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INCOME TAXES: (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Income Taxes    
Statutory income tax provision $ 214.7 $ 212.3
Peruvian royalty 0.4 1.7
Mexican royalty 16.7 16.1
Peruvian special mining tax 6.1 6.5
Total income tax provision $ 237.9 $ 236.6
Effective income tax rate (as a percent) 38.10% 33.60%
Changes in the entity's uncertain tax positions $ 0.0  
Peru    
Income Taxes    
Peruvian royalty $ 0.4 $ 1.7
Increment in operating income margin for royalty tax (as a percent) 5.00%  
Increase in royalty tax rate for each 5% increase up to 12% increase in operating income margin (as a percent) 0.75%  
Royalty charges $ 6.0 7.1
Increase in special mining tax rate for each 5% increase up to 85% increase in operating income margin (as a percent) 0.40%  
Increment in operating income margin (as a percent) 5.00%  
Accrued special mining tax $ 6.1 6.5
Peru | Minimum    
Income Taxes    
Mining royalty tax (as a percent) 1.00%  
Royalty charge assessed as a percentage of net sales 1.00%  
Special mine tax (as a percent) 2.00%  
Increment in operating income margin (as a percent) 2.00%  
Peru | Maximum    
Income Taxes    
Mining royalty tax (as a percent) 12.00%  
Operating income margin for royalty tax (as a percent) 10.00%  
Increment in operating income margin for royalty tax (as a percent) 12.00%  
Special mine tax (as a percent) 8.40%  
Operating income margin (as a percent) 10.00%  
Increment in operating income margin (as a percent) 85.00%  
Mexico    
Income Taxes    
Mexican royalty $ 16.7 $ 16.1
Mining royalty tax (as a percent) 7.50%  
Additional royalty tax over net sales from sales of gold, silver and platinum (as a percent) 0.50%  
Royalty expense paid $ 85.9