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COMMITMENTS AND CONTINGENCIES: - Environmental Costs (Details)
$ in Millions, MXN in Billions
 1 Months Ended 3 Months Ended 6 Months Ended 12 Months Ended
Sep. 26, 2016
lawsuit
Sep. 15, 2014
MXN
Sep. 15, 2014
USD ($)
Apr. 30, 2017
item
m³
Mar. 31, 2014
Mar. 31, 2017
USD ($)
lawsuit
item
Sep. 30, 2016
lawsuit
Mar. 31, 2016
USD ($)
Jun. 30, 2015
lawsuit
item
Dec. 31, 2015
USD ($)
lawsuit
item
Dec. 31, 2014
item
m³
Dec. 31, 2011
category
person
Dec. 31, 2016
lawsuit
Environmental costs                          
Environmental capital investment | $           $ 38.6   $ 39.2          
Number of collective action lawsuits                 6        
Number of subsidiaries against which lawsuits were filed | item                 2        
Number of lawsuits dismissed                 4        
Remaining number of Collective action lawsuits                 2        
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                          
Volume of copper sulfate solution (in cubic meters) | m³                     40,000    
Excess provision reversed on closure of trust | $           $ 10.2              
Estimated total damages | $                   $ 136.4      
Number of civil actions seeking damages           26       8     3
Buenavista del Cobre, S.A. de C.V | Maximum                          
Environmental costs                          
Amount committed to the Mexican Federal Government to establish a trust   MXN 2 $ 150.0                    
Buenavista del Cobre, S.A. de C.V | Minimum                          
Environmental costs                          
Number of workers to clean the river | item                     1,200    
Peru                          
Environmental costs                          
Environmental capital investment | $           $ 16.5   10.1          
Number of atmospheric basins established that require further attention | item           3              
AQS, new hours | item       24                  
AQS, new micrograms | m³       250                  
AQS, old hours | item                     24    
Revised air quality standards for sulfur dioxide as per new environmental regulation (in micrograms per cubic meter) | m³                     20    
Number of months for identification of contaminated sites and around in facilities         12 months                
Period for preparation of decontamination plan         P24M                
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 | $           $ 22.1   $ 29.1          
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  
Acciones Colectivas de Sinaloa                          
Environmental costs                          
Number of collective action lawsuits                 2        
Number of lawsuits dismissed 1                        
Francisca Garcia Enriquez                          
Environmental costs                          
Number of constitutional action lawsuits                   2      
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             3