STOCKHOLDERS' EQUITY - AOCI (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss)
Total after-tax amount	$ 828,593	$ 763,043	$ 717,682	$ 651,462
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss)
Total pre-tax amount	1,586	260	(13,836)
Total after-tax amount	1,190	203	(14,238)	$ (20,574)
Unrecognized Net Periodic Benefit Credit (costs)
Accumulated Other Comprehensive Income (Loss)
Total pre-tax amount	4,390	2,898	(11,821)
Total after-tax amount	3,260	2,152	(12,749)
Amount remaining in unrecognized net periodic benefit costs portion of accumulated other comprehensive income for previous valuation allowance on deferred tax assets			4,000
Interest Rate Swap
Accumulated Other Comprehensive Income (Loss)
Total pre-tax amount	(1,622)	(563)
Total after-tax amount	(1,198)	(416)	591
Interest Rate Swap, 2018
Accumulated Other Comprehensive Income (Loss)
Total pre-tax amount			801
Total after-tax amount			591
Foreign Currency Translation
Accumulated Other Comprehensive Income (Loss)
Total pre-tax amount	(1,182)	(2,075)	(2,816)
Total after-tax amount	$ (872)	$ (1,533)	$ (2,080)