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INCOME TAXES: - Effective tax rate (Details) - item
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Reconciliation of the statutory income tax rate to the effective tax rate        
Expected tax at U.S. statutory rate (as a percent) 21.00% 21.00% 21.00% 35.00%
Foreign tax at other than statutory rate, net of foreign tax credit benefit (as a percent) 13.70% 14.00% 13.90%  
Percentage depletion (as a percent) (2.20%) (2.70%) (2.10%)  
Other permanent differences (as a percent) 0.10% 0.10% 0.40%  
Change in 2017 valuation allowance on U.S. deferred tax assets, foreign tax credits and U.S. tax effect on Peruvian deferred taxes     7.50%  
Additional valuation allowance on U.S. deferred tax assets, foreign tax credits and U.S. tax effect on Peruvian deferred taxes 8.90% 10.10% 1.00%  
Decrease in U.S. deferred tax asset due to tax rate changes     (0.20%)  
U.S. one time transition tax on accumulated foreign earnings     0.40%  
Increase (decrease) in unrecognized tax benefits for uncertain tax positions (as a percent) (0.10%) (2.10%)    
Amounts (over) / under provided in prior years (0.10%) (1.10%) (1.30%)  
Other (as a percent) 1.50% (0.30%) 0.10%  
Effective income tax rate 42.80% 39.00% 40.70%  
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) 29.50% 29.50% 29.50%