COMMITMENTS AND CONTINGENCIES: (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended		0 Months Ended	9 Months Ended		12 Months Ended	9 Months Ended				12 Months Ended	43 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Jan. 02, 2009 Peruvian operations	Sep. 30, 2013 Peruvian operations	Sep. 30, 2012 Peruvian operations	Dec. 31, 2007 Peruvian operations	Sep. 30, 2013 Peruvian operations Minimum	Sep. 30, 2013 Peruvian operations Maximum	Sep. 30, 2013 Mexican Operations	Sep. 30, 2012 Mexican Operations	Dec. 31, 2011 Mexican Operations person category	Sep. 30, 2013 Mexican Operations
Environmental costs
Environmental capital expenditure	$ 33.2	$ 16.8		$ 2.9	$ 3.0				$ 30.3	$ 13.8
Cost of the modernization of the smelter						570.0
Air quality standards for sulfur dioxide as per environmental regulation (in micrograms per cubic meter)			80	20
World air quality standards for sulfur dioxide (in micrograms per cubic meter)							105	250
Sulfur dioxide recapture percentage before modernization of Ilo smelter						33.00%
Sulfur dioxide recapture percentage after modernization of Ilo smelter						92.00%
Sulfur dioxide recapture percentage Ilo smelter				95.00%
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												35.7
Plant demolition and soil cost recognized												$ 33.9