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Derivatives, Hedges, Financial Instruments and Carbon Credits - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $)
In Thousands, unless otherwise specified
Jun. 30, 2013
Dec. 31, 2012
Liabilities-Current and noncurrent accrued and other liabilities:    
Total $ 2,074 $ 1,965
Recurring [Member]
    
Assets-Supplies, prepaid items and other:    
Total 625 170
Commodities contracts [Member] | Recurring [Member]
    
Assets-Supplies, prepaid items and other:    
Derivative Assets    79
Carbon credits [Member] | Recurring [Member]
    
Assets-Supplies, prepaid items and other:    
Carbon credits 625 91
Liabilities-Current and noncurrent accrued and other liabilities:    
Contractual obligations - carbon credits 625 91
Interest rate contracts [Member] | Recurring [Member]
    
Liabilities-Current and noncurrent accrued and other liabilities:    
Derivative Financial Instruments, Liabilities 1,449 1,874
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
    
Liabilities-Current and noncurrent accrued and other liabilities:    
Total     
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Recurring [Member]
    
Assets-Supplies, prepaid items and other:    
Total     
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Commodities contracts [Member] | Recurring [Member]
    
Assets-Supplies, prepaid items and other:    
Derivative Assets     
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Carbon credits [Member] | Recurring [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] | Recurring [Member]
    
Liabilities-Current and noncurrent accrued and other liabilities:    
Derivative Financial Instruments, Liabilities     
Significant Other Observable Inputs (Level 2) [Member]
    
Liabilities-Current and noncurrent accrued and other liabilities:    
Total 1,449  
Significant Other Observable Inputs (Level 2) [Member] | Recurring [Member]
    
Assets-Supplies, prepaid items and other:    
Total     
Significant Other Observable Inputs (Level 2) [Member] | Commodities contracts [Member] | Recurring [Member]
    
Assets-Supplies, prepaid items and other:    
Derivative Assets     
Significant Other Observable Inputs (Level 2) [Member] | Carbon credits [Member] | Recurring [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] | Recurring [Member]
    
Liabilities-Current and noncurrent accrued and other liabilities:    
Derivative Financial Instruments, Liabilities 1,449  
Significant Unobservable Inputs (Level 3) [Member]
    
Liabilities-Current and noncurrent accrued and other liabilities:    
Total 625  
Significant Unobservable Inputs (Level 3) [Member] | Recurring [Member]
    
Assets-Supplies, prepaid items and other:    
Total 625  
Significant Unobservable Inputs (Level 3) [Member] | Commodities contracts [Member] | Recurring [Member]
    
Assets-Supplies, prepaid items and other:    
Derivative Assets     
Significant Unobservable Inputs (Level 3) [Member] | Carbon credits [Member] | Recurring [Member]
    
Assets-Supplies, prepaid items and other:    
Carbon credits 625  
Liabilities-Current and noncurrent accrued and other liabilities:    
Contractual obligations - carbon credits 625  
Significant Unobservable Inputs (Level 3) [Member] | Interest rate contracts [Member] | Recurring [Member]
    
Liabilities-Current and noncurrent accrued and other liabilities:    
Derivative Financial Instruments, Liabilities