POSTRETIREMENT BENEFIT PLANS - Components of cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Supplemental Benefit Plan
Components of net periodic benefit cost (credit)
Interest cost	$ 2	$ 1
Amortization of net actuarial (gain) loss	2	2
Net periodic benefit cost (credit)	4	3
Postretirement Health Benefit Plan
Components of net periodic benefit cost (credit)
Service cost	39	48
Interest cost	110	107
Amortization of net actuarial (gain) loss	(191)	(137)
Net periodic benefit cost (credit)	$ (42)	$ 18