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Tax Effects of Significant Temporary Differences Creating Deferred Tax Assets and Liabilities (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount $ 700 $ 600
Deferred tax assets:    
Employee benefits 3,940 6,255
Postretirement employee benefits 17,132 27,370
Incentive compensation 5,194 11,356
Inventories 7,959 8,859
Pensions 2,516 8,338
Federal and state credit carryforwards 11,752 8,369
Net operating losses 3,088 1,462
Other 1,949 3,774
Total deferred tax assets 53,530 75,783
Valuation allowance (3,733) (4,407)
Deferred tax assets, net of valuation allowance 49,797 71,376
Deferred tax liabilities:    
Plant and equipment (153,885) (206,502)
Intangible assets (7,577) (14,136)
Deferred Tax Liabilities, Gross 161,462 220,638
Total deferred tax liabilities (111,665) $ (149,262)
Valuation allowances, State Tax Credits [Member]    
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount $ 11,100