Receivables from customers, net and notes receivable, net - Bad Debt Expense and Recoveries (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2013
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision for Doubtful Accounts	$ 4.4	$ 7.3	$ 5.5
Allowance for Doubtful Accounts Receivable, Period Increase (Decrease)	4.2	6.6	5.1
Allowance for Doubtful Accounts Receivable, Charge-offs	0.4	5.7	0.6	$ 0.7
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)	3.6		3.8
Allowance for Doubtful Accounts Receivable, Charge-offs		0.7	0.6
Allowance for Doubtful Accounts Receivable, Recoveries			0.2
Commercial Hedging [Member] | OTC Customer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for Doubtful Accounts Receivable, Period Increase (Decrease)		2.3
Commercial Hedging [Member] | LME Customer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for Doubtful Accounts Receivable, Period Increase (Decrease)		0.6
Physical Commodities Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for Doubtful Accounts Receivable, Period Increase (Decrease)	0.4	2.8	0.9
Securities Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for Doubtful Accounts Receivable, Period Increase (Decrease)	$ 0.4	0.5	$ 0.7
Securities Segment [Member] | Provident Group Asset Acquisition [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for Doubtful Accounts Receivable, Period Increase (Decrease)		$ 1.1