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Derivatives Hedges and Financial Instruments - Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($)
$ in Thousands
Jun. 30, 2021
Dec. 31, 2020
Liabilities - Current and noncurrent accrued and other liabilities:    
Embedded derivative $ 2,181 $ 1,029
Fair Value, Recurring    
Assets - Supplies, prepaid items and other:    
Natural gas contracts   80
Total   80
Liabilities - Current and noncurrent accrued and other liabilities:    
Natural gas contracts   1,285
Embedded derivative 2,181 1,029
Total 2,181 $ 2,314
Fair Value, Recurring | Significant Unobservable Inputs (Level 3) [Member]    
Liabilities - Current and noncurrent accrued and other liabilities:    
Embedded derivative 2,181  
Total $ 2,181