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COMMITMENTS AND CONTINGENCIES: - Environmental Costs (Details)
$ in Millions
 1 Months Ended 3 Months Ended 6 Months Ended 12 Months Ended
Mar. 31, 2014
Sep. 30, 2016
lawsuit
Jun. 30, 2018
USD ($)
lawsuit
Jun. 30, 2017
USD ($)
Jun. 30, 2015
lawsuit
item
Dec. 31, 2015
lawsuit
item
Dec. 31, 2011
category
person
Dec. 31, 2017
lawsuit
Dec. 31, 2016
item
Environmental costs                  
Environmental capital investment | $     $ 44.5 $ 80.3          
Number of collective action lawsuits         6        
Number of subsidiaries against which lawsuits were filed | item         2        
Number of collective action lawsuits dismissed         2        
Remaining number of lawsuits         4        
Number of civil actions seeking damages               33  
Number of constitutional action lawsuits           4      
Number of subsidiaries to which environmental impact authorizations were granted | item           1      
Buenavista del Cobre, S.A. de C.V                  
Environmental costs                  
Number of civil actions seeking damages | item           8     3
Peru                  
Environmental costs                  
Environmental capital investment | $     18.0 44.5          
Number of months for identification of contaminated sites and around in facilities 12 months                
Period for preparation of decontamination plan P30M                
Period of extension for preparation of decontamination plan 1 year                
Number of days in which results of soil confirmation tests are to be presented to authorities 30 days                
Mexico                  
Environmental costs                  
Environmental capital investment | $     $ 26.5 $ 35.8          
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    
Francisca Garcia Enriquez                  
Environmental costs                  
Number of additional constitutional lawsuits filed           2      
Mario Alberto Salcido et al; Maria Elena Heredia Bustamante et al; Martin Eligio Ortiz Gamez et al and Maria de los Angeles Enriquez Bacame et al                  
Environmental costs                  
Number of additional constitutional lawsuits filed   4              
Norberto Bustamante et al                  
Environmental costs                  
Number of additional constitutional lawsuits filed     2