COMMITMENTS AND CONTINGENCIES: (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		1 Months Ended	3 Months Ended		12 Months Ended	0 Months Ended	3 Months Ended			12 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014 Peruvian operations	Mar. 31, 2014 Peruvian operations item	Mar. 31, 2013 Peruvian operations	Dec. 31, 2013 Peruvian operations	Apr. 08, 2014 Peruvian operations Subsequent event	Mar. 31, 2014 Peruvian operations Maximum	Mar. 31, 2014 Mexican operations	Mar. 31, 2013 Mexican operations	Dec. 31, 2011 Mexican operations person category
Environmental costs
Environmental capital expenditure	$ 16.2	$ 12.3		$ 9.8	$ 1.8				$ 6.4	$ 10.5
Period required for ambient air monitoring as per environmental regulation						12 months
Revised air quality standards for sulfur dioxide as per new environmental regulation (in micrograms per cubic meter)								20
Air quality standards level for sulfur dioxide to be maintained when emission is excess than specified as per new environmental regulation (in micrograms per cubic meter)				80
Number of atmospheric basins established that require further attention				3
Number of months for identification of contaminated sites and around in facilities			12 months
Period for preparation of decontamination plan			P24M
Period of extension for preparation of decontamination plan			1 year
Number of days in which results of soil confirmation tests are to be presented to authorities			30 days
Period within which the plan must be reported to the government							1 year
Number of categories of collective actions											3
Minimum number of people claiming injury due to collective action initiative in Civil Federal Procedures Code (CFPC)											30