INCOME TAXES: - Effective tax rate (Details) - item	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
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%
Foreign tax at other than statutory rate, net of foreign tax credit benefit (as a percent)	14.70%	13.80%	13.70%
Percentage depletion (as a percent)	(2.10%)	(1.80%)	(2.20%)
Other permanent differences (as a percent)	(0.10%)	(0.30%)	0.10%
Additional valuation allowance on U.S. deferred tax assets, foreign tax credits and U.S. tax effect on Peruvian deferred taxes	5.50%	8.40%	8.90%
Increase (decrease) in unrecognized tax benefits for uncertain tax positions (as a percent)	1.50%	(0.50%)	(0.10%)
Amounts (over) / under provided in prior years	(1.70%)		(0.10%)
Other (as a percent)	(1.20%)	(0.20%)	1.50%
Effective income tax rate	37.60%	40.40%	42.80%
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%