Significant Components of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Deferred tax assets:
Employee benefits	$ 1,296	$ 1,440
Inventories	7,071	6,966
Natural gas derivatives	4,183
Goodwill	3,980	874
Net Operating Loss	39,360
Other	1,723	2,942
Foreign losses	1,230	4,300
Foreign tax assets valuation allowance	(1,230)	(4,300)
Total deferred tax assets	57,613	12,222
Deferred tax liabilities:
Depreciation	71,976	81,628
Total deferred tax liabilities	71,976	81,628
Net deferred tax liabilities	$ 14,363	$ 69,406