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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.

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- Definition Amount of gain (loss) recognized in net periodic benefit (cost) credit of defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(4) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of prior service cost (credit) recognized in net periodic benefit cost (credit) of defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(5) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of transition asset (obligation) recognized in net periodic benefit (cost) credit of defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(6) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Weighted average rate increase of compensation, used to determine benefit obligation of defined benefit plan. Plan includes, but is not limited to, pay-related defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(2) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Weighted average rate for present value of future retirement benefits cash flows, used to determine net periodic benefit cost of defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(1) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Weighted average rate of return on plan assets, reflecting average rate of earnings expected on existing plan assets and expected contributions, used to determine net periodic benefit cost of defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(3) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Weighted average rate of compensation increase used to determine net periodic benefit cost of defined benefit plan. Plan includes, but is not limited to, pay-related defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(2) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of expected return (loss) recognized in net periodic benefit (cost) credit, calculated based on expected long-term rate of return and market-related value of plan assets of defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(3) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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