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RETIREMENT PLANS (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Components of net periodic benefit cost [Abstract]        
Service period of participants not impacted by pension plan amendment     15 years  
Age of plan participants not impacted by pension plan amendment     50 years  
Service period of participants who are age fifty not impacted by pension plan amendment     10 years  
Decrease in pension plan projected benefit obligation     $ (33,433)  
Pension plan defined benefit plan cost increase limit percentage     4.00%  
Pension Benefits [Member]        
Components of net periodic benefit cost [Abstract]        
Service cost $ 2,865 $ 3,482 $ 7,192 $ 6,702
Interest cost 3,409 3,651 7,089 7,063
Expected return on plan assets (4,568) (4,251) (9,005) (8,404)
Amortization of actuarial (gain) loss 1,069 1,525 2,438 2,743
Net periodic benefit cost 2,775 4,407 7,714 8,104
SERP [Member]        
Components of net periodic benefit cost [Abstract]        
Service cost 0 0 0 0
Interest cost 15 17 29 33
Expected return on plan assets 0 0 0 0
Amortization of actuarial (gain) loss 7 6 14 13
Net periodic benefit cost 22 23 43 46
Other Postretirement Benefits [Member]        
Components of net periodic benefit cost [Abstract]        
Service cost 0 0 0 0
Interest cost 7 3 14 15
Amortization of actuarial (gain) loss (168) (186) (335) (374)
Net periodic benefit cost $ (161) $ (183) $ (321) $ (359)