Critical and Significant Accounting Policies (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Exploration costs included in cost of product sold
Exploration Costs	$ 985	$ 1,179	$ 1,778
Building And Building Improvements [Member] | Minimum [Member]
Property, Plant And Equipment [Line Items]
Plant and equipment, estimated useful life	5 years
Building And Building Improvements [Member] | Maximum [Member]
Property, Plant And Equipment [Line Items]
Plant and equipment, estimated useful life	25 years
Machinery And Equipment [Member] | Minimum [Member]
Property, Plant And Equipment [Line Items]
Plant and equipment, estimated useful life	3 years
Machinery And Equipment [Member] | Maximum [Member]
Property, Plant And Equipment [Line Items]
Plant and equipment, estimated useful life	20 years
Furniture And Fixtures [Member]
Property, Plant And Equipment [Line Items]
Plant and equipment, estimated useful life	3 years