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PROPERTY, PLANT AND MINE DEVELOPMENT - Geographic Information (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Property, plant and mine development      
Property, plant and equipment $ 5,626,552 $ 5,106,036 $ 5,088,967
Canada      
Property, plant and mine development      
Property, plant and equipment 3,730,809 3,266,594  
Finland      
Property, plant and mine development      
Property, plant and equipment 889,610 853,445  
Sweden      
Property, plant and mine development      
Property, plant and equipment 13,812 13,812  
Mexico      
Property, plant and mine development      
Property, plant and equipment 982,115 961,943  
United States      
Property, plant and mine development      
Property, plant and equipment $ 10,206 $ 10,242