RETIREMENT PLANS - Net Periodic Pension Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Plans
Components of the plan's net periodic pension cost
Service cost	$ 439	$ 1,709	$ 1,794
Interest cost	2,585	3,472	3,279
Expected return on plan assets	(3,060)	(4,571)	(4,835)
Amortization of actuarial (gain) loss		29	121
Settlement	89	(35)
Net periodic pension expense	53	604	359
Postretirement Benefits
Components of the plan's net periodic pension cost
Service cost	139	126	147
Interest cost	163	182	161
Amortization of actuarial (gain) loss	(11)	(58)	(67)
Net periodic pension expense	$ 291	$ 250	$ 241