Summary of significant accounting policies - Types of Assets (Details)	12 Months Ended
	Dec. 31, 2017
Production Vessels and Kilns, Storage Tanks, Piping
Property, Plant and Equipment
Percentage of machinery and equipment	59.00%
PPE useful life (in years)	20 years
Control systems, Instrumentation, Metering Equipment
Property, Plant and Equipment
Percentage of machinery and equipment	12.00%
PPE useful life (in years)	15 years
Blending Equipment, Storage Tanks, Piping, Shipping Equipment and Platforms, Safety Equipment
Property, Plant and Equipment
Percentage of machinery and equipment	8.00%
Blending Equipment, Storage Tanks, Piping, Shipping Equipment and Platforms, Safety Equipment | Minimum
Property, Plant and Equipment
PPE useful life (in years)	25 years
Blending Equipment, Storage Tanks, Piping, Shipping Equipment and Platforms, Safety Equipment | Maximum
Property, Plant and Equipment
PPE useful life (in years)	30 years
Production Control System Equipment and Hardware, Laboratory Testing Equipment
Property, Plant and Equipment
Percentage of machinery and equipment	18.00%
Production Control System Equipment and Hardware, Laboratory Testing Equipment | Minimum
Property, Plant and Equipment
PPE useful life (in years)	5 years
Production Control System Equipment and Hardware, Laboratory Testing Equipment | Maximum
Property, Plant and Equipment
PPE useful life (in years)	10 years
Machinery & equipment support structures and foundations
Property, Plant and Equipment
Percentage of machinery and equipment	3.00%
PPE useful life (in years)	40 years