INCOME TAXES: - Tax assets and liabilities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Assets:
Inventories	$ 62.4	$ 55.7
Capitalized exploration expenses	13.0	14.0
U.S. foreign tax credit carryforward, net of FIN 48 liability	974.1	731.8
U.S. tax effect of Peruvian deferred tax liability	111.2	131.3
Provisions	169.5	145.0
Mexican tax on consolidated dividends		9.0
Deferred workers participation	37.8	24.7
Accrued salaries, wages and vacations	8.5	10.0
Sales price adjustment (PUI)	(5.8)	3.0
Social responsibility expenses	4.3	2.1
Deferred charges	41.8	14.5
Valuation allowance on U.S. deferred tax assets, foreign tax credits and U.S. tax effect on Peruvian deferred	(1,343.2)	(1,097.0)
Other	19.5	9.9
Total deferred tax assets	93.1	54.0
Liabilities:
Property, plant and equipment	(17.2)	(45.4)
Other	(5.3)	(3.0)
Total deferred tax liabilities	(22.5)	(48.4)
Total net deferred tax assets	70.6	$ 5.6
Increase in valuation allowance	$ 246.2
U.S. statutory rate (as a percent)	21.00%	21.00%	21.00%	35.00%