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Employee Benefit Plans 4 (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Union plan      
Weighted-average assumptions used to determine net periodic cost:      
Expected long-term return on plan assets 6.00% 6.00% 6.00%
Non-union plan      
Weighted-average assumptions used to determine net periodic cost:      
Expected long-term return on plan assets 4.20% 4.20% 4.20%
Pension Benefits      
Components of Net Periodic Benefits Cost:      
Service cost $ 5,558 $ 4,441 $ 5,342
Interest cost 7,880 8,527 7,646
Expected return on plan assets (11,798) (10,374) (11,172)
Amortization of prior service cost (credit) 435 434 0
Amortization of actuarial (gain) loss 1,935 1,419 1,254
Net periodic pension cost under accounting standards 4,010 4,447 3,070
Regulatory adjustment (483) (593) 0
Total expense recognized, before surcharges and allocation to overhead pool $ 3,527 $ 3,854 $ 3,070
Weighted-average assumptions used to determine net periodic cost:      
Discount rate 3.43% 4.43% 3.76%
Expected long-term return on plan assets 6.25% 6.50% 6.50%
Pension Benefits | Minimum      
Weighted-average assumptions used to determine net periodic cost:      
Rate of compensation increase 3.00% 3.00% 3.00%
Pension Benefits | Maximum      
Weighted-average assumptions used to determine net periodic cost:      
Rate of compensation increase 8.00% 8.00% 8.00%
Post-Retirement Medical Benefits      
Components of Net Periodic Benefits Cost:      
Service cost $ 171 $ 186 $ 218
Interest cost 208 285 292
Expected return on plan assets (510) (449) (493)
Amortization of prior service cost (credit) 0 0
Amortization of actuarial (gain) loss (977) (801) (769)
Net periodic pension cost under accounting standards (1,108) (779) (752)
Regulatory adjustment 0 0 0
Total expense recognized, before surcharges and allocation to overhead pool $ (1,108) $ (779) $ (752)
Weighted-average assumptions used to determine net periodic cost:      
Discount rate 3.12% 4.20% 3.52%