STOCKHOLDERS' EQUITY (AOCI) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Loss
Total after-tax amount	$ 665,474	$ 651,462
Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Loss
Total pre-tax amount	(21,566)	(27,181)
Total after-tax amount	(20,078)	(20,574)	$ (22,807)	$ (23,417)
Unrecognized Net Periodic Benefit Costs
Accumulated Other Comprehensive Loss
Total pre-tax amount	(20,774)	(25,768)
Total after-tax amount	(19,398)	(19,715)	(21,530)	(21,886)
Interest Rate Swap
Accumulated Other Comprehensive Loss
Total pre-tax amount	1,199	481
Total after-tax amount	791	292	(194)	(329)
Foreign Currency Translation
Accumulated Other Comprehensive Loss
Total pre-tax amount	(1,991)	(1,894)
Total after-tax amount	$ (1,471)	$ (1,151)	$ (1,083)	$ (1,202)