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PROPERTY, PLANT AND EQUIPMENT | 12 Months Ended
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Dec. 31, 2019
PROPERTY, PLANT AND EQUIPMENT |
PROPERTY, PLANT AND EQUIPMENT | 6.PROPERTY, PLANT AND EQUIPMENT ​ Property, plant and equipment consist of the following: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | December 31, |
​ | ​ | 2019 |  | 2018 | ​
​ | ​ | (in thousands) | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Mining equipment and processing facilities | ​ | $ | 1,937,642 | ​ | $ | 1,851,479 | ​
Land and coal mineral rights | ​ |  | 453,237 | ​ |  | 445,411 | ​
Oil & gas mineral interests (1) | ​ | ​ | 618,282 | ​ | ​ | — | ​
Buildings, office equipment and improvements | ​ |  | 304,111 | ​ |  | 287,053 | ​
Construction and mine development in progress | ​ |  | 86,876 | ​ |  | 71,190 | ​
Mine development costs | ​ |  | 283,860 | ​ |  | 270,675 | ​
Property, plant and equipment, at cost | ​ |  | 3,684,008 | ​ |  | 2,925,808 | ​
Less accumulated depreciation, depletion and amortization | ​ |  | (1,675,022) | ​ |  | (1,513,450) | ​
Total property, plant and equipment, net | ​ | $ | 2,008,986 | ​ | $ | 1,412,358 | ​
| (1) | Oil & gas mineral interests acquired in the AllDale and Wing Acquisitions. See Note 3 – Acquisitions for more information.
​ At December 31, 2019 and 2018, land and coal mineral rights above include $40.1 million and $49.2 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, 2019, our oil & gas mineral interests noted in the table above includes the carrying value of our unproved oil & gas mineral interests totaling $376.2 million. 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 2019, we incurred $13.2 million in mine development costs, primarily related to the development of our Excel #5 mine at our MC Mining complex. During 2018, we reduced mine development costs by $9.9 million primarily related to the impairment of our Dotiki mine. Regarding the mine development costs for MC Mining's Excel #5 mine, we had no incidental production during this development phase and thus no related capitalized development costs associated with incidental production. 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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- Definition The entire disclosure for long-lived, physical assets used in the normal conduct of business and not intended for resale. Includes, but is not limited to, accounting policies and methodology, roll forwards, depreciation, depletion and amortization expense, including composite depreciation, accumulated depreciation, depletion and amortization expense, useful lives and method used, income statement disclosures, assets held for sale and public utility disclosures. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -URI http://asc.fasb.org/topic&trid=2155823 + Details
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