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Derivatives, Hedges, Financial Instruments and Carbon Credits - Net Gains (Losses) Included in Continuing Operating Results and Statement of Operations Classifications (Detail) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Derivative Instruments, Gain (Loss) [Line Items]        
Total net gains (losses) included in operating results $ (1,132) $ 726 $ 25 $ 326
Other Income [Member] | Carbon Credits [Member]        
Derivative Instruments, Gain (Loss) [Line Items]        
Total net gains (losses) included in operating results 1,147 1,164 1,681 2,031
Other Expense [Member] | Contractual Obligations Relating to Carbon Credits [Member]        
Derivative Instruments, Gain (Loss) [Line Items]        
Total net gains (losses) included in operating results (1,147) (1,164) (1,575) (1,851)
Non-operating Other Expense [Member]        
Derivative Instruments, Gain (Loss) [Line Items]        
Non-operating other income (expense) - embedded derivative $ (1,132) $ 726 $ (81) $ 146