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Employee Benefit Plans Employee Benefit Plans 4 (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Pension Benefits      
Components of Net Periodic Benefits Cost:      
Service cost $ 5,342 $ 4,999 $ 5,094
Interest cost 7,646 7,904 7,910
Expected return on plan assets (11,172) (9,705) (9,838)
Amortization of prior service cost (credit) 0 0 49
Amortization of actuarial (gain) loss 1,254 923 911
Net periodic pension cost under accounting standards 3,070 4,121 4,126
Regulatory adjustment 0 (465) (859)
Total expense recognized, before surcharges and allocation to overhead pool $ 3,070 $ 4,586 $ 4,985
Weighted-average assumptions used to determine net periodic cost:      
Discount rate 3.76% 4.44% 4.65%
Expected long-term return on plan assets 6.50% 6.50% 7.00%
Expected long-term return on plan assets 6.50% 6.50% 7.00%
Post-Retirement Medical Benefits      
Components of Net Periodic Benefits Cost:      
Service cost $ 218 $ 227 $ 247
Interest cost 292 324 371
Expected return on plan assets (493) (466) (489)
Amortization of prior service cost (credit) 0 0 (34)
Amortization of actuarial (gain) loss (769) (775) (701)
Net periodic pension cost under accounting standards (752) (690) (606)
Regulatory adjustment 0 0 0
Total expense recognized, before surcharges and allocation to overhead pool $ (752) $ (690) $ (606)
Weighted-average assumptions used to determine net periodic cost:      
Discount rate 3.52% 3.97% 4.25%
Union plan      
Weighted-average assumptions used to determine net periodic cost:      
Expected long-term return on plan assets 6.00% 6.00% 7.00%
Expected long-term return on plan assets 6.00% 6.00% 7.00%
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%
Expected long-term return on plan assets 4.20% 4.20% 4.20%
Minimum | Pension Benefits      
Weighted-average assumptions used to determine net periodic cost:      
Rate of compensation increase 3.00% 3.00%  
Maximum | Pension Benefits      
Weighted-average assumptions used to determine net periodic cost:      
Rate of compensation increase 8.00% 8.00%