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PENSION AND OTHER POSTRETIREMENT BENEFIT PLANS (Components of cost) (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Components of net periodic benefit cost    
Pension settlement expense $ 1,957 $ 900
Nonunion Defined Benefit Pension Plan    
Components of net periodic benefit cost    
Interest cost 1,240 1,305
Expected return on plan assets (2,167) (1,946)
Pension settlement expense 1,957 900
Amortization of net actuarial loss 886 955
Net periodic benefit cost 1,916 1,214
Supplemental Benefit Plan    
Components of net periodic benefit cost    
Interest cost 26 32
Amortization of net actuarial loss 20 38
Net periodic benefit cost 46 70
Postretirement Health Benefit Plan    
Components of net periodic benefit cost    
Service cost 122 107
Interest cost 265 254
Amortization of prior service credit (47) (47)
Amortization of net actuarial loss 173 176
Net periodic benefit cost $ 513 $ 490