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INCOME TAXES: (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Income Taxes        
Statutory income tax provision     $ 698.5 $ 473.0
GILTI tax     15.2  
Peruvian royalty     6.7 0.5
Mexican royalty     60.9 67.7
Peruvian special mining tax     22.3 15.4
Total income tax provision $ 257.9 $ 220.1 $ 803.6 $ 556.6
Effective income tax rate (as a percent)     39.20% 35.70%
Changes in the entity's uncertain tax positions     $ 0.0  
Peru        
Income Taxes        
Peruvian royalty     $ 6.7 $ 0.5
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     $ 24.2 15.4
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     $ 22.3 15.4
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     $ 60.9 $ 67.7
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%