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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 |  |

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