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Summary of Significant Accounting Policies - Estimated Useful Life of Property, Plant and Equipment (Detail)
9 Months Ended
Sep. 30, 2017
Land and Improvements  
Property Plant And Equipment [Line Items]  
Property, plant and equipment, useful life 10 years
Plant and Building | Minimum  
Property Plant And Equipment [Line Items]  
Property, plant and equipment, useful life 5 years
Plant and Building | Maximum  
Property Plant And Equipment [Line Items]  
Property, plant and equipment, useful life 15 years
Real Estate Properties | Minimum  
Property Plant And Equipment [Line Items]  
Property, plant and equipment, useful life 10 years
Real Estate Properties | Maximum  
Property Plant And Equipment [Line Items]  
Property, plant and equipment, useful life 40 years
Rail Spur  
Property Plant And Equipment [Line Items]  
Property, plant and equipment, useful life 30 years
Vehicles | Minimum  
Property Plant And Equipment [Line Items]  
Property, plant and equipment, useful life 3 years
Vehicles | Maximum  
Property Plant And Equipment [Line Items]  
Property, plant and equipment, useful life 5 years
Machinery, Equipment and Tooling | Minimum  
Property Plant And Equipment [Line Items]  
Property, plant and equipment, useful life 3 years
Machinery, Equipment and Tooling | Maximum  
Property Plant And Equipment [Line Items]  
Property, plant and equipment, useful life 15 years
Furniture and Fixtures | Minimum  
Property Plant And Equipment [Line Items]  
Property, plant and equipment, useful life 3 years
Furniture and Fixtures | Maximum  
Property Plant And Equipment [Line Items]  
Property, plant and equipment, useful life 10 years
Deferred Mining Costs  
Property Plant And Equipment [Line Items]  
Property, plant and equipment, useful life 3 years