Derivatives Hedges and Financial Instruments - Liabilities Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Recurring - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
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	1,029	$ 1,084
Total	2,314	$ 1,084
Level 2 [Member]
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
Total	1,285
Significant Unobservable Inputs (Level 3) [Member]
Liabilities - Current and noncurrent accrued and other liabilities:
Embedded derivative	1,029
Total	$ 1,029