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Allowance for Doubtful Accounts - Narrative (Details) - USD ($) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2019 | Jun. 30, 2019 | Mar. 31, 2019 | Dec. 31, 2018 | Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2018 | Sep. 30, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |
Allowance for doubtful accounts - deposits with and receivables from broker-dealers, clearings organizations, and counterparties | $ 36,900,000 |  |  |  | $ 1,300,000 | $ 36,900,000 |  |
Accounts receivable, allowance for credit loss, write-off |  |  |  |  | 35,600,000 | 300,000 | $ 300,000 |
Allowance for doubtful accounts receivable | 11,700,000 |  |  |  | 25,800,000 | 11,700,000 |  |
Allowance for doubtful accounts | 0 |  |  |  | 0 | 0 |  |
Provision for bad debts, net of recoveries and impairment |  |  |  |  | 13,000,000.0 | 2,500,000 | 3,100,000 |
(Recovery) bad debt on physical coal | $ (10,000,000.0) | $ 0 | $ 0 | $ (2,400,000) | 0 | (12,400,000) | 1,000,000.0 | $ 47,000,000.0
Net increase in provision for bad debts |  |  |  |  |  | 2,600,000 | 2,900,000 |
Allowance for doubtful accounts receivable, recoveries |  |  |  |  |  | 400,000 | 100,000 |
Commerical Segment [Member] |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |
Provision for bad debts, net of recoveries and impairment |  |  |  |  | 3,500,000 |  |  |
(Recovery) bad debt on physical coal |  |  |  |  | 3,200,000 |  |  |
Net increase in provision for bad debts |  |  |  |  |  | 2,700,000 | 2,800,000 |
Institutional Segment [Member] |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |
Provision for bad debts, net of recoveries and impairment |  |  |  |  | 5,400,000 |  |  |
Net increase in provision for bad debts |  |  |  |  |  | $ 1,400,000 | $ 400,000 |
Retail [Member] |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |
Provision for bad debts, net of recoveries and impairment |  |  |  |  | $ 600,000 |  |  |

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- Definition Allowance For Doubtful Accounts - Deposits with and Receivables from Broker-Dealers, Clearings Organizations, and Counterparties + References No definition available. + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition Amount of allowance for credit loss on accounts receivable. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 50 -Paragraph 13 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919249-210447 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=121611835&loc=d3e5074-111524 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 50 -Paragraph 13 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919249-210447 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=121599878&loc=SL82895884-210446 + Details
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- Definition Amount of increase (decrease) in allowance for credit loss on accounts receivable. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 50 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919249-210447 + Details
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- Definition Amount of increase in allowance for credit loss on accounts receivable, from recovery. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 50 -Paragraph 13 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919249-210447 + Details
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- Definition Amount of allowance for credit loss on financing receivable. Excludes allowance for financing receivable covered under loss sharing agreement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=121611835&loc=SL6953423-111524 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(7)(2)) -URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=121599878&loc=SL82895884-210446 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 50 -Paragraph 13 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919249-210447 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 50 -Paragraph 13 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919249-210447 + Details
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