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Commitments and Contingencies - Narrative (Details) - USD ($) | 1 Months Ended | 3 Months Ended | 6 Months Ended |
--- | --- | --- | --- | ---
Mar. 31, 2020 | Jun. 30, 2021 | Jun. 30, 2020 | Jun. 30, 2021 | Jun. 30, 2020 | Dec. 31, 2020
Commitments and Contingencies Disclosure [Abstract] |  |  |  |  |  |
Settlement proceeds | $ 1,800,000 |  |  |  |  |
Throughput obligation |  | $ 0 |  | $ 0 |  | $ 0
Coal royalty expense |  | $ 15,700,000 | $ 13,600,000 | $ 28,400,000 | $ 27,800,000 |

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- Definition Amount awarded from other party in judgment or settlement of litigation. + References No definition available. + Details
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- Definition Amount of the recorded obligation to transfer funds in the future for fixed or minimum amounts or quantities of goods or services at fixed or minimum prices (for example, as in take-or-pay contracts or throughput contracts). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 440 -SubTopic 10 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=123406679&loc=d3e25383-109308 + Details
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- Definition Amount of expense related to royalty payments under a contractual arrangement such as payment for mineral and drilling rights and use of technology or intellectual property. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.3) -URI http://asc.fasb.org/extlink&oid=123367319&loc=SL114868664-224227 + Details
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