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CONCENTRATION OF RISK: (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
CONCENTRATION OF RISK:      
Number of open-pit copper mines 4    
Number of underground poly metallic mines 5    
Number of smelters 2    
Number of refineries 8    
Cash equivalents
      
Concentration of risk      
Total cash and short-term investment $ 1,881.0    
Percentage of total cash 100.00%    
Percentage of cash in US dollars 98.70%    
Cash equivalents | United States
      
Concentration of risk      
Total cash and short-term investment 1,020.0    
Percentage of total cash 54.20%    
Percentage invested in one institution of country 45.80%    
Percentage invested in one institution of total cash 24.90%    
Cash equivalents | Peru
      
Concentration of risk      
Total cash and short-term investment 94.0    
Percentage of total cash 5.00%    
Percentage invested in one institution of country 73.70%    
Percentage invested in one institution of total cash 3.70%    
Cash equivalents | Mexican Operations
      
Concentration of risk      
Total cash and short-term investment 147.1    
Percentage of total cash 7.80%    
Percentage invested in one institution of country 95.10%    
Percentage invested in one institution of total cash 7.40%    
Cash equivalents | Switzerland
      
Concentration of risk      
Total cash and short-term investment $ 619.9    
Percentage of total cash 33.00%    
Percentage invested in one institution of country 100.00%    
Percentage invested in one institution of total cash 33.00%    
Accounts receivable trade | Five largest customers
      
Concentration of risk      
Percentage of accounts receivable and total sales 37.10% 40.00% 35.90%
Accounts receivable trade | Largest customer
      
Concentration of risk      
Percentage of accounts receivable and total sales 12.20% 10.40% 10.80%
Total sales | Five largest customers
      
Concentration of risk      
Percentage of accounts receivable and total sales 28.70% 28.80% 28.70%
Total sales | Largest customer
      
Concentration of risk      
Percentage of accounts receivable and total sales 8.40% 7.40% 7.30%