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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, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Assets [Abstract]        
Assets, Total net realized and unrealized gains (losses) included in operating results $ 1,147 $ 1,164 $ 1,681 $ 2,031
Assets, Sales     (534) (867)
Assets, Ending balance 1,147 1,164 1,147 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 1,147 1,164 1,147 1,164
Liabilities, Beginning balance (1,838) (3,137) (2,660) (2,557)
Liabilities, Total net realized and unrealized gains (losses) included in operating results (2,279) (438) (1,656) (1,705)
Liabilities, Issuances     (229)  
Liabilities, Settlements     428 687
Liabilities, Ending balance (4,117) (3,575) (4,117) (3,575)
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 $ (2,278) $ (438) $ (1,227) $ (1,018)