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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
 3 Months Ended 6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Jun. 30, 2015
Jun. 30, 2014
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Assets [Abstract]        
Assets, Beginning balance $ 2,716 $ 1,673 $ 2,779 $ 1,284
Assets, Transfers into Level 3 0 0 0 0
Assets, Transfers out of Level 3 0 0 0 0
Assets, Total net realized and unrealized gains (losses) included in earnings 1,364 0 1,334 1,925
Assets, Purchases 0 0 0 0
Assets, Issuances 0 0 0 0
Assets,Sales (31) 0 (64) (1,536)
Assets, Settlements 0 0 0 0
Assets, Ending balance 4,049 1,673 4,049 1,673
Total net gains (losses) for the period included in earnings attributed to the change in unrealized gains or losses on assets and liabilities still held at the reporting date 1,334 0 1,334 1,004
Liabilities, Beginning balance (2,716) (1,673) (2,779) (1,284)
Liabilities, Transfers into Level 3 0 0 0 0
Liabilities, Transfers out of Level 3 0 0 0 0
Liabilities, Total net realized and unrealized gains (losses) included in earnings (1,364)   (1,334) (1,634)
Liabilities, Purchases 0 0 0 0
Liabilities, Issuances 0 0 0 0
Liabilities, Sales 0 0 0 0
Liabilities, Settlements 31   64 1,245
Liabilities, Ending balance (4,049) $ (1,673) (4,049) (1,673)
Total net gains (losses) for the period included in earnings attributed to the change in unrealized gains or losses on assets and liabilities still held at the reporting date $ (1,334)   $ (1,334) $ (1,004)