Allowance for Doubtful Accounts - Narrative (Details) - USD ($)	3 Months Ended				12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for doubtful accounts - deposits with and receivables from broker-dealers, clearings organizations, and counterparties	$ 36,900,000				$ 1,300,000	$ 36,900,000
Accounts receivable, allowance for credit loss, write-off					35,600,000	300,000	$ 300,000
Allowance for doubtful accounts receivable	11,700,000				25,800,000	11,700,000
Allowance for doubtful accounts	0				0	0
Provision for bad debts, net of recoveries and impairment					13,000,000.0	2,500,000	3,100,000
(Recovery) bad debt on physical coal	$ (10,000,000.0)	$ 0	$ 0	$ (2,400,000)	0	(12,400,000)	1,000,000.0	$ 47,000,000.0
Net increase in provision for bad debts						2,600,000	2,900,000
Allowance for doubtful accounts receivable, recoveries						400,000	100,000
Commerical Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision for bad debts, net of recoveries and impairment					3,500,000
(Recovery) bad debt on physical coal					3,200,000
Net increase in provision for bad debts						2,700,000	2,800,000
Institutional Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision for bad debts, net of recoveries and impairment					5,400,000
Net increase in provision for bad debts						$ 1,400,000	$ 400,000
Retail [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision for bad debts, net of recoveries and impairment					$ 600,000