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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. 15, 2014
MXN
Sep. 15, 2014
USD ($)
Aug. 06, 2014
m³
Dec. 31, 2017
lawsuit
item
m³
Aug. 31, 2014
item
Mar. 31, 2014
Sep. 30, 2016
lawsuit
Jun. 30, 2015
lawsuit
item
Dec. 31, 2017
USD ($)
lawsuit
item
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
lawsuit
item
Dec. 31, 2011
category
person
Mar. 31, 2016
lawsuit
Mar. 02, 2015
USD ($)
Environmental costs                            
Environmental capital investment | $                 $ 222.6 $ 250.4 $ 120.8      
Number of collective action lawsuits               6            
Number of subsidiaries against which lawsuits were filed               2            
Number of collective action lawsuits dismissed               2            
Remaining number of lawsuits | lawsuit               4            
Number of civil actions seeking damages | lawsuit       33         33          
Number of constitutional action lawsuits                     4      
Number of subsidiaries to which environmental impact authorizations were granted                     1      
Buenavista del Cobre, S.A. de C.V                            
Environmental costs                            
Volume of copper sulfate solution (in cubic meters) | m³     40,000                      
Excess provision credited to income on closure of trust | $                 $ 10.2          
Estimated total damages | $                           $ 136.4
Number of civil actions seeking damages                     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         1,200                  
Peru                            
Environmental costs                            
Environmental capital investment | $                 $ 93.7 110.3 $ 98.8      
Number of atmospheric basins established that require further attention                 3          
AQS, new hours       24                    
AQS, new micrograms of SO2 | m³       250                    
AQS, old hours       24                    
Revised air quality standards for sulfur dioxide as per new environmental regulation (in micrograms per cubic meter) | m³       20                    
AQS, new micrograms of gaseous mercury | m³       2                    
Mean monthly AQS | m³       1.5                    
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 | $                 $ 128.9 $ 140.1 $ 22.0      
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                            
Remaining number of Collective action lawsuits | lawsuit                     2      
Number of additional constitutional lawsuits filed | lawsuit               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 constitutional action lawsuits | lawsuit             4