EMPLOYEE BENEFIT PLANS - Postretirement Benefits Other Than Pensions, Obligations (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($) plan
Medical Plans
Postretirement medical benefit and postemployment medical benefit obligations
End of period	$ 10,644
Postretirement Medical Benefit Plans
Postretirement medical benefit and postemployment medical benefit obligations
Beginning of period	10,195
Service cost	141
Interest cost	88
Benefit payments (net of contributions by participants)	(155)
End of period	$ 10,269
Threshold percentage, unrecognized amount of actuarial gains and losses as a percentage of projected benefit obligation	10.00%
Postemployment Medical Benefit Plans
Postretirement medical benefit and postemployment medical benefit obligations
End of period	$ 375
Non-bargaining unit employees | Medical Plans
POSTRETIREMENT BENEFITS OTHER THAN PENSIONS
Number of plans | plan	3
Non-bargaining unit employees | Medical Plans | Maximum
POSTRETIREMENT BENEFITS OTHER THAN PENSIONS
Qualifying age	65 years
Non-bargaining unit employees | Postretirement Medical Benefit Plans
POSTRETIREMENT BENEFITS OTHER THAN PENSIONS
Number of plans | plan	2
Non-bargaining unit employees | Postretirement Medical Benefit Plans | Minimum
POSTRETIREMENT BENEFITS OTHER THAN PENSIONS
Qualifying age	55 years
Non-bargaining unit employees | Postemployment Medical Benefit Plans
POSTRETIREMENT BENEFITS OTHER THAN PENSIONS
Number of plans | plan	3