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Major Customers and Suppliers (Details) - supplier
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Sales | Customer concentration | Petroleum | Customer A      
Major Customers and Suppliers      
Concentration risk 19.00% 15.00% 14.00%
Sales | Customer concentration | Nitrogen Fertilizer      
Major Customers and Suppliers      
Concentration risk 16.00% 20.00% 24.00%
Sales | Customer concentration | Nitrogen Fertilizer | Customer B      
Major Customers and Suppliers      
Concentration risk 5.00% 10.00% 10.00%
Sales | Customer concentration | Nitrogen Fertilizer | Customer C      
Major Customers and Suppliers      
Concentration risk 11.00% 10.00% 14.00%
Contracted volume | Supplier concentration | Petroleum      
Major Customers and Suppliers      
Concentration risk 55.00% 61.00% 61.00%
Number of major suppliers 1 1 1