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Derivatives Hedges and Financial Instruments - 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, 2018
Dec. 31, 2017
Dec. 31, 2016
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Assets [Abstract]      
Assets, Beginning balance     $ 1,154
Assets, Total net realized and unrealized gains (losses) included in operating results $ 2,214 $ 2,031 1,256
Assets, Sales (1,681) (2,031) (2,410)
Assets, Ending balance 533    
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 533    
Liabilities, Beginning balance (2,660) (2,557) (1,154)
Liabilities, Transfers into Level 3     (5,817)
Liabilities, Total net realized and unrealized gains (losses) included in operating results (606) (1,690) 802
Liabilities, Issuances (229)    
Liabilities, Settlements 1,320 1,587 3,612
Liabilities, Ending balance (2,175) (2,660) (2,557)
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 $ (1,780) $ (103) $ (983)