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COMMITTMENTS AND CONTINGENCIES: (Details 3) (USD $)
1 Months Ended9 Months Ended
Sep. 30, 2010
Year
Worker
Sep. 30, 2011
Union
Peruvian operations.
  
Labor matters  
Percentage of labor unionized (as a percent) 60.00%
Number of labor unions 8
Number of labor unions represent majority of workers 3
Number of labor unions at each production area represent majority of workers 1
Term of collective bargaining agreement (in years)3 
Percentage of Annual salary increase as per collective bargaining agreement (as a percent)5.00% 
Signing bonus given to each worker as per collective bargaining agreement$ 6,700 
Number of workers covered under collective bargaining agreement2,000 
Number of labor unions other than majority workers unions 5
Mexican operations
  
Labor matters  
Percentage of labor unionized (as a percent) 72.30%
Number of labor unions 3
Period of other benefits negotiated under collective bargaining agreement (in years) 2
Mexican operations | Buenavista mine
  
Labor matters  
Number of workers covered under collective bargaining agreement 780
Mine rehabilitation and reconstruction cost 164,000,000
Mine rehabilitation and reconstruction cost capitalized 97,500,000
Mine rehabilitation and reconstruction cost charged to operating cost 66,500,000
Significant total estimated cost of repairs of Buenavista mine 202,000,000
Number of workers operating in mine and plants as well as in growth program 2,100
Original production capacity of copper (in tons) 180,000
Expected production capacity of copper (in tons) 450,000
Investment in expansion program $ 3,700,000,000