Significant Accounting Policies - Property and Equipment, Net (Details)	12 Months Ended
	Dec. 31, 2018
Railroad and equipment | Minimum
Property, plant and equipment
Estimated useful life (in years)	15 years
Railroad and equipment | Maximum
Property, plant and equipment
Estimated useful life (in years)	30 years
Furniture and fixtures | Minimum
Property, plant and equipment
Estimated useful life (in years)	5 years
Furniture and fixtures | Maximum
Property, plant and equipment
Estimated useful life (in years)	10 years
Machinery and equipment | Minimum
Property, plant and equipment
Estimated useful life (in years)	3 years
Machinery and equipment | Maximum
Property, plant and equipment
Estimated useful life (in years)	10 years
Office equipment | Minimum
Property, plant and equipment
Estimated useful life (in years)	5 years
Office equipment | Maximum
Property, plant and equipment
Estimated useful life (in years)	10 years
Autos and trucks
Property, plant and equipment
Estimated useful life (in years)	5 years