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CONCENTRATION OF RISK: (Details)
$ in Millions
12 Months Ended
Dec. 31, 2019
USD ($)
item
Dec. 31, 2018
Dec. 31, 2017
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 $ 2,005.8    
Percentage of total cash 100.00%    
Percentage of cash in US dollars 97.40%    
Cash equivalents | United States      
Concentration of risk      
Total cash and short-term investment $ 1,726.7    
Percentage of total cash 86.10%    
Percentage invested in one institution of country 14.20%    
Percentage invested in one institution of total cash 12.20%    
Cash equivalents | Switzerland      
Concentration of risk      
Total cash and short-term investment $ 126.3    
Percentage of total cash 6.30%    
Percentage invested in one institution of country 100.00%    
Percentage invested in one institution of total cash 6.30%    
Cash equivalents | Peru      
Concentration of risk      
Total cash and short-term investment $ 24.6    
Percentage of total cash 1.20%    
Percentage invested in one institution of country 72.50%    
Percentage invested in one institution of total cash 0.90%    
Cash equivalents | Mexico      
Concentration of risk      
Total cash and short-term investment $ 128.2    
Percentage of total cash 6.40%    
Percentage invested in one institution of country 87.00%    
Percentage invested in one institution of total cash 5.60%    
Accounts receivable trade | Five largest customers      
Concentration of risk      
Percentage of accounts receivable and total sales 37.60% 34.20% 28.40%
Accounts receivable trade | Largest customer      
Concentration of risk      
Percentage of accounts receivable and total sales 13.00% 10.90% 8.20%
Total sales | Five largest customers      
Concentration of risk      
Percentage of accounts receivable and total sales 28.00% 28.40% 25.10%
Total sales | Largest customer      
Concentration of risk      
Percentage of accounts receivable and total sales 10.40% 9.30% 8.80%