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COMMITTMENTS AND CONTINGENCIES: (Details 3) (USD $)
 1 Months Ended 12 Months Ended
Sep. 30, 2010
person
Dec. 31, 2011
union
Peru
    
Labor matters    
Percentage of labor unionized   61.00%
Number of workers who were unionized   4,159
Number of labor unions represent majority of workers   3
Number of labor unions at each production area represent majority of workers   1
Percentage of annual salary increase as per collective bargaining agreement 5.00%  
Bonus given to each workers as per collective bargaining agreement $ 6,700  
Number of workers covered under collective bargaining agreement 2,000  
Number of labor unions other than majority workers unions   5
Mexican Operations.
    
Labor matters    
Percentage of labor unionized   75.00%
Number of workers who were unionized   7,975
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   212,800,000
Mine rehabilitation and reconstruction cost capitalized   131,700,000
Mine rehabilitation and reconstruction cost charged to operating cost   81,100,000