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BENEFIT PLANS - Net Periodic Benefit Cost (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Prior service costs [Abstract]      
Maximum percentage of net gain or loss over the greater of the projected benefit obligation or the market-related value of assets without requiring amortization 10.00%    
Pension Benefits [Member]      
Net periodic benefit cost [Abstract]      
Service cost $ 5,750 $ 5,081 $ 4,561
Interest cost 5,657 6,264 5,676
Expected return on plan assets (16,306) (14,311) (14,820)
Plan amendment (637) 0 13
Amortization of unrecognized net loss/(gain) 3,239 2,568 1,193
Amortization of prior service cost 241 64 (293)
Net periodic (benefit) (2,056) (334) (3,670)
Post-retirement Benefits [Member]      
Net periodic benefit cost [Abstract]      
Service cost 0 0 0
Interest cost 57 70 69
Expected return on plan assets 0 0 0
Plan amendment 0 0 0
Amortization of unrecognized net loss/(gain) 40 38 21
Amortization of prior service cost (179) (179) (179)
Net periodic (benefit) $ (82) $ (71) $ (89)