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EMPLOYEE BENEFIT PLANS (Components of cost) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Components of net periodic benefit cost      
Pension settlement expense $ 4,156 $ 3,229 $ 3,202
Nonunion Defined Benefit Pension Plan      
Components of net periodic benefit cost      
Interest cost 4,514 4,572 5,200
Expected return on plan assets (5,712) (8,607) (9,180)
Pension settlement expense 4,156 3,023 3,202
Amortization of net actuarial loss 3,132 4,087 3,218
Net periodic benefit cost 6,090 3,075 2,440
Supplemental Benefit Plan      
Components of net periodic benefit cost      
Interest cost 102 130 123
Pension settlement expense   206  
Amortization of net actuarial loss 82 152 159
Net periodic benefit cost 184 488 282
Postretirement Health Benefit Plan      
Components of net periodic benefit cost      
Service cost 489 429 406
Interest cost 1,060 1,017 913
Amortization of prior service credit (190) (190) (190)
Amortization of net actuarial loss 694 705 853
Net periodic benefit cost $ 2,053 $ 1,961 $ 1,982