EMPLOYEE BENEFIT PLANS - Benefit Obligations (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Amounts recognized on consolidated balance sheets:
Noncurrent assets recognized		$ 2,646	$ 3,619
Amounts recognized to accumulated other comprehensive income:
Total accumulated other comprehensive income		(380)	(1,405)
Pension Plans
Benefit obligation:
Interest cost		0	0	$ 816
Postretirement Benefit Plans
Benefit obligation:
Balance at beginning of period	$ 6,488	5,447	6,488
Service cost		70	94	392
Interest cost		227	223	466
Actuarial loss		413	917
Plan amendments		0	(422)
Curtailment gain	(800)	0	(477)
Benefits paid		(221)	(1,376)
Balance at end of period		5,936	5,447	6,488
Plan assets:
Fair value of plan assets at beginning of period	$ 10,369	9,066	10,369
Actual return (loss) on plan assets		(263)	73
Benefits paid		(221)	(1,376)
Fair value of plan assets at end of period		8,582	9,066	$ 10,369
Funded status of plans		2,646	3,619
Amounts recognized on consolidated balance sheets:
Noncurrent assets recognized		2,646	3,619
Amounts recognized to accumulated other comprehensive income:
Prior service credit, net of taxes		(1,680)	(2,111)
Net actuarial loss, net of taxes		1,300	643
Total accumulated other comprehensive income		(380)	$ (1,468)
Actual loss on plan assets, tax payments		$ 900