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BENEFIT PLANS (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Mar. 31, 2019
Jun. 30, 2020
Jun. 30, 2019
Pension Benefits [Member]          
Net periodic benefit cost [Abstract]          
Service cost $ 1,438 $ 1,270   $ 2,875 $ 2,540
Interest cost 1,356 1,565   2,712 3,133
Expected return on plan assets (3,932) (3,578)   (7,864) (7,155)
Amortization of unrecognized net loss 810 643   1,620 1,284
Amortization of prior service cost 60 16   120 31
Net periodic benefit (268) (84)   (537) (167)
Pension Benefits [Member] | Nonqualified Plan [Member] | Unfunded Plan [Member]          
Pension Plans [Abstract]          
Contribution made to defined benefit pension plan by employer 3,900   $ 7,300    
Post-retirement Benefits [Member]          
Net periodic benefit cost [Abstract]          
Service cost 0 0   0 0
Interest cost 14 18   28 35
Expected return on plan assets 0 0   0 0
Amortization of unrecognized net loss 10 9   20 19
Amortization of prior service cost (44) (45)   (89) (89)
Net periodic benefit $ (20) $ (18)   $ (41) $ (35)