisitions - Cotton Distributors Inc. Preliminary Purchase Price Allocation (Details) - USD ($) $ in Millions		12 Months Ended
	Oct. 31, 2022	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Business Acquisition [Line Items]
Gain on acquisition		$ 23.5	$ 0.0	$ 3.3
Cotton Distributors Inc.
Business Acquisition [Line Items]
Cash and cash equivalents	$ 8.2
Deposits with and receivables from broker-dealers, clearing organizations, and counterparties	7.7
Receivable 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
Payables to broker-dealers, clearing organizations, and counterparties	0.4
Payables to clients	2.6
Income taxes payable	0.8
Total fair value of 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
Business Acquisition [Line Items]
Total fair value of intangible assets acquired	4.7
Cotton Distributors Inc. | Supplier relationships
Business Acquisition [Line Items]
Total fair value of intangible assets acquired	3.7
Cotton Distributors Inc. | Trade name
Business Acquisition [Line Items]
Total fair value of intangible assets acquired	0.4
Cotton Distributors Inc. | Non-compete
Business Acquisition [Line Items]
Total fair value of intangible assets acquired	$ 0.1