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Property, Plant and Equipment, Net - Schedule of Net Property, Plant and Equipment (Detail) - USD ($)
$ in Thousands
Mar. 31, 2017
Dec. 31, 2016
Property Plant And Equipment [Line Items]    
Property, plant and equipment, gross $ 125,400 $ 123,477
Less: accumulated depreciation and depletion 20,997 19,381
Total property, plant and equipment, net 104,403 104,096
Machinery, Equipment and Tooling    
Property Plant And Equipment [Line Items]    
Property, plant and equipment, gross 5,474 4,841
Vehicles    
Property Plant And Equipment [Line Items]    
Property, plant and equipment, gross 1,035 953
Furniture and Fixtures    
Property Plant And Equipment [Line Items]    
Property, plant and equipment, gross 308 305
Plant and Building    
Property Plant And Equipment [Line Items]    
Property, plant and equipment, gross 64,439 64,390
Real Estate Properties    
Property Plant And Equipment [Line Items]    
Property, plant and equipment, gross 3,607 3,503
Railroad and Sidings    
Property Plant And Equipment [Line Items]    
Property, plant and equipment, gross 7,932 7,927
Land and Improvements    
Property Plant And Equipment [Line Items]    
Property, plant and equipment, gross 13,339 13,317
Asset Retirement Obligation    
Property Plant And Equipment [Line Items]    
Property, plant and equipment, gross 1,324 1,324
Mineral Properties    
Property Plant And Equipment [Line Items]    
Property, plant and equipment, gross 9,785 9,785
Deferred Mining Costs    
Property Plant And Equipment [Line Items]    
Property, plant and equipment, gross 417 417
Construction in Progress    
Property Plant And Equipment [Line Items]    
Property, plant and equipment, gross $ 17,740 $ 16,715