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Derivatives, Hedges, Financial Instruments and Carbon Credits - Additional Information (Detail) (USD $)
 1 Months Ended 3 Months Ended 12 Months Ended
Feb. 28, 2011
Mar. 31, 2013
CreditFacility
lb
Dec. 31, 2012
lb
Contractual weighted-average pay rate   3.23%  
Estimated market weighted-average receive rate   0.51%  
Carbon credit fair value per unit   0.50 0.50
Derivative future or forward contract in pounds   625,000 625,000
Weighted-average cost per pound of future or forward copper contracts   $ 3.53 $ 3.53
Future or forward copper contract period   May 2013 May 2013
LIBOR rate 3.23%    
Interest rate swap declining balance beginning balance 23,800,000    
Interest rate swap declining balance ending balance 18,800,000    
Interest rate contracts fixed rate description Fixed three-month LIBOR rate of 3.23% on a declining balance    
Maturity period interest rate contracts held March 2016    
Number of carbon credits   625,000  
Assets or liabilities measured at fair value on a recurring basis transferred between Level 1 and Level 2 classifications