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PROPERTY, PLANT AND EQUIPMENT | 12 Months Ended
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Dec. 31, 2020
PROPERTY, PLANT AND EQUIPMENT |
PROPERTY, PLANT AND EQUIPMENT | 7.PROPERTY, PLANT AND EQUIPMENT ​ Property, plant and equipment consist of the following: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | 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.
​ At December 31, 2020 and 2019, land and coal mineral rights above include $37.5 million and $40.1 million, respectively, of carrying value associated with coal reserves attributable to properties where we or a third party to which we lease reserves are not currently engaged in mining operations or leasing to third parties, and therefore, the coal reserves are not currently being depleted. We believe that the carrying value of these coal reserves will be recovered. ​ At December 31, 2020 and 2019, our oil & gas mineral interests noted in the table above includes the carrying value of our unproved oil & gas mineral interests totaling $340.5 million and $376.2 million, respectively. As discussed in Note 2 – Summary of Significant Accounting Policies, we generally do not record depletion expense for our unproved oil & gas mineral interests; however, we do review for impairment as needed throughout the year. ​ During 2020 and 2019, we incurred $13.1 million and $13.2 million, respectively, in mine development costs, primarily related to the development of our Excel Mine No. 5 at our MC Mining complex. All past capitalized mine development costs are associated with other mines that shifted to the production phase in past years and we are amortizing these costs accordingly. We believe that the carrying value of the past development costs will be recovered. For information regarding long-lived asset impairments please see Note 4 – Long-Lived Asset Impairments. ​ See Note 2 – Summary of Significant Accounting Policies for more information on our accounting policy for property, plant and equipment.

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