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EMPLOYEE BENEFITS - POSTRETIREMENT - Schedule of Net Periodic Benefit Costs Charged to Operating Expenses (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Abstract]    
Service cost $ 1,629 $ 1,897
Interest cost 1,712 2,149
Expected return on assets (3,315) (2,857)
Amortization of unrecognized gains (840) (655)
Amortization of prior service credits (2,424) (1,622)
Net periodic benefit $ (3,238) $ (1,088)