Income Taxes - Significant Components of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Post-retirement obligations	$ 3,283	$ 2,833
Tax credits	787	1,693
Expenses not currently deductible	8,461	12,987
Net operating loss carryover	3,670	5,228
Other	1,045	994
Total deferred tax assets	17,246	23,735
Valuation allowance for deferred tax assets	(3,311)	(6,183)
Net deferred tax assets	13,935	17,552
Deferred tax liabilities:
Property, plant and equipment	17,279	21,982
Intangible assets	38,982	39,044
Basis difference - convertible debt		7,670
Goodwill	7,316	7,430
Total deferred liabilities	63,577	76,126
Net deferred tax liabilities	$ 49,642	$ 58,574