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Property, Plant and Equipment, Net - Schedule of Net Property, Plant and Equipment (Detail) - USD ($)
$ in Thousands
Mar. 31, 2021
Dec. 31, 2020
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross $ 363,521 $ 359,664
Less: accumulated depreciation and depletion 91,324 84,988
Total property, plant and equipment, net 272,197 274,676
Machinery, equipment and tooling    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 29,089 29,002
SmartSystems    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 24,067 22,352
Vehicles    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 2,899 2,893
Furniture and fixtures    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 1,325 1,302
Plant and building    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 199,923 199,867
Real estate properties    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 6,462 6,458
Railroad and sidings    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 27,703 27,703
Land and land improvements    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 33,155 33,040
Asset retirement obligation    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 20,730 19,993
Mineral properties    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 7,442 7,442
Deferred mining costs    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 2,425 2,123
Construction in progress    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross $ 8,301 $ 7,489