Income Taxes - Significant Components of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Post-retirement obligations	$ 6,562	$ 3,283
Tax credits	782	787
Expenses not currently deductible	14,930	8,461
Net operating loss carryover	4,798	3,670
Debt and derivative instruments	8,852
Other	685	1,045
Total deferred tax assets	36,609	17,246
Valuation allowance for deferred tax assets	(4,108)	(3,311)
Net deferred tax assets	32,501	13,935
Deferred tax liabilities:
Property, plant and equipment	42,357	17,279
Intangible assets	370,121	38,982
Goodwill	8,309	7,316
Total deferred liabilities	420,787	63,577
Net deferred tax liabilities	$ 388,286	$ 49,642