Receivables from customers, net and notes receivable, net - Bad Debt Expense and Recoveries (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision for Doubtful Accounts		$ 3.1	$ 4.3	$ 4.4
Allowance for Doubtful Accounts Receivable, Period Increase (Decrease)		2.9	4.2	4.2
Provision for Other Losses		0.3	0.1
Bad debt on physical coal	$ 1.0	1.0	47.0	0.0
Allowance for Doubtful Accounts Receivable, Charge-offs		0.3	6.1	5.7
Allowance for Doubtful Accounts Receivable, Recoveries		0.1		0.2
Commercial Hedging [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for Doubtful Accounts Receivable, Period Increase (Decrease)		2.8	3.8	3.6
Allowance for Doubtful Accounts Receivable, Charge-offs				0.4
Physical Commodities Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for Doubtful Accounts Receivable, Period Increase (Decrease)			0.2	0.4
Clearing and Execution Services Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for Doubtful Accounts Receivable, Period Increase (Decrease)		$ 0.4	$ 0.3
Securities Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for Doubtful Accounts Receivable, Period Increase (Decrease)				$ 0.4