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Derivatives, Hedges, Financial Instruments and Carbon Credits - Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Liabilities - Current and noncurrent accrued and other liabilities:    
Embedded derivative $ (1,838) $ (2,660)
Recurring [Member]    
Liabilities - Current and noncurrent accrued and other liabilities:    
Embedded derivative (1,838) (2,660)
Total (1,838) $ (2,660)
Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member]    
Liabilities - Current and noncurrent accrued and other liabilities:    
Embedded derivative (1,838)  
Total $ (1,838)