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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 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Assets [Abstract]        
Assets, Beginning balance   $ 6   $ 1,154
Assets, Total net realized and unrealized gains (losses) included in operating results $ 1,164 1,196 $ 2,031 1,256
Assets, Sales   (1,202) (867) (2,410)
Assets, Ending balance 1,164   1,164  
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 1,164   1,164  
Liabilities, Beginning balance (3,137) (1,871) (2,557) (1,154)
Liabilities, Transfers into Level 3       (5,817)
Liabilities, Total net realized and unrealized gains (losses) included in operating results (438) (3,395) (1,705) 754
Liabilities, Settlements   2,661 687 3,612
Liabilities, Ending balance (3,575) (2,605) (3,575) (2,605)
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 $ (438) $ (2,199) $ (1,018) $ (816)