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DERIVATIVE INSTRUMENTS: (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended 6 Months Ended
Jun. 30, 2012
Jun. 30, 2012
Zero cost collar contracts
Second quarter 2012
lbs
Jun. 30, 2011
Zero cost collar contracts
Second quarter 2011
lbs
Jun. 30, 2012
Zero cost collar contracts
First six months 2012
lbs
Jun. 30, 2011
Zero cost collar contracts
First six months 2011
lbs
Jun. 30, 2011
Swap contracts
Second quarter 2011
lbs
Jun. 30, 2011
Swap contracts
First six months 2011
lbs
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              
Pounds   0 105,800,000 46,300,000 211,600,000 112,400,000 232,000,000
Average LME cap price   0.00 4.84 5.18 4.84    
Average LME floor price   0.00 3.02 3.50 3.02    
Weighted average COMEX price           4.08 4.08
Realized loss on copper derivatives           $ 8.6 $ 44.3