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INCOME TAXES: (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Dec. 31, 2015
Dec. 31, 2014
Income Taxes        
Statutory income tax provision $ 82.6 $ 111.2    
Peruvian royalty   0.5    
Mexican royalty 9.3 13.7    
Peruvian special mining tax 0.4 3.8    
Income tax provision $ 92.3 $ 129.2    
Effective income tax rate (as a percent) 33.80% 31.50%    
Increase in tax rate as effect of forecasted dividends 1.00%      
Changes in the entity's uncertain tax positions $ 0.0      
Peru        
Income Taxes        
Peruvian royalty $ 0.0   $ 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 $ 4.1 $ 5.7    
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 $ 0.4 3.8    
Income tax rate ( as a percent)       30.00%
Dividend tax rate       4.10%
Peru | Minimum        
Income Taxes        
Mining royalty tax (as a percent) 1.00%      
Royalty charge assessed as a percentage of net sales 1.00%      
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 $ 9.3 $ 13.7    
Mining royalty tax (as a percent) 7.50%      
Royalty charges $ 33.9      
Additional royalty tax over net sales from sales of gold, silver and platinum (as a percent) 0.50%      
2015 - 2016        
Income Taxes        
Income tax rate ( as a percent)     28.00%  
Dividend tax rate     6.80%  
2017 - 2018        
Income Taxes        
Income tax rate ( as a percent)     27.00%  
Dividend tax rate     8.00%  
2019 and later        
Income Taxes        
Income tax rate ( as a percent)     26.00%  
Dividend tax rate     9.30%