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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 |  |

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