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DERIVATIVE INSTRUMENTS: (Details) (USD $)
 12 Months Ended 12 Months Ended 12 Months Ended
Dec. 31, 2011
mmbtu
Dec. 31, 2010
Dec. 31, 2011
Zero cost collar and swap contracts
Dec. 31, 2010
Zero cost collar contracts
Dec. 31, 2011
Zero cost collar contracts
2011
lbs
priceperunit
Dec. 31, 2011
Zero cost collar contracts
1st Quarter 2012
priceperunit
lbs
Dec. 31, 2011
Swap contracts
2011
lbs
priceperunit
Dec. 31, 2010
Swap contracts
2011
lbs
priceperunit
Dec. 31, 2009
Exchange rate derivatives, U.S. Dollar/ Mexican peso contracts
DERIVATIVE INSTRUMENTS:                  
Minimum correlation coefficient between average monthly copper prices on LME and COMEX exchanges (as a percent) 99.90%                
Copper derivative activity related to copper sales transactions                  
Pound         423,300,000 46,300,000 390,800,000 276,700,000  
Average LME cap price         3.63 5.18      
Average LME floor price         2.27 3.50      
Weighted average COMEX price             3.46 3.79  
Realized gain (loss) on copper derivatives $ 13,500,000 $ (41,900,000) $ 13,500,000 $ (41,900,000)          
Estimated percentage of copper sales covered           13.60%      
Unrealized gain recognized in other comprehensive income net of income taxes 131,014,000 (125,562,000)       3,500,000      
Accumulated unrealized loss recognized in other comprehensive income, income tax (3,500,000) 71,400,000       5,400,000      
Estimated unrealized loss to be reclassified from accumulated other comprehensive loss into earnings within next twelve months 5,400,000                
Gas consumption protected through gas swap contracts (in MMBTUs) 306,000                
Fixed price for gas swap contracts $ 3.6350                
Gain on derivative instruments                 $ 4,200,000