INCOME TAXES - Temporary Differences Related to Deferred Income Taxes (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred income tax assets:
Post-retirement liability	$ 446	$ 717
Deferred income	250	300
Share-based compensation	2,123	1,238
Capital loss carryforwards	0	91
State tax credit carryforwards	2,986	3,198
State operating loss carryforwards	1,264	1,529
Inventories	2,077	1,738
Operating lease liabilities	1,322	1,582
Deferred compensation	1,250	0
Other	1,036	1,291
Gross deferred income tax assets	12,754	11,684
Less: valuation allowance	(862)	(1,284)
Net deferred income tax assets	11,892	10,400
Deferred income tax liabilities:
Fixed assets	(12,205)	(10,332)
Operating lease right-of-use assets	(1,318)	(1,577)
Other	(667)	(420)
Gross deferred income tax liabilities	(14,190)	(12,329)
Net deferred income tax liability	$ (2,298)	$ (1,929)