INCOME TAXES: - Effective tax rate (Details) - item	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of the statutory income tax rate to the effective tax rate
Expected tax at U.S. statutory rate (as a percent)	21.00%	35.00%	35.00%
Foreign tax at other than statutory rate, net of foreign tax credit benefit (as a percent)	13.90%	0.60%	2.00%
Percentage depletion (as a percent)	(2.10%)	(2.90%)	(2.70%)
Other permanent differences (as a percent)	0.40%	5.20%
Change in 2017 valuation allowance on U.S. deferred tax assets, foreign tax credits and U.S. tax effect on Peruvian deferred	7.50%	26.90%
Additional valuation allowance on 2018 U.S. deferred tax assets, foreign tax credits and U.S. tax effect on Peruvian deferred	1.00%
Decrease in U.S. deferred tax asset due to tax rate changes	(0.20%)	4.80%
U.S. one time transition tax on accumulated foreign earnings	0.40%	2.40%
Increase (decrease) in unrecognized tax benefits for uncertain tax positions (as a percent)		0.70%	0.20%
Repatriated foreign earnings (as a percent)		(2.00%)	3.60%
Amounts (over) / under provided in prior years	(1.30%)	(0.10%)	(0.50%)
Other (as a percent)	0.10%	(1.40%)	2.30%
Effective income tax rate	40.70%	69.20%	39.90%
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%	28.00%