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PROPERTY, PLANT AND EQUIPMENT - Summary (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019
PROPERTY, PLANT AND EQUIPMENT |  |
Property, plant and equipment, at cost | $ 3,554,090 | $ 3,684,008
Less accumulated depreciation, depletion and amortization | (1,753,845) | (1,675,022)
Total property, plant and equipment, net | 1,800,245 | 2,008,986
Mining equipment and processing facilities |  |
PROPERTY, PLANT AND EQUIPMENT |  |
Property, plant and equipment, at cost | 1,896,324 | 1,937,642
Land and coal mineral rights |  |
PROPERTY, PLANT AND EQUIPMENT |  |
Property, plant and equipment, at cost | 454,310 | 453,237
Coal reserves not subject to depletion | 37,500 | 40,100
Oil and gas mineral interests |  |
PROPERTY, PLANT AND EQUIPMENT |  |
Property, plant and equipment, at cost | 616,904 | 618,282
Unproved properties | 340,500 | 376,200
Buildings, office equipment and improvements |  |
PROPERTY, PLANT AND EQUIPMENT |  |
Property, plant and equipment, at cost | 279,938 | 304,111
Construction and mine development in progress |  |
PROPERTY, PLANT AND EQUIPMENT |  |
Property, plant and equipment, at cost | 25,799 | 86,876
Mine development costs |  |
PROPERTY, PLANT AND EQUIPMENT |  |
Property, plant and equipment, at cost | 280,815 | 283,860
Mine development costs | $ 13,100 | $ 13,200

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- Definition Capitalized costs of unproved properties incurred for any combination mineral interests acquisitions and other costs not previously disclosed within this table. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -URI http://asc.fasb.org/topic&trid=2145477 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e61901-109447 + Details
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- Definition Amount of accumulated depreciation and amortization from plant, property, and equipment and right-of-use asset from finance lease. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 842 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=121609121&loc=SL77918686-209980 Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 842 -SubTopic 20 -Section 45 -Paragraph 4 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=121603541&loc=SL77918638-209977 + Details
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