Employee Benefits - Amounts in Balance Sheets (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Amounts Recognized in the Consolidated Balance Sheets:
Current accrued expenses and other current liabilities	$ 1,913	$ 1,913
Non-current other liabilities	17,833	17,301
Total accrued liabilities	19,746	19,214
Other comprehensive loss - gross actuarial (gains) losses	$ 1,600	$ 1,200	$ (900)