POSTRETIREMENT BENEFIT PLANS - Components of cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Components of net periodic benefit cost
Pension settlement expense		$ 89
Supplemental Benefit Plan
Components of net periodic benefit cost
Interest cost	$ 1	3
Pension settlement expense		89
Amortization of net actuarial (gain) loss	2	4
Net periodic benefit cost	3	96
Postretirement Health Benefit Plan
Components of net periodic benefit cost
Service cost	48	47
Interest cost	107	144
Amortization of net actuarial (gain) loss	(137)	(149)
Net periodic benefit cost	$ 18	$ 42