STOCKHOLDERS' EQUITY - AOCI (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Accumulated Other Comprehensive Income
Total after-tax amount	$ 851,909	$ 828,593	$ 759,206	$ 763,043
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income
Total pre-tax amount	2,677	1,586
Total after-tax amount	1,994	1,190	(2,264)	203
Unrecognized Net Periodic Benefit Credit
Accumulated Other Comprehensive Income
Total pre-tax amount	4,255	4,390
Total after-tax amount	3,160	3,260	2,102	2,152
Interest Rate Swap
Accumulated Other Comprehensive Income
Total pre-tax amount	(843)	(1,622)
Total after-tax amount	(623)	(1,198)	(1,413)	(416)
Foreign Currency Translation
Accumulated Other Comprehensive Income
Total pre-tax amount	(735)	(1,182)
Total after-tax amount	$ (543)	$ (872)	$ (2,953)	$ (1,533)