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EMPLOYEE BENEFIT PLANS (Plan summary) (Details)
$ in Thousands
 3 Months Ended 12 Months Ended
Sep. 30, 2019
USD ($)
plan
person
Mar. 31, 2017
USD ($)
person
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Pension and other postretirement benefit plans          
Noncash charge to pension settlement expense     $ 4,000    
Nonunion Defined Benefit Pension Plan          
Pension and other postretirement benefit plans          
Nonparticipating annuity contract purchase price $ 14,000 $ 7,600 14,000   $ 7,600
Number of plan participants for which vested pension benefits were settled | person 120 50      
Transfer of remaining obligation to PGBC $ 1,500   1,500    
Number of plan participants for which benefit obligation was transferred to PBGC. | plan 30        
Employer contributions $ 7,700   7,710 $ 5,500  
Noncash charge to pension settlement expense     4,000    
Postretirement Health Benefit Plan          
Pension and other postretirement benefit plans          
Employer contributions     $ 848 $ 769  
Unrecognized prior service credit, amortization period     9 years