Annual report pursuant to Section 13 and 15(d)

Shareholders' Equity - Components of Accumulated Other Comprehensive Income (Loss) (Details)

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Shareholders' Equity - Components of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Total Shareholders’ Equity $ 329,927 $ 337,449 $ 300,225  
Foreign Currency Translation Adjustments        
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Total Shareholders’ Equity (4,465) (8,597)    
Retirement Liability Adjustment        
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Retirement Liability Adjustment – Before Tax (12,988) (10,611)    
Retirement Liability Adjustment – Before Tax 4,101 3,714    
Total Shareholders’ Equity (8,887) (6,897)    
Accumulated Other Comprehensive Loss        
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Total Shareholders’ Equity $ (13,352) $ (15,494) $ (15,064) $ (11,949)