STOCKHOLDERS' EQUITY - AOCI components (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Changes in accumulated other comprehensive income (loss), net of tax, by component
Balances	$ 763,043	$ 717,682	$ 651,462
OTHER COMPREHENSIVE INCOME, net of tax	987	14,441	9,912
Balances	828,593	763,043	717,682
Accumulated Other Comprehensive Income (Loss)
Changes in accumulated other comprehensive income (loss), net of tax, by component
Balances	203	(14,238)	(20,574)
Other comprehensive income (loss) before reclassifications	1,359	6,197
Amounts reclassified from accumulated other comprehensive income (loss)	(372)	8,244
OTHER COMPREHENSIVE INCOME, net of tax	987	14,441	9,912
Balances	1,190	203	(14,238)
Unrecognized Net Periodic Benefit Credit (costs)
Changes in accumulated other comprehensive income (loss), net of tax, by component
Balances	2,152	(12,749)
Other comprehensive income (loss) before reclassifications	1,480	6,657
Amounts reclassified from accumulated other comprehensive income (loss)	(372)	8,244
OTHER COMPREHENSIVE INCOME, net of tax	1,108	14,901
Balances	3,260	2,152	(12,749)
Interest Rate Swap
Changes in accumulated other comprehensive income (loss), net of tax, by component
Balances	(416)	591
Other comprehensive income (loss) before reclassifications	(782)	(1,007)
OTHER COMPREHENSIVE INCOME, net of tax	(782)	(1,007)
Balances	(1,198)	(416)	591
Interest Rate Swap, 2018
Changes in accumulated other comprehensive income (loss), net of tax, by component
Balances		591
Balances			591
Foreign Currency Translation
Changes in accumulated other comprehensive income (loss), net of tax, by component
Balances	(1,533)	(2,080)
Other comprehensive income (loss) before reclassifications	661	547
OTHER COMPREHENSIVE INCOME, net of tax	661	547
Balances	$ (872)	$ (1,533)	$ (2,080)

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Amount after tax, before reclassification adjustments, of other comprehensive income (loss), attributable to parent.

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Amount after tax of other comprehensive income (loss) attributable to parent entity.

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Total of all stockholders' equity (deficit) items, net of receivables from officers, directors, owners, and affiliates of the entity which are attributable to the parent. The amount of the economic entity's stockholders' equity attributable to the parent excludes the amount of stockholders' equity which is allocable to that ownership interest in subsidiary equity which is not attributable to the parent (noncontrolling interest, minority interest). This excludes temporary equity and is sometimes called permanent equity.

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