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CONCENTRATION OF RISK: (Details)
$ in Millions
12 Months Ended
Dec. 31, 2018
USD ($)
item
Dec. 31, 2017
Dec. 31, 2016
Concentration of risk      
Number of open-pit copper mines | item 4    
Number of underground poly metallic mines | item 5    
Number of smelters | item 2    
Number of refineries | item 10    
Cash equivalents      
Concentration of risk      
Total cash and short-term investment $ 1,058.4    
Percentage of total cash 100.00%    
Percentage of cash in US dollars 95.80%    
Cash equivalents | United States      
Concentration of risk      
Total cash and short-term investment $ 697.1    
Percentage of total cash 65.90%    
Percentage invested in one institution of country 27.70%    
Percentage invested in one institution of total cash 18.30%    
Cash equivalents | Switzerland      
Concentration of risk      
Total cash and short-term investment $ 315.7    
Percentage of total cash 29.80%    
Percentage invested in one institution of country 61.20%    
Percentage invested in one institution of total cash 18.30%    
Cash equivalents | Peru      
Concentration of risk      
Total cash and short-term investment $ 7.6    
Percentage of total cash 0.70%    
Percentage invested in one institution of country 75.60%    
Percentage invested in one institution of total cash 0.50%    
Cash equivalents | Mexico      
Concentration of risk      
Total cash and short-term investment $ 38.0    
Percentage of total cash 3.60%    
Percentage invested in one institution of country 89.30%    
Percentage invested in one institution of total cash 3.20%    
Accounts receivable trade | Five largest customers      
Concentration of risk      
Percentage of accounts receivable and total sales 34.20% 28.40% 31.10%
Accounts receivable trade | Largest customer      
Concentration of risk      
Percentage of accounts receivable and total sales 10.90% 8.20% 11.90%
Total sales | Five largest customers      
Concentration of risk      
Percentage of accounts receivable and total sales 28.40% 25.10% 30.40%
Total sales | Largest customer      
Concentration of risk      
Percentage of accounts receivable and total sales 9.30% 8.80% 8.40%