STOCKHOLDERS' EQUITY - AOCI components (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Changes in accumulated other comprehensive income (loss), net of tax, by component
Balances	$ 828,593	$ 763,043
OTHER COMPREHENSIVE INCOME (LOSS), net of tax	804	(2,467)
Balances	851,909	759,206
Accumulated Other Comprehensive Income (Loss)
Changes in accumulated other comprehensive income (loss), net of tax, by component
Balances	1,190	203
Other comprehensive income (loss) before reclassifications	904	(2,425)
Amounts reclassified from accumulated other comprehensive income (loss)	(100)	(42)
OTHER COMPREHENSIVE INCOME (LOSS), net of tax	804	(2,467)
Balances	1,994	(2,264)
Unrecognized Net Periodic Benefit Credit
Changes in accumulated other comprehensive income (loss), net of tax, by component
Balances	3,260	2,152
Other comprehensive income (loss) before reclassifications		(8)
Amounts reclassified from accumulated other comprehensive income (loss)	(100)	(42)
OTHER COMPREHENSIVE INCOME (LOSS), net of tax	(100)	(50)
Balances	3,160	2,102
Interest Rate Swap
Changes in accumulated other comprehensive income (loss), net of tax, by component
Balances	(1,198)	(416)
Other comprehensive income (loss) before reclassifications	575	(997)
OTHER COMPREHENSIVE INCOME (LOSS), net of tax	575	(997)
Balances	(623)	(1,413)
Foreign Currency Translation
Changes in accumulated other comprehensive income (loss), net of tax, by component
Balances	(872)	(1,533)
Other comprehensive income (loss) before reclassifications	329	(1,420)
OTHER COMPREHENSIVE INCOME (LOSS), net of tax	329	(1,420)
Balances	$ (543)	$ (2,953)