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Employee Benefit Plans Employee Benefit Plans 4 (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Pension Benefits      
Components of Net Periodic Benefits Cost:      
Service cost $ 5,094 $ 6,276 $ 5,643
Interest cost 7,910 7,686 7,520
Expected return on plan assets (9,838) (9,795) (8,898)
Amortization of transition 0 0 0
Amortization of prior service cost (credit) 49 118 118
Amortization of actuarial (gain) loss 911 1,790 0
Net periodic pension cost under accounting standards 4,126 6,075 4,383
Regulatory adjustment - over collection (859) (523) (1,622)
Total expense recognized, before allocation to overhead pool $ 4,985 $ 6,598 $ 6,005
Weighted-average assumptions used to determine net periodic cost:      
Discount rate 4.65% 4.25% 5.10%
Expected long-term return on plan assets 7.00% 7.00% 7.00%
Rate of compensation increase   4.00% 4.00%
Expected long-term return on plan assets 7.00% 7.00% 7.00%
Post-Retirement Medical Benefits      
Components of Net Periodic Benefits Cost:      
Service cost $ 247 $ 340 $ 348
Interest cost 371 435 495
Expected return on plan assets (489) (493) (453)
Amortization of transition 0 0 418
Amortization of prior service cost (credit) (34) (200) (200)
Amortization of actuarial (gain) loss (701) (316) (330)
Net periodic pension cost under accounting standards (606) (234) 278
Regulatory adjustment - over collection 0 0 0
Total expense recognized, before allocation to overhead pool $ (606) $ (234) $ 278
Weighted-average assumptions used to determine net periodic cost:      
Discount rate 4.25% 3.80% 4.65%
Union plan      
Weighted-average assumptions used to determine net periodic cost:      
Expected long-term return on plan assets 7.00% [1] 7.00% 7.00%
Expected long-term return on plan assets 7.00% [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] *7.0% for union plan, 4.2% for non-union, net of income taxes in 2016, 2015 and 2014.