Income Taxes - Components of Deferred Income Tax Balances (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred income tax liabilities:
Plant, equipment, and intangibles	$ (114,413)	$ (120,171)
Deferred income tax assets:
Postretirement, pensions, and stock compensation	30,896	28,736
Reserves and accruals	25,641	29,297
Net operating loss and tax credit carryforwards	164,823	228,815
Valuation allowances	(90,872)	(151,841)
Deferred tax assets	130,488	135,007
Net deferred income tax asset	$ 16,075	$ 14,836