SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES: (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011 segment dollarsperpound Y	Dec. 31, 2010 dollarsperpound	Dec. 31, 2009 dollarsperpound
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES:
Estimated useful lives of buildings and equipment, low end of range (in years)	5
Estimated useful lives of buildings and equipment, high end of range (in years)	40
Increase in total reserves at a property to qualify as a major expansion (as a percent)	10.00%
Period considered for calculation of average metal price (in years)	3
Current price of copper (in dollars per pound)	3.26	2.97	2.90
Long-term price assumptions for copper for internal ore reserve estimation		1.80	1.80
Long-term price assumptions for molybdenum for internal ore reserve estimation		11.00	11.00
Long-term price assumption for copper for internal ore reserve estimation to reflect the changes in market trends	2.00
Long-term price assumptions for molybdenum for internal ore reserve estimation to reflect the changes in market trends	12.00
Number of years of average prices for copper used for ore reserve estimation	10
Number of years of average prices for molybdenum used for ore reserve estimation	15
Average price of copper (in dollars per pound)	3.00
Average price of molybdenum (in dollars per pound)	14.74
Number of years of average prices for copper and molybdenum used for impairment analysis	3
Number of reportable segments	3
Financial information related to segments
Total property, net	$ 4,419,885	$ 4,094,993	$ 3,969,600
Maximum
Financial information related to segments
Maximum period after shipping within which pricing is based by customer contracts in most cases (in months)	3
Maximum maturity period of cash equivalents (in months)	3
Percentage applied in measuring hedge effectiveness	125.00%
Minimum
Financial information related to segments
Percentage applied in measuring hedge effectiveness	80.00%
Mine development
Financial information related to segments
Total property, net	39,800	42,100
Mexican IMMSA Unit
Financial information related to segments
Number of underground poly metal mines	5
Total property, net	320,100	296,300	278,400
Peruvian Operations
Financial information related to segments
Number of open-pit copper mines	2
Total property, net	$ 2,225,900	$ 2,164,700	$ 2,026,300