Allowance for Doubtful Accounts - Narrative (Details) - USD ($)	12 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for doubtful accounts - deposits with and receivables from broker-dealers, clearings organizations, and counterparties	$ 1,400,000	$ 1,300,000
Allowance for doubtful accounts receivable	46,400,000	38,500,000
Allowance for doubtful accounts	0	0
Accounts receivable, allowance for doubtful accounts write-off	5,600,000	5,900,000
Accounts receivable, allowance for doubtful accounts related to physical coal business	5,600,000	5,900,000	$ 35,600,000
Provision for bad debts, net of recoveries and impairment	12,400,000	10,400,000	13,000,000
Commerical Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision for bad debts, net of recoveries and impairment	11,600,000	8,500,000
Bad debt expense (recovery)		4,600,000	3,200,000
Net increase in provision for bad debts			3,500,000
Retail Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision for bad debts, net of recoveries and impairment	2,300,000	1,100,000	600,000
Retail Segment, Physical Commodity Accounts
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision for bad debts, net of recoveries and impairment	$ 2,500,000
Institutional Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision for bad debts, net of recoveries and impairment		600,000
Net increase in provision for bad debts			$ 5,400,000
Global Payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision for bad debts, net of recoveries and impairment		$ 200,000