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INCOME TAXES: (Details 2) - item
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Reconciliation of the statutory income tax rate to the effective tax rate      
Expected tax at U.S. statutory rate (as a percent) 35.00% 35.00% 35.00%
Foreign tax at other than statutory rate, net of foreign tax credit benefit (as a percent) 3.60% 3.60% 3.50%
Percentage depletion (as a percent) (5.90%) (5.20%) (5.00%)
Other permanent differences (as a percent) 0.70% 0.10% (0.20%)
Increase (decrease) in unrecognized tax benefits for uncertain tax positions (as a percent) 6.70% 0.50%  
Repatriated foreign earnings (as a percent)   (0.40%) (1.40%)
Amounts (over) under provided in prior years (as a percent) (2.20%) 2.20% 0.40%
Other (as a percent) 1.20% 0.70% 0.10%
Effective income tax rate 39.10% 36.50% 32.40%
Number of jurisdictions where company files income tax returns 3    
Duration for which statutory tax rates for Peru and Mexico are considered to determine effective tax rate 3 years    
Mexico      
Reconciliation of the statutory income tax rate to the effective tax rate      
Expected tax at U.S. statutory rate (as a percent) 30.00% 30.00% 30.00%
Peru      
Reconciliation of the statutory income tax rate to the effective tax rate      
Expected tax at U.S. statutory rate (as a percent) 28.00% 30.00% 30.00%