Receivables from customers, net and notes receivable, net - Bad Debt Expense and Recoveries (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2013
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision for Doubtful Accounts	$ 4.3	$ 4.4	$ 7.3
Allowance for Doubtful Accounts Receivable, Period Increase (Decrease)	4.2	4.2	6.6
Allowance for Doubtful Accounts Receivable, Charge-offs	0.1		6.1	$ 5.7	$ 0.6
Allowance for Doubtful Accounts Receivable, Recoveries		0.2
Commercial Hedging [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for Doubtful Accounts Receivable, Period Increase (Decrease)	3.8	3.6	2.3
Allowance for Doubtful Accounts Receivable, Charge-offs		0.4	0.7
Physical Commodities Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for Doubtful Accounts Receivable, Period Increase (Decrease)	0.2	0.4	2.8
Clearing and Execution Services Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for Doubtful Accounts Receivable, Period Increase (Decrease)	$ 0.3
Securities Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for Doubtful Accounts Receivable, Period Increase (Decrease)		$ 0.4	$ 0.5