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Derivatives, Hedges, Financial Instruments and Carbon Credits - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Liabilities - Current and noncurrent accrued and other liabilities:    
Embedded derivative $ (2,970) $ (2,660)
Recurring [Member]    
Assets - Supplies, prepaid items and other:    
Total 1,147  
Liabilities - Current and noncurrent accrued and other liabilities:    
Embedded derivative (2,970) (2,660)
Total (4,117) $ (2,660)
Recurring [Member] | Carbon Credits [Member]    
Assets - Supplies, prepaid items and other:    
Carbon credits 1,147  
Liabilities - Current and noncurrent accrued and other liabilities:    
Contractual obligations - carbon credits (1,147)  
Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member]    
Assets - Supplies, prepaid items and other:    
Total 1,147  
Liabilities - Current and noncurrent accrued and other liabilities:    
Embedded derivative (2,970)  
Total (4,117)  
Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Carbon Credits [Member]    
Assets - Supplies, prepaid items and other:    
Carbon credits 1,147  
Liabilities - Current and noncurrent accrued and other liabilities:    
Contractual obligations - carbon credits $ (1,147)