STOCKHOLDERS' EQUITY (AOCI) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Loss
Total after-tax amount	$ 721,755	$ 717,682	$ 665,474	$ 651,462
Amount remaining in unrecognized net periodic benefit costs portion of accumulated other comprehensive income for previous valuation allowance on deferred tax assets	4,000
Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Loss
Total pre-tax amount	(11,346)	(13,836)
Total after-tax amount	(12,388)	(14,238)	(20,078)	(20,574)
Unrecognized Net Periodic Benefit Costs
Accumulated Other Comprehensive Loss
Total pre-tax amount	(9,185)	(11,821)
Total after-tax amount	(10,792)	(12,749)	(19,398)	(19,715)
Interest Rate Swap, 2019
Accumulated Other Comprehensive Loss
Total pre-tax amount	351
Total after-tax amount	259	591
Interest Rate Swap, 2018
Accumulated Other Comprehensive Loss
Total pre-tax amount		801
Total after-tax amount		591	791	292
Foreign Currency Translation
Accumulated Other Comprehensive Loss
Total pre-tax amount	(2,512)	(2,816)
Total after-tax amount	$ (1,855)	$ (2,080)	$ (1,471)	$ (1,151)