Income Taxes - Components of Deferred Income Tax Balances (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred income tax liabilities:
Plant, equipment, and intangibles	$ (92,271)	$ (96,254)
Plant, equipment, and intangibles	(17,610)	(16,906)
Total Deferred Tax Liabilities, Gross	(109,881)	(113,160)
Deferred income tax assets:
Postretirement, pensions, and stock compensation	35,394	28,169
Reserves and accruals	24,388	15,395
Net operating loss, capital loss, and tax credit carryforwards	107,028	76,456
Lease liability	18,515	17,882
Valuation allowances	(84,308)	(48,251)
Deferred tax assets	101,017	89,651
Net deferred income tax asset	$ (8,864)	$ (23,509)