PENSION AND OTHER POSTRETIREMENT PLANS - Changes in Obligations and Plan Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Benefits
Change in benefit obligation:
Benefit obligation at beginning of year	$ 28,757	$ 23,611
Service cost	940	952	$ 1,106
Interest cost	794	908	794
Benefits paid and expected expenses	(609)	(475)
Assumption changes and other	3,865	3,761
Benefit obligation at end of year	33,747	28,757	23,611
Change in plan assets:
Fair value of plan assets at beginning of year	39,745	33,874
Actual return on plan assets	3,764	6,346
Employer contribution to the plan	1,160
Benefits paid and actual expenses	(609)	(475)
Fair value of plan assets at end of year	44,060	39,745	33,874
Funded status at end of year	10,313	10,988
SERP Benefits
Change in benefit obligation:
Benefit obligation at beginning of year	5,323	3,811
Service cost	371	261	290
Interest cost	149	147	127
Benefits paid and expected expenses	(112)	(112)
Assumption changes and other	238	1,216
Benefit obligation at end of year	5,969	5,323	$ 3,811
Change in plan assets:
Fair value of plan assets at beginning of year	0
Actual return on plan assets	0	0
Employer contribution to the plan	112	112
Benefits paid and actual expenses	(112)	(112)
Fair value of plan assets at end of year	0	0
Funded status at end of year	$ (5,969)	$ (5,323)