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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
Mar. 31, 2014
Mar. 31, 2013
Assets    
Beginning balance $ 1,284 $ 91
Transfers into Level 3      
Transfers out of Level 3      
Total realized and unrealized gain (loss) included in operating results 1,925 233
Purchases      
Issuances      
Sales (1,536) (12)
Settlements      
Ending balance 1,673 312
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 1,004 221
Liabilities    
Beginning balance (1,284) (91)
Transfers into Level 3      
Transfers out of Level 3      
Total realized and unrealized gain (loss) included in operating results (1,634) (233)
Purchases      
Issuances      
Sales      
Settlements 1,245 12
Ending balance (1,673) (312)
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) $ (221)