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EMPLOYEE BENEFIT PLANS - Postretirement Benefits Other Than Pensions, Plans, Amendments and Obligations (Details)
$ in Thousands
12 Months Ended
Dec. 31, 2016
USD ($)
plan
Dec. 31, 2015
USD ($)
plan
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 $ 6,901  
Postretirement and/or postemployment medical benefit obligation at end of year 8,174 $ 6,901
Postretirement Medical Benefit Plans    
Postretirement medical benefit and postemployment medical benefit obligations    
Postretirement medical benefit obligation at beginning of year 6,723 7,713
Service cost 549 593
Interest cost 285 270
Plan amendments - prior service cost   (896)
Benefit payments (net of contributions by participants) (382) (325)
Actuarial (gain) loss 821 (632)
Postretirement and/or postemployment medical benefit obligation at end of year 7,996 $ 6,723
Postretirement Medical Benefit Plans Amendment    
POSTRETIREMENT BENEFITS OTHER THAN PENSIONS    
Number of plans | plan   3
Age after which participants are no longer eligible for benefit coverage   65 years
Percentage reduction in liability   11.50%
Annual amortization of prior service cost   $ 100
Postemployment Medical Benefit Plans    
Postretirement medical benefit and postemployment medical benefit obligations    
Postretirement medical benefit obligation at beginning of year 178  
Postretirement and/or postemployment medical benefit obligation at end of year $ 178 $ 178
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