Income Taxes - Components of Company's Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
Postretirement and other employee benefits	$ 103,699	$ 94,581
Product liability	24,023	30,791
Net operating loss, capital loss, and tax credit carryforwards	17,862	14,291
All other items	41,439	37,296
Total deferred tax assets	187,023	176,959
Deferred tax liabilities:
Property, plant and equipment	(114,641)	(117,148)
All other items	(8,134)	(6,141)
Total deferred tax liabilities	(122,775)	(123,289)
Deferred tax assets (liabilities), net, total	64,248	53,670
Valuation allowances	(35,145)	(27,270)
Net deferred tax asset	$ 29,103	$ 26,400