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INCOME TAXES: (Details 2)
12 Months Ended
Dec. 31, 2013
item
Dec. 31, 2012
Dec. 31, 2011
Reconciliation of the statutory income tax rate to the effective tax rate      
Expected tax (as a percent) 30.00% 30.00% 30.00%
Effect of income taxed at a rate other than the statutory rate (as a percent) 1.70% 1.60% 4.30%
Percentage depletion (5.00%) (4.20%) (4.00%)
Other permanent differences (as a percent) 3.50% 3.70% 1.20%
Peru tax on net income deemed distributed (as a percent) 1.20% 1.30% 1.30%
Special mining tax 2.00% 1.60% 0.50%
Increase (decrease) in unrecognized tax benefits for uncertain tax positions (as a percent)   5.00%  
Repatriated foreign earnings (as a percent) (1.40%) (1.70%) 2.10%
Amounts (over) / under provided in prior years (as a percent) 0.40% (0.60%) (3.50%)
Other (as a percent)   (0.40%) 0.10%
Effective income tax rate (as a percent) 32.40% 36.30% 32.00%
Number of jurisdictions where company files income tax returns 3    
U.S. statutory rate (as a percent) 35.00% 35.00% 35.00%
Duration for which statutory tax rates for Peru and Mexico are considered to determine effective tax rate 3 years