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EMPLOYEE RETIREMENT PLANS - Components of Net Periodic Pension Cost (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Components of net periodic pension cost [Abstract]      
Service cost $ 1,214 $ 1,415  
Interest cost 1,013 875  
Supplemental Executive Retirement Plan [Member]      
Components of net periodic pension cost [Abstract]      
Service cost 1,214 1,415 $ 1,186
Interest cost 1,013 875 618
Amortization of prior service cost 1,667 1,660 1,550
Amortization of loss 457 15  
Total net periodic benefit cost $ 4,351 $ 3,965 $ 3,354