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Pensions and Other Postretirement Benefits (Narrative) (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Estimated actuarial items that will be amortized from AOCI loss $ 700    
Ultimate health care cost trend rate 4.00%    
Defined benefit plan, employer contribution in 2018 $ 1,500    
Defined contribution plan matching expense 700 $ 700 $ 600
Accrued pension and other postretirement costs 16,424 14,713  
Pension Plans      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Accrued pension and other postretirement costs 11,163 10,634  
Other Retirement Obligations      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Other retirement obligations 1,200 800  
Accrued pension and other postretirement costs 4,726 3,833  
Nonqualified Plan | Deferred Compensation, Excluding Share-based Payments and Retirement Benefits      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Assets held-in-trust 3,300 3,200  
Accrued pension and other postretirement costs 4,400 4,100  
Unfunded Plan | Nonqualified Plan | Pension Plans      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Assets held-in-trust 1,700 1,600  
Non qualified pension plan liabilities   $ 2,000  
Unfunded Plan | Nonqualified Plan | Domestic Plan      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Non qualified pension plan liabilities $ 2,300