COMMITMENTS AND CONTINGENCIES: (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended			46 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011 category person	Dec. 31, 2013
Environmental costs
Environmental capital expenditure	$ 60.7	$ 27.4	$ 14.0
Peruvian operations
Environmental costs
Environmental capital expenditure	20.9	6.7	2.5
Period required for ambient air monitoring as per environmental regulation	12 months
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
Period for the soil decontamination plan, as a result of any contamination being identified	3 years
Number of months after the issuance of the regulations to update approved environmental programs to comply with the requirements of the rule	12 months
Peruvian operations | Maximum
Environmental costs
Revised air quality standards for sulfur dioxide as per new environmental regulation (in micrograms per cubic meter)	20
Mexican operations
Environmental costs
Environmental capital expenditure	39.8	20.7	11.5
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
Plant demolition and soil remediation budgeted cost				62.4
Plant demolition and soil cost recognized				$ 35.7