XML 77 R56.htm IDEA: XBRL DOCUMENT v3.8.0.1
INCOME TAXES: - Effective tax rate (Details) - item
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
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) 0.60% 2.00% 3.60%
Percentage depletion (as a percent) (2.90%) (2.70%) (5.90%)
Other permanent differences (as a percent) 5.20%   0.70%
Valuation allowance on US deferred tax assets, foreign tax credits and US tax effect on Peruvian deferred 26.90%    
Decrease in US deferred tax asset due to tax rate changes 4.80%    
US one time transition tax on accumulated foreign earnings 2.40%    
Increase (decrease) in unrecognized tax benefits for uncertain tax positions (as a percent) 0.70% 0.20% 6.70%
Repatriated foreign earnings (as a percent) (2.00%) 3.60%  
Amounts (over) / under provided in prior years (0.10%) (0.50%) (2.20%)
Other (as a percent) (1.40%) 2.30% 1.20%
Effective income tax rate 69.20% 39.90% 39.10%
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% 28.00% 28.00%