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EMPLOYEE RETIREMENT PLANS AND POSTRETIREMENT BENEFITS - Net periodic pension benefit cost (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Net periodic pension benefit cost      
Service cost $ 1,068 $ 272 $ 841
Interest cost 1,716 1,211 802
Expected return on plan assets (683) (615) (665)
Amortization of actuarial gain and losses 459 411 478
Net periodic pension cost $ 2,560 $ 1,279 $ 1,456