EMPLOYEE BENEFIT PLANS - Components of cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Components of net periodic benefit cost
Pension settlement expense, including termination expense	$ 89	$ 8,505	$ 12,925
Nonunion Defined Benefit Pension Plan
Components of net periodic benefit cost
Interest cost		624	4,269
Expected return on plan assets		(31)	(1,582)
Pension settlement expense, including termination expense		4,164	12,925
Amortization of net actuarial (gain) loss		260	2,583
Net periodic benefit cost		5,017	18,195
Noncash charge to pension settlement expense		4,000
Supplemental Benefit Plan
Components of net periodic benefit cost
Interest cost	9	39	108
Pension settlement expense, including termination expense	89	370
Amortization of net actuarial (gain) loss	8	95	81
Net periodic benefit cost	106	504	189
Postretirement Health Benefit Plan
Components of net periodic benefit cost
Service cost	187	320	366
Interest cost	576	1,212	837
Amortization of prior service credit	(1)	(33)	(93)
Amortization of net actuarial (gain) loss	(597)	898	304
Net periodic benefit cost	$ 165	$ 2,397	$ 1,414