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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, 2014
Unit
MMBTU
Dec. 31, 2013
Unit
MMBTU
Derivative Instruments and Hedging Activities Disclosures [Line Items]      
Contractual weighted-average cost   3.95 3.98
Estimated weighted-average market value   4.29  
Contractual weighted-average pay rate   3.23%  
Estimated market weighted-average receive rate   0.53%  
Carbon credit fair value per unit   2.50 1.00
Derivative future or forward contracts in volume   2,140,000 1,530,000
The month future or forward natural gas contract period ends   October 2014 October 2014
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   669,000 1,284,000
Assets or liabilities measured at fair value on a recurring basis transferred between Level 1 and Level 2 classifications      31,000
Senior Secured Notes [Member]
      
Derivative Instruments and Hedging Activities Disclosures [Line Items]      
Excess of estimated fair value over carrying value   $ 35,000,000 $ 20,000,000
Senior Secured Notes [Member] | Significant Other Observable Inputs (Level 2) [Member]
      
Derivative Instruments and Hedging Activities Disclosures [Line Items]      
Notes lower range of ask or bid price   $ 107.8 $ 104.5
Notes higher range of ask or bid price   $ 108.8 $ 104.9