STOCKHOLDERS' EQUITY (AOCI) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Loss
Total after-tax amount	$ 668,515	$ 651,462
Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Loss
Total pre-tax amount	(18,683)	(27,181)
Total after-tax amount	(17,842)	(20,574)	$ (22,026)	$ (23,417)
Unrecognized Net Periodic Benefit Costs
Accumulated Other Comprehensive Loss
Total pre-tax amount	(17,929)	(25,768)
Total after-tax amount	(17,285)	(19,715)	(20,622)	(21,886)
Interest Rate Swap
Accumulated Other Comprehensive Loss
Total pre-tax amount	1,535	481
Total after-tax amount	1,134	292	(113)	(329)
Foreign Currency Translation
Accumulated Other Comprehensive Loss
Total pre-tax amount	(2,289)	(1,894)
Total after-tax amount	$ (1,691)	$ (1,151)	$ (1,291)	$ (1,202)