Income Taxes - Components of Deferred Income Tax Balances (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred income tax liabilities:
Plant, equipment, and intangibles	$ (120,171)	$ (179,229)
Deferred income tax assets:
Postretirement, pensions, and stock compensation	28,736	35,500
Reserves and accruals	29,297	22,795
Net operating loss and tax credit carryforwards	228,815	245,135
Valuation allowances	(151,841)	(104,771)
Deferred tax assets	135,007	198,659
Net deferred income tax asset	$ 14,836	$ 19,430