EMPLOYEE BENEFIT PLANS - Postretirement Benefits Other Than Pensions, Plans, Amendments and Obligations (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) plan	Dec. 31, 2018 USD ($)
Postretirement medical benefit and postemployment medical benefit obligations
Threshold percentage, unrecognized amount of actuarial gains and losses as a percentage of projected benefit obligation	10.00%
Medical Plans
Postretirement medical benefit and postemployment medical benefit obligations
Postretirement medical benefit obligation at beginning of year	$ 8,927
Total postretirement and/or postemployment medical obligations at end of period	10,570	$ 8,927
Postretirement Medical Benefit Plans
Postretirement medical benefit and postemployment medical benefit obligations
Postretirement medical benefit obligation at beginning of year	8,556	8,455
Service cost	563	677
Interest cost	352	288
Benefit payments (net of contributions by participants)	(617)	(408)
Actuarial (gain) loss	1,341	(456)
Total postretirement and/or postemployment medical obligations at end of period	10,195	8,556
Postemployment Medical Benefit Plans
Postretirement medical benefit and postemployment medical benefit obligations
Postretirement medical benefit obligation at beginning of year	371
Total postretirement and/or postemployment medical obligations at end of period	$ 375	$ 371
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