Income Taxes - Components of Company's Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Postretirement and other employee benefits	$ 116,321	$ 191,099
Product liability	32,668	67,528
Net operating loss, capital loss, and tax credit carryforwards	14,174	15,274
All other items	33,423	48,718
Total deferred tax assets	196,586	322,619
Deferred tax liabilities:
Property, plant and equipment	(101,720)	(160,075)
All other items	(6,099)	(9,685)
Total deferred tax liabilities	(107,819)	(169,760)
Deferred tax assets (liabilities), net, total	88,767	152,859
Valuation allowances	(30,102)	(20,228)
Net deferred tax asset	$ 58,665	$ 132,631