RETIREMENT PLANS - Net Periodic Pension Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Plans
Components of the plan's net periodic pension cost
Service cost	$ 1,676	$ 1,308	$ 652
Interest cost	3,388	2,002	766
Expected return on plan assets	(4,470)	(2,024)	(813)
Amortization of unrecognized net actuarial loss	716	1,271	245
Curtailment		128
Net periodic pension cost	1,310	2,685	$ 850
Postretirement Benefits
Components of the plan's net periodic pension cost
Service cost	183	97
Interest cost	206	97
Net periodic pension cost	$ 389	$ 194