STOCKHOLDERS' EQUITY (AOCI) (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss)
Total after-tax amount	$ 775,091	$ 759,206	$ 763,043	$ 743,159	$ 721,755	$ 717,682
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss)
Total pre-tax amount	(2,843)		260
Total after-tax amount	(2,090)	$ (2,264)	203	(12,429)	$ (12,388)	(14,238)
Unrecognized Net Periodic Benefit Credit (cost)
Accumulated Other Comprehensive Income (Loss)
Total pre-tax amount	2,683		2,898
Total after-tax amount	1,993		2,152	(10,421)		(12,749)
Interest Rate Swap
Accumulated Other Comprehensive Income (Loss)
Total pre-tax amount	(2,148)		(563)
Total after-tax amount	(1,587)		(416)	(269)		591
Foreign Currency Translation
Accumulated Other Comprehensive Income (Loss)
Total pre-tax amount	(3,378)		(2,075)
Total after-tax amount	$ (2,496)		$ (1,533)	$ (1,739)		$ (2,080)