Receivables from customers, net and notes receivable, net - Bad Debt Expense and Recoveries (Details) - USD ($)	3 Months Ended								12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision for bad debts									$ (2,500,000)	$ (3,100,000)	$ (4,300,000)
Allowance for Doubtful Accounts Receivable, Period Increase (Decrease)									2,600,000	2,900,000	4,200,000
Insurance Recoveries									10,000,000
Gain (Loss) Related to Litigation Settlement									2,400,000
(Recovery) bad debt on physical coal	$ (10,000,000)	$ 0	$ 0	$ (2,400,000)	$ 0	$ 0	$ 0	$ 1,000,000	(12,400,000.0)	1,000,000.0	47,000,000.0
Allowance for Doubtful Accounts Receivable, Charge-offs									300,000	300,000	100,000
Allowance for Doubtful Accounts Receivable, Recoveries									400,000	100,000
Commercial Hedging [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for Doubtful Accounts Receivable, Period Increase (Decrease)									2,700,000	2,800,000	3,800,000
Clearing and Execution Services Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for Doubtful Accounts Receivable, Period Increase (Decrease)									1,400,000	$ 400,000	300,000
Physical Commodities Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for Doubtful Accounts Receivable, Period Increase (Decrease)											$ 200,000
Allowance for Doubtful Accounts Receivable, Recoveries									$ 12,400,000