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Derivatives, Hedges, Financial Instruments and Carbon Credits - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (Recurring [Member], USD $)
In Thousands, unless otherwise specified
Mar. 31, 2013
Dec. 31, 2012
Assets-Supplies, prepaid items and other:    
Total $ 312 $ 170
Liabilities-Current and noncurrent accrued and other liabilities:    
Total 2,101 1,965
Commodities contracts [Member]
    
Assets-Supplies, prepaid items and other:    
Derivative Assets   79
Liabilities-Current and noncurrent accrued and other liabilities:    
Derivative Financial Instruments, Liabilities 83   
Carbon credits [Member]
    
Assets-Supplies, prepaid items and other:    
Carbon credits 312 91
Liabilities-Current and noncurrent accrued and other liabilities:    
Contractual obligations-carbon credits 312 91
Interest rate contracts [Member]
    
Liabilities-Current and noncurrent accrued and other liabilities:    
Derivative Financial Instruments, Liabilities 1,706 1,874
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
    
Assets-Supplies, prepaid items and other:    
Total     
Liabilities-Current and noncurrent accrued and other liabilities:    
Total 83  
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Commodities contracts [Member]
    
Assets-Supplies, prepaid items and other:    
Derivative Assets     
Liabilities-Current and noncurrent accrued and other liabilities:    
Derivative Financial Instruments, Liabilities 83  
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Carbon credits [Member]
    
Assets-Supplies, prepaid items and other:    
Carbon credits     
Liabilities-Current and noncurrent accrued and other liabilities:    
Contractual obligations-carbon credits     
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Interest rate contracts [Member]
    
Liabilities-Current and noncurrent accrued and other liabilities:    
Derivative Financial Instruments, Liabilities     
Significant Other Observable Inputs (Level 2) [Member]
    
Assets-Supplies, prepaid items and other:    
Total     
Liabilities-Current and noncurrent accrued and other liabilities:    
Total 1,706  
Significant Other Observable Inputs (Level 2) [Member] | Commodities contracts [Member]
    
Assets-Supplies, prepaid items and other:    
Derivative Assets     
Liabilities-Current and noncurrent accrued and other liabilities:    
Derivative Financial Instruments, Liabilities     
Significant Other Observable Inputs (Level 2) [Member] | Carbon credits [Member]
    
Assets-Supplies, prepaid items and other:    
Carbon credits     
Liabilities-Current and noncurrent accrued and other liabilities:    
Contractual obligations-carbon credits     
Significant Other Observable Inputs (Level 2) [Member] | Interest rate contracts [Member]
    
Liabilities-Current and noncurrent accrued and other liabilities:    
Derivative Financial Instruments, Liabilities 1,706  
Significant Unobservable Inputs (Level 3) [Member]
    
Assets-Supplies, prepaid items and other:    
Total 312  
Liabilities-Current and noncurrent accrued and other liabilities:    
Total 312  
Significant Unobservable Inputs (Level 3) [Member] | Commodities contracts [Member]
    
Assets-Supplies, prepaid items and other:    
Derivative Assets     
Liabilities-Current and noncurrent accrued and other liabilities:    
Derivative Financial Instruments, Liabilities     
Significant Unobservable Inputs (Level 3) [Member] | Carbon credits [Member]
    
Assets-Supplies, prepaid items and other:    
Carbon credits 312  
Liabilities-Current and noncurrent accrued and other liabilities:    
Contractual obligations-carbon credits 312  
Significant Unobservable Inputs (Level 3) [Member] | Interest rate contracts [Member]
    
Liabilities-Current and noncurrent accrued and other liabilities:    
Derivative Financial Instruments, Liabilities