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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
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Assets [Abstract]      
Assets, Beginning balance   $ 1,154 $ 2,779
Assets, Total net realized and unrealized gains (losses) included in operating results $ 2,031 1,256 2,351
Assets, Sales (2,031) (2,410) (3,976)
Assets, Ending balance     1,154
Total net 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     1,143
Liabilities, Beginning balance (2,557) (1,154) (2,779)
Liabilities, Transfers into Level 3   (5,817)  
Liabilities, Total net realized and unrealized gains (losses) included in operating results (1,690) 802 (1,447)
Liabilities, Settlements 1,587 3,612 3,072
Liabilities, Ending balance (2,660) (2,557) (1,154)
Total net 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 $ (103) $ (983) $ (1,143)