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Derivatives, Hedges, Financial Instruments and Carbon Credits - Reconciliation of Beginning and Ending Balances for Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended 9 Months Ended
Sep. 30, 2014
Sep. 30, 2013
Sep. 30, 2014
Sep. 30, 2013
Assets        
Beginning balance $ 1,673 $ 625 $ 1,284 $ 91
Transfers into Level 3            
Transfers out of Level 3            
Total realized and unrealized gains (losses) included in operating results       1,925 546
Purchases            
Issuances            
Sales       (1,536) (12)
Settlements            
Ending balance 1,673 625 1,673 625
Total gains (losses) for the period included in operating results attributed to the change in unrealized gains or losses on assets and liabilities still held at the reporting date       1,004 534
Liabilities        
Beginning balance (1,673) (625) (1,284) (91)
Transfers into Level 3            
Transfers out of Level 3            
Total realized and unrealized gains (losses) included in operating results       (1,634) (546)
Purchases            
Issuances            
Sales            
Settlements       1,245 12
Ending balance (1,673) (625) (1,673) (625)
Total gains (losses) for the period included in operating results attributed to the change in unrealized gains or losses on assets and liabilities still held at the reporting date     $ (1,004) $ (534)