PENSION AND POSTRETIREMENT PLANS - Components of Net Periodic Benefit Cost (Credit) - (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Benefits
Components of net periodic benefit (credit) cost and other amounts recognized in other comprehensive income:
Service cost	$ 940	$ 952	$ 1,106
Interest cost	794	908	794
Expected return on plan assets	(2,905)	(2,445)	(2,547)
Amortization of net loss	431	494	335
Amortization of prior service credit	(77)	(77)	(77)
Amortization of transition obligation	0	0	0
Net periodic benefit (credit) cost	(817)	(168)	(389)
SERP Benefits
Components of net periodic benefit (credit) cost and other amounts recognized in other comprehensive income:
Service cost	371	261	290
Interest cost	149	147	127
Expected return on plan assets	0	0	0
Amortization of net loss	227	70	121
Amortization of prior service credit	0	0	0
Amortization of transition obligation	0	0	5
Net periodic benefit (credit) cost	$ 747	$ 478	$ 543