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PROPERTY, PLANT AND EQUIPMENT (Tables) | 12 Months Ended
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Dec. 31, 2020
PROPERTY, PLANT AND EQUIPMENT |
Schedule of property, plant and equipment | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | December 31, |
​ | ​ | 2020 |  | 2019 | ​
​ | ​ | (in thousands) | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Mining equipment and processing facilities | ​ | $ | 1,896,324 | ​ | $ | 1,937,642 | ​
Land and coal mineral rights | ​ |  | 454,310 | ​ |  | 453,237 | ​
Oil & gas mineral interests (1) | ​ | ​ | 616,904 | ​ | ​ | 618,282 | ​
Buildings, office equipment and improvements | ​ |  | 279,938 | ​ |  | 304,111 | ​
Construction and mine development in progress | ​ |  | 25,799 | ​ |  | 86,876 | ​
Mine development costs | ​ |  | 280,815 | ​ |  | 283,860 | ​
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 | ​
| (1) | Oil & gas mineral interests acquired in the AllDale and Wing Acquisitions. See Note 3 – Acquisitions for more information.

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- Definition Tabular disclosure of physical assets used in the normal conduct of business and not intended for resale. Includes, but is not limited to, balances by class of assets, depreciation and depletion expense and method used, including composite depreciation, and accumulated deprecation. + 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(13)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229 + Details
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