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EMPLOYEE RETIREMENT PLANS - Schedule of components of net periodic pension cost (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
EMPLOYEE RETIREMENT PLANS    
Service cost $ 383 $ 463
Interest cost 2,012 2,460
Expected return on plan assets (2,761) (2,502)
Net amortization and deferral 763 626
Pension expense $ 397 $ 1,047