PENSION AND OTHER POSTRETIREMENT PLANS (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits
Change in benefit obligation:
Benefit obligation at beginning of year	$ 20,844	$ 18,515
Service cost	1,129	1,153	$ 1,134
Interest cost	750	794	706
Benefits paid and expected expenses	(285)	(279)
Assumption changes and other	2,321	661
Benefit obligation at end of year	24,759	20,844	18,515
Change in plan assets:
Fair value of plan assets at beginning of year	27,914	24,562
Actual return on plan assets	4,859	1,416
Employer contribution	2,207	2,215
Benefits paid and actual expenses	(285)	(279)
Fair value of plan assets at end of year	34,695	27,914	24,562
Funded status at end of year	9,936	7,070
SERP Benefits
Change in benefit obligation:
Benefit obligation at beginning of year	3,004	2,555
Service cost	212	176	168
Interest cost	105	105	91
Benefits paid and expected expenses	(112)	(112)
Assumption changes and other	710	280
Benefit obligation at end of year	3,919	3,004	2,555
Change in plan assets:
Fair value of plan assets at beginning of year	0	0
Actual return on plan assets	0	0
Employer contribution	112	112
Benefits paid and actual expenses	(112)	(112)
Fair value of plan assets at end of year	0	0	$ 0
Funded status at end of year	$ (3,919)	$ (3,004)