PENSION AND OTHER POSTRETIREMENT PLANS - Components (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Components of net periodic benefit cost and other amounts recognized in Other Comprehensive Income
Total recognized in other comprehensive income	$ 1,076	$ 1,567	$ 302
Pension Benefits
Components of net periodic benefit cost and other amounts recognized in Other Comprehensive Income
Service cost	952	1,106	1,129
Interest cost	908	794	750
Expected return on plan assets	(2,445)	(2,547)	(2,129)
Amortization of net loss	494	335	479
Amortization of prior service credit	(77)	(77)	(77)
Amortization of transition obligation	0	0	0
Net periodic benefit (credit) cost	(168)	(389)	152
Net loss (gain)	(140)	1,980	(409)
Amortization of net loss	(494)	(335)	(479)
Amortization of prior service credit	77	77	77
Amortization of transition obligation	0	0	0
Total recognized in other comprehensive income	(557)	1,722	(811)
SERP Benefits
Components of net periodic benefit cost and other amounts recognized in Other Comprehensive Income
Service cost	261	290	212
Interest cost	147	127	105
Expected return on plan assets	0	0	0
Amortization of net loss	70	121	51
Amortization of prior service credit	0	0	0
Amortization of transition obligation	0	5	27
Net periodic benefit (credit) cost	478	543	395
Net loss (gain)	1,216	(413)	710
Amortization of net loss	(70)	(121)	(51)
Amortization of prior service credit	0	0	0
Amortization of transition obligation	0	(5)	(27)
Total recognized in other comprehensive income	$ 1,146	$ (539)	$ 632