PENSION AND POSTRETIREMENT PLANS - Components of Net Periodic Benefit (Credit) Cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Pension Benefits
Components of net periodic benefit cost and other amounts recognized in Other Comprehensive Income
Service cost	$ 325	$ 291	$ 975	$ 876
Interest cost	198	187	593	557
Expected return on plan assets	(626)	(521)	(1,876)	(1,561)
Amortization of net loss	83	113	248	338
Amortization of prior service credit	(20)	(20)	(58)	(58)
Amortization of transition obligation	0	0	0	0
Net periodic benefit (credit) cost	(40)	50	(118)	152
SERP Benefits
Components of net periodic benefit cost and other amounts recognized in Other Comprehensive Income
Service cost	73	54	218	159
Interest cost	32	26	95	78
Expected return on plan assets	0	0	0	0
Amortization of net loss	30	12	91	38
Amortization of prior service credit	0	0	0	0
Amortization of transition obligation	1	7	3	21
Net periodic benefit (credit) cost	$ 136	$ 99	$ 407	$ 296