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Inventories (Details) - USD ($) $ in Thousands | Sep. 30, 2021 | Dec. 31, 2020
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Inventory Disclosure [Abstract] |  |
Coal | $ 17,172 | $ 17,695
Mining supplies | 33,783 | 29,856
Total inventories | $ 50,955 | $ 47,551

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- Definition Amount before valuation and LIFO reserves of other supplies used within the manufacturing or production process expected to be consumed within one year or operating cycle, if longer. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.6(a)(5)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
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