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COMMITMENTS AND CONTINGENCIES: - Environmental matters (Details)
$ in Millions
6 Months Ended 12 Months Ended
Jun. 30, 2020
USD ($)
Jun. 30, 2019
USD ($)
Dec. 31, 2011
category
person
Environmental costs      
Environmental capital investment $ 13.9 $ 32.3  
Peru      
Environmental costs      
Environmental capital investment   13.9  
Environmental capital investment (5.5)    
Mexico      
Environmental costs      
Environmental capital investment $ 19.4 $ 18.4  
Number of categories of collective actions | category     3
Minimum number of people claiming injury due to collective action initiative in Civil Federal Procedures Code (CFPC) | person     30