RETIREMENT PLANS - Net Periodic Pension Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Plans
Components of the plan's net periodic pension cost
Service cost	$ 1,709	$ 1,794	$ 1,676
Interest cost	3,472	3,279	3,388
Expected return on plan assets	(4,571)	(4,835)	(4,470)
Actuarial gain/ loss	29	121	716
Settlement charge	(35)
Net periodic pension expense	604	359	1,310
Postretirement Benefits
Components of the plan's net periodic pension cost
Service cost	126	147	183
Interest cost	182	161	206
Actuarial gain/ loss	(58)	(67)
Net periodic pension expense	$ 250	$ 241	$ 389