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EMPLOYEE BENEFIT PLANS (Plans) (Details)
$ in Thousands
 1 Months Ended 3 Months Ended 12 Months Ended
Sep. 30, 2018
USD ($)
Mar. 31, 2019
USD ($)
Mar. 31, 2017
USD ($)
person
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Nonunion Defined Benefit Pension, Supplemental Benefit, and Postretirement Health Benefit Plans            
Pension settlement expense, pre-tax       $ 12,925 $ 4,156 $ 3,229
Nonunion Defined Benefit Pension Plan            
Nonunion Defined Benefit Pension, Supplemental Benefit, and Postretirement Health Benefit Plans            
Nonparticipating annuity contract     $ 7,600   7,600  
Pension settlement expense, pre-tax       12,925 4,156 $ 3,023
Employer contributions $ 5,500     5,500    
Number of plan participants for which vested pension benefits were settled | person     50      
Postretirement Health Benefit Plan            
Nonunion Defined Benefit Pension, Supplemental Benefit, and Postretirement Health Benefit Plans            
Employer contributions       $ 769 $ 733  
Unrecognized prior service credit, amortization period       9 years    
Forecast | Nonunion Defined Benefit Pension Plan            
Nonunion Defined Benefit Pension, Supplemental Benefit, and Postretirement Health Benefit Plans            
Pension settlement expense, pre-tax   $ 4,000        
Pension settlement expense, after-tax   3,000        
Employer contributions   $ 7,000