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DERIVATIVE INSTRUMENTS: (Details) (USD $)
3 Months Ended 6 Months Ended
Jun. 30, 2011
Jun. 30, 2011
DERIVATIVE INSTRUMENTS:    
Minimum correlation coefficient between average monthly copper prices on LME and COMEX exchanges 99.90% 99.90%
Copper derivative activity related to copper sales transactions    
Accumulated unrealized loss recognized in other comprehensive income net of income taxes $ 23,313,000 $ 89,040,000
Zero cost collar contracts | Second quarter 2011
   
Copper derivative activity related to copper sales transactions    
Pounds (in millions) 105,800,000 105,800,000
Average LME cap price 4.84 4.84
Average LME floor price 3.02 3.02
Zero cost collar contracts | Six months 2011
   
Copper derivative activity related to copper sales transactions    
Pounds (in millions) 211,600,000 211,600,000
Average LME cap price 4.84 4.84
Average LME floor price 3.02 3.02
Zero cost collar contracts | Second half of 2011
   
Copper derivative activity related to copper sales transactions    
Pounds (in millions) 211,600,000 211,600,000
Average LME cap price 4.84 4.84
Average LME floor price 3.02 3.02
Estimated percentage of copper sales covered (as a percent)   30.00%
Accumulated unrealized loss recognized in other comprehensive income net of income taxes   8,500,000
Accumulated unrealized loss recognized in other comprehensive income, income tax   5,400,000
Zero cost collar contracts | 1st Quarter 2012
   
Copper derivative activity related to copper sales transactions    
Pounds (in millions) 46,300,000 46,300,000
Average LME cap price 5.18 5.18
Average LME floor price 3.50 3.50
Estimated percentage of copper sales covered (as a percent)   13.00%
Swap contracts | Second quarter 2011
   
Copper derivative activity related to copper sales transactions    
Pounds (in millions) 112,400,000 112,400,000
Weighted average COMEX price 4.08 4.08
Swap contracts | Six months 2011
   
Copper derivative activity related to copper sales transactions    
Pounds (in millions) 232,000,000 232,000,000
Weighted average COMEX price 4.08 4.08
Swap contracts | Second half of 2011
   
Copper derivative activity related to copper sales transactions    
Pounds (in millions) 224,900,000 224,900,000
Weighted average COMEX price 4.08 4.08
Estimated percentage of copper sales covered (as a percent)   32.00%
Accumulated unrealized loss recognized in other comprehensive income net of income taxes   28,000,000
Accumulated unrealized loss recognized in other comprehensive income, income tax   17,600,000
Second quarter 2011
   
Copper derivative activity related to copper sales transactions    
Realized (loss) gain on copper derivatives 8,600,000  
Six months 2011
   
Copper derivative activity related to copper sales transactions    
Realized (loss) gain on copper derivatives   $ 44,300,000