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Property, Plant and Equipment, net | 12 Months Ended
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Dec. 31, 2017
Property Plant And Equipment [Abstract] |
Property, Plant and Equipment, net | 7. Property, Plant and Equipment, net Net property, plant and equipment consists of:
|  | December 31, |
|  | 2017 |  |  | 2016 |
Machinery, equipment and tooling |  | $ | 8,242 |  |  | $ | 4,841 |
Vehicles |  |  | 1,546 |  |  |  | 953 |
Furniture and fixtures |  |  | 720 |  |  |  | 305 |
Plant and building |  |  | 81,561 |  |  |  | 64,390 |
Real estate properties |  |  | 4,432 |  |  |  | 3,503 |
Railroad and sidings |  |  | 10,254 |  |  |  | 7,927 |
Land and improvements |  |  | 16,378 |  |  |  | 13,317 |
Asset retirement obligation |  |  | 8,408 |  |  |  | 1,324 |
Mineral properties |  |  | 9,878 |  |  |  | 9,785 |
Deferred mining costs |  |  | 657 |  |  |  | 417 |
Construction in progress |  |  | 56,493 |  |  |  | 16,715 |
|  |  | 198,569 |  |  |  | 123,477 |
Less: accumulated depreciation and depletion |  |  | 26,367 |  |  |  | 19,381 |
Total property, plant and equipment, net |  | $ | 172,202 |  |  | $ | 104,096 |
Depreciation expense was $7,267, $6,441 and $5,276 for the years ended December 31, 2017, 2016 and 2015, respectively. Depletion expense was $33, $3 and $13 for the years ended December 31, 2017, 2016 and 2015, respectively. The Company capitalized $139 and $1,808 of interest expense associated with the construction of new plant and equipment for the years ended December 31, 2016 and 2015, respectively. No amounts of interest expense were capitalized in the year ended December 31, 2017.

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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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -URI http://asc.fasb.org/topic&trid=2155823 + Details
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