INCOME TAXES: - Tax assets and liabilities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Assets:
Inventories	$ 61.5	$ 47.0
Capitalized exploration expenses	11.1	9.5
U.S. foreign tax credit carryforward, net of FIN 48 liability	483.4	280.2
U.S. tax effect of Peruvian deferred tax liability	165.1	174.1
Provisions	155.0	152.6
Mexican tax on consolidated dividends	14.8	14.5
Deferred workers participation	27.0	3.0
Accrued salaries, wages and vacations	9.1	7.2
Sales price adjustment (PUI)	2.7
Social responsibility expenses	2.1	2.1
Deferred charges	6.1
Valuation allowance on U.S. deferred tax assets, foreign tax credits and U.S. tax effect on Peruvian deferred	(819.1)	(619.6)
Other	10.1	1.6
Total deferred tax assets	128.9	72.2
Liabilities:
Property, plant and equipment	(137.3)	(130.3)
Deferred charges		(16.9)
Sales price adjustment (PUI)		(0.4)
Other	(0.4)	(5.3)
Total deferred tax liabilities	(137.7)	(152.9)
Total net deferred tax (liabilities) / assets	(8.8)	$ (80.7)
Increase in valuation allowance	$ 199.5
U.S. statutory rate (as a percent)	21.00%	35.00%	35.00%