INCOME TAXES - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred Tax Assets:
Net operating loss carryforwards	$ 19,353	$ 24,941
Operating and finance leases	24,529	25,440
Pension liabilities	36,085	24,925
Share-based compensation	5,946	6,082
U.S. federal tax credits	8,826	8,575
U.S. state tax credits	7,011	7,881
Vacation and bonus	12,307	7,645
Research and development	8,992	7,539
Inventory	4,854	5,993
Workers compensation	3,353	3,835
Other	16,643	16,496
Total gross deferred tax assets	147,899	139,352
Less valuation allowance	(23,105)	(23,320)
Net deferred tax assets	124,794	116,032
Deferred Tax Liabilities:
Acquisition related intangibles	57,295	62,851
Depreciation and amortization	27,737	28,284
Operating and finance leases	26,549	27,555
Other	8,044	6,215
Total gross deferred tax liabilities	119,625	124,905
Net deferred tax assets (liabilities)	$ 5,169
Net deferred tax assets (liabilities)		$ (8,873)