PENSION AND OTHER POSTRETIREMENT PLANS - Components of Net Periodic Benefit (Credit) Cost and Other Amounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Components of net periodic benefit (credit) cost and other amounts recognized in other comprehensive income:
Total recognized in other comprehensive income	$ 3,245	$ 1,076	$ 1,567
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)
Net loss (gain)	3,006	(140)	1,980
Amortization of net loss	(431)	(494)	(335)
Amortization of prior service credit	77	77	77
Amortization of transition obligation	0	0	0
Total recognized in other comprehensive income	2,652	(557)	1,722
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
Net loss (gain)	239	1,216	(413)
Amortization of net loss	(227)	(70)	(121)
Amortization of prior service credit	0	0	0
Amortization of transition obligation	0	0	(5)
Total recognized in other comprehensive income	$ 12	$ 1,146	$ (539)