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CONCENTRATION OF RISK: (Details)
$ in Millions
12 Months Ended
Dec. 31, 2020
USD ($)
item
Dec. 31, 2019
Dec. 31, 2018
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    
Total cash and short-term investment $ 2,594.4    
Cash equivalents      
Concentration of risk      
Total cash and short-term investment $ 2,594.4    
Percentage of total cash 100.00%    
Percentage of cash in US dollars 97.00%    
Cash equivalents | United States      
Concentration of risk      
Total cash and short-term investment $ 1,968.8    
Percentage of total cash 75.90%    
Percentage invested in one institution of country 14.10%    
Percentage invested in one institution of total cash 10.70%    
Cash equivalents | Switzerland      
Concentration of risk      
Total cash and short-term investment $ 340.3    
Percentage of total cash 13.10%    
Percentage invested in one institution of country 79.40%    
Percentage invested in one institution of total cash 10.40%    
Cash equivalents | Peru      
Concentration of risk      
Total cash and short-term investment $ 35.5    
Percentage of total cash 1.40%    
Percentage invested in one institution of country 81.40%    
Percentage invested in one institution of total cash 1.10%    
Cash equivalents | Mexico      
Concentration of risk      
Total cash and short-term investment $ 249.8    
Percentage of total cash 9.60%    
Percentage invested in one institution of country 53.80%    
Percentage invested in one institution of total cash 5.20%    
Accounts receivable trade | Five largest customers      
Concentration of risk      
Percentage of accounts receivable and total sales 35.30% 37.60% 34.20%
Accounts receivable trade | Largest customer      
Concentration of risk      
Percentage of accounts receivable and total sales 12.90% 13.00% 10.90%
Total sales | Five largest customers      
Concentration of risk      
Percentage of accounts receivable and total sales 28.10% 28.00% 28.40%
Total sales | Largest customer      
Concentration of risk      
Percentage of accounts receivable and total sales 10.00% 10.40% 9.30%