FINANCIAL INSTRUMENTS AND FAIR VALUE MEASUREMENTS - Assets and Liabilities (Details) - Recurring basis - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Assets:
Assets	$ 39,369	$ 37,156
Cash and cash equivalents
Assets:
Money market funds	36,001	34,201
Other long-term assets
Assets:
Equity, bond, and money market mutual funds held in trust related to the Voluntary Savings Plan	3,017	2,955
Interest rate swaps	351
Other long-term liabilities
Liabilities:
Interest rate swap	1,194	1,622
Level 1
Assets:
Assets	39,018	37,156
Level 1 | Cash and cash equivalents
Assets:
Money market funds	36,001	34,201
Level 1 | Other long-term assets
Assets:
Equity, bond, and money market mutual funds held in trust related to the Voluntary Savings Plan	3,017	2,955
Level 2
Assets:
Assets	351
Level 2 | Other long-term assets
Assets:
Interest rate swaps	351
Level 2 | Other long-term liabilities
Liabilities:
Interest rate swap	$ 1,194	$ 1,622