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Employee Benefit Plans Employee Benefit Plans 4 (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Pension Benefits      
Components of Net Periodic Benefits Cost:      
Service cost $ 4,999 $ 5,094 $ 6,276
Interest cost 7,904 7,910 7,686
Expected return on plan assets (9,705) (9,838) (9,795)
Amortization of transition 0 0 0
Amortization of prior service cost (credit) 0 49 118
Amortization of actuarial (gain) loss 923 911 1,790
Net periodic pension cost under accounting standards 4,121 4,126 6,075
Regulatory adjustment - over collection (465) (859) (523)
Total expense recognized, before allocation to overhead pool $ 4,586 $ 4,985 $ 6,598
Weighted-average assumptions used to determine net periodic cost:      
Discount rate 4.44% 4.65% 4.25%
Expected long-term return on plan assets 6.50% 7.00% 7.00%
Rate of compensation increase     4.00%
Expected long-term return on plan assets 6.50% 7.00% 7.00%
Post-Retirement Medical Benefits      
Components of Net Periodic Benefits Cost:      
Service cost $ 227 $ 247 $ 340
Interest cost 324 371 435
Expected return on plan assets (466) (489) (493)
Amortization of transition 0 0 0
Amortization of prior service cost (credit) 0 (34) (200)
Amortization of actuarial (gain) loss (775) (701) (316)
Net periodic pension cost under accounting standards (690) (606) (234)
Regulatory adjustment - over collection 0 0 0
Total expense recognized, before allocation to overhead pool $ (690) $ (606) $ (234)
Weighted-average assumptions used to determine net periodic cost:      
Discount rate 3.97% 4.25% 3.80%
Union plan      
Weighted-average assumptions used to determine net periodic cost:      
Expected long-term return on plan assets 6.00% [1] 7.00% 7.00%
Expected long-term return on plan assets 6.00% [1] 7.00% 7.00%
Non-union plan      
Weighted-average assumptions used to determine net periodic cost:      
Expected long-term return on plan assets 4.20% [1] 4.20% 4.20%
Expected long-term return on plan assets 4.20% [1] 4.20% 4.20%
Minimum | Pension Benefits      
Weighted-average assumptions used to determine net periodic cost:      
Rate of compensation increase 3.00%    
Maximum | Pension Benefits      
Weighted-average assumptions used to determine net periodic cost:      
Rate of compensation increase 8.00%    
[1] *6.0% for union plan and 4.2% for non-union (net of income taxes) in 2017, and 7.00% for union plan and 4.20% for non-union (net of income taxes) for 2016 and 2015.