FINANCIAL INSTRUMENTS AND FAIR VALUE MEASUREMENTS - Assets and Liabilities (Details) - Recurring basis - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Assets:
Assets	$ 37,156	$ 22,967
Cash and cash equivalents
Assets:
Money market funds	34,201	20,540
Other long-term assets
Assets:
Equity, bond, and money market mutual funds held in trust related to the Voluntary Savings Plan	2,955	2,427
Other long-term liabilities
Liabilities:
Interest rate swap	1,622	563
Level 1
Assets:
Assets	37,156	22,967
Level 1 | Cash and cash equivalents
Assets:
Money market funds	34,201	20,540
Level 1 | Other long-term assets
Assets:
Equity, bond, and money market mutual funds held in trust related to the Voluntary Savings Plan	2,955	2,427
Level 2 | Other long-term liabilities
Liabilities:
Interest rate swap	$ 1,622	$ 563