Significant Components of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Deferred tax assets:
Employee benefits	$ 1,349	$ 1,296
Inventories	8,811	7,071
Natural gas derivatives	1,281	4,183
Goodwill & other intangibles	4,881	3,980
Net operating loss	51,722	39,360
Other	2,027	1,723
Foreign losses		1,230
Foreign tax assets valuation allowance		(1,230)
Total deferred tax assets	70,071	57,613
Deferred tax liabilities:
Depreciation	71,308	71,976
Total deferred tax liabilities	71,308	71,976
Net deferred tax liabilities	$ 1,237	$ 14,363