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STOCKHOLDERS' EQUITY (AOCI) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Accumulated Other Comprehensive Loss        
Total after-tax amount $ 651,462 $ 599,055 $ 588,728 $ 560,883
Impact on unrecognized net actuarial loss        
Change in the unrecognized net actuarial loss, after tax (2,640) (1,267) (7,535)  
Accumulated Other Comprehensive Loss        
Accumulated Other Comprehensive Loss        
Total pre-tax amount (27,181) (31,840) (38,507)  
Total after-tax amount (20,574) (23,417) (27,496) $ (23,479)
Unrecognized Net Periodic Benefit Costs        
Accumulated Other Comprehensive Loss        
Total pre-tax amount (25,768) (29,320) (35,231)  
Total after-tax amount (19,715) (21,886) (25,497)  
Interest Rate Swap        
Accumulated Other Comprehensive Loss        
Total pre-tax amount 481 (542) (897)  
Total after-tax amount 292 (329) (545)  
Foreign Currency Translation        
Accumulated Other Comprehensive Loss        
Total pre-tax amount (1,894) (1,978) (2,379)  
Total after-tax amount $ (1,151) $ (1,202) $ (1,454)