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Pension Plans and Other Postretirement Benefit Plans - Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Settlement loss |  |  | $ 11,800 |
Pension Plan And SERP [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Service cost | [1] | $ 14,350 | 23,877 | $ 25,306
Interest cost |  | 33,245 | 26,536 | 26,160
Expected return on plan assets |  | $ (43,656) | $ (43,872) | $ (39,088)
Defined Benefit Plan, Net Periodic Benefit (Cost) Credit, Expected Return (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration] |  | Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax | Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax | Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax
Amortization of prior service cost |  | $ 491 | $ 257 | $ 257
Net loss recognition |  | 4,915 | 4,180 | 6,645
Settlement loss | [2] |  | 11,828 |
Net periodic benefit cost |  | 9,345 | 22,806 | 19,280
Other Postretirement Benefits [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Service cost | [1] | 2,394 | 4,369 | 4,114
Interest cost |  | 6,766 | 5,503 | 5,139
Expected return on plan assets |  | $ (3,562) | $ (2,799) | $ (2,400)
Defined Benefit Plan, Net Periodic Benefit (Cost) Credit, Expected Return (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration] |  | Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax | Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax | Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax
Amortization of prior service cost |  | $ (1,050) | $ (1,050) | $ (921)
Net loss recognition |  | 319 | 3,344 | 3,865
Net periodic benefit cost |  | $ 4,867 | $ 9,367 | $ 9,797
[1] | Total service costs in the table above are recorded to the same accounts as labor expense. Labor and benefits expense is recorded to various projects based on whether the work is a capital project or an operating expense. Approximately 40 percent of all labor and benefits is capitalized to utility property and 60 percent is expensed to utility other operating expenses.
[2] | The settlement loss was deferred as a regulatory asset and is being amortized over 12 years in accordance with regulatory accounting orders.

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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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