STOCKHOLDERS' EQUITY (AOCI) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Loss
Total after-tax amount	$ 705,091	$ 651,462
Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Loss
Total pre-tax amount	(25,115)	(27,181)
Total after-tax amount	(22,620)	(20,574)	$ (22,201)	$ (23,417)
Unrecognized Net Periodic Benefit Costs
Accumulated Other Comprehensive Loss
Total pre-tax amount	(24,759)	(25,768)
Total after-tax amount	(22,357)	(19,715)	(21,116)	(21,886)
Interest Rate Swap
Accumulated Other Comprehensive Loss
Total pre-tax amount	1,761	481
Total after-tax amount	1,301	292	(88)	(329)
Foreign Currency Translation
Accumulated Other Comprehensive Loss
Total pre-tax amount	(2,117)	(1,894)
Total after-tax amount	$ (1,564)	$ (1,151)	$ (997)	$ (1,202)