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Employee Benefit Plans - Components of Net Periodic Pension Costs (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Union plan |  |  |
Weighted-average assumptions used to determine net periodic cost: |  |  |
Expected long-term return on plan assets | 5.50% | 5.50% | 5.75%
Non-union plan |  |  |
Weighted-average assumptions used to determine net periodic cost: |  |  |
Expected long-term return on plan assets | 3.90% | 3.90% | 4.00%
Pension Benefits |  |  |
Components of Net Periodic Benefits Cost: |  |  |
Service cost | $ 3,196 | $ 5,644 | $ 6,316
Interest cost | 10,142 | 7,401 | 6,833
Expected return on plan assets | (10,483) | (13,166) | (12,541)
Amortization of prior service cost (credit) | 434 | 434 | 434
Amortization of actuarial (gain) loss | 0 | 0 | 3,817
Net periodic pension cost under accounting standards | 3,289 | 313 | 4,859
Regulatory adjustment | (281) | 0 | (1,277)
Total expense recognized, before surcharges and allocation to overhead pool | $ 3,008 | $ 313 | $ 3,582
Weighted-average assumptions used to determine net periodic cost: |  |  |
Discount rate | 5.41% | 2.89% | 2.55%
Expected long-term return on plan assets | 5.75% | 5.75% | 6.00%
Pension Benefits | Minimum |  |  |
Weighted-average assumptions used to determine net periodic cost: |  |  |
Rate of compensation increase | 2.50% | 2.50% | 2.50%
Pension Benefits | Maximum |  |  |
Weighted-average assumptions used to determine net periodic cost: |  |  |
Rate of compensation increase | 7.00% | 7.00% | 7.00%
Post-Retirement Medical Benefits |  |  |
Components of Net Periodic Benefits Cost: |  |  |
Service cost | $ 130 | $ 129 | $ 149
Interest cost | 106 | 60 | 110
Expected return on plan assets | (477) | (587) | (537)
Amortization of prior service cost (credit) | 0 | 0 | 0
Amortization of actuarial (gain) loss | (969) | (1,734) | (1,417)
Net periodic pension cost under accounting standards | (1,210) | (2,132) | (1,695)
Regulatory adjustment | 0 | 0 | 0
Total expense recognized, before surcharges and allocation to overhead pool | $ (1,210) | $ (2,132) | $ (1,695)
Weighted-average assumptions used to determine net periodic cost: |  |  |
Discount rate | 5.34% | 2.46% | 2.20%

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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/exampleRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 17 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480482/715-20-55-17 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 45 -Paragraph 3A -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480535/715-20-45-3A Reference 3: http://www.xbrl.org/2003/role/exampleRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 18 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480482/715-20-55-18 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (h)(4) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Subparagraph (a)(4) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-6 + 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/exampleRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 17 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480482/715-20-55-17 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 45 -Paragraph 3A -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480535/715-20-45-3A Reference 3: http://www.xbrl.org/2003/role/exampleRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 18 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480482/715-20-55-18 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (h)(5) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Subparagraph (a)(5) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-6 + 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/exampleRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 17 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480482/715-20-55-17 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 45 -Paragraph 3A -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480535/715-20-45-3A Reference 3: http://www.xbrl.org/2003/role/exampleRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 18 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480482/715-20-55-18 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (h)(3) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Subparagraph (a)(3) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-6 + Details
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