PENSION AND OTHER POSTRETIREMENT PLANS - Components (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Components of net periodic benefit cost and other amounts recognized in Other Comprehensive Income
Total recognized in other comprehensive income	$ 1,567	$ 302	$ 1,452
Pension Benefits
Components of net periodic benefit cost and other amounts recognized in Other Comprehensive Income
Service cost	1,106	1,129	1,153
Interest cost	794	750	794
Expected return on plan assets	(2,547)	(2,129)	(1,927)
Amortization of net loss	335	479	406
Amortization of prior service credit	(77)	(77)	(77)
Amortization of transition obligation	0	0	0
Net periodic benefit (credit) cost	(389)	152	349
Net loss (gain)	1,980	(409)	1,172
Amortization of net loss	(335)	(479)	(406)
Amortization of prior service credit	77	77	77
Amortization of transition obligation	0	0	0
Total recognized in other comprehensive income	1,722	(811)	843
SERP Benefits
Components of net periodic benefit cost and other amounts recognized in Other Comprehensive Income
Service cost	290	212	176
Interest cost	127	105	105
Expected return on plan assets	0	0	0
Amortization of net loss	121	51	27
Amortization of prior service credit	0	0	0
Amortization of transition obligation	5	27	28
Net periodic benefit (credit) cost	543	395	336
Net loss (gain)	(413)	710	280
Amortization of net loss	(121)	(51)	(27)
Amortization of prior service credit	0	0	0
Amortization of transition obligation	(5)	(27)	(28)
Total recognized in other comprehensive income	$ (539)	$ 632	$ 225