POSTRETIREMENT BENEFIT PLANS - Components of cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Components of net periodic benefit cost
Pension settlement expense				$ 89
Supplemental Benefit Plan
Components of net periodic benefit cost
Interest cost	$ 1	$ 2	$ 2	5
Pension settlement expense				89
Amortization of net actuarial (gain) loss	3	1	5	5
Net periodic benefit cost	4	3	7	99
Postretirement Health Benefit Plan
Components of net periodic benefit cost
Service cost	48	47	96	94
Interest cost	107	144	214	288
Amortization of net actuarial (gain) loss	(137)	(149)	(274)	(298)
Net periodic benefit cost	$ 18	$ 42	$ 36	$ 84