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Retirement Plans and Other Postretirement Benefits (Tables) | 6 Months Ended
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Jun. 30, 2026
Retirement Benefits [Abstract] |
Schedule of Net Periodic Benefit Costs and the Portion of these Costs Charged to Expense | The following table provides detail of the plans’ net periodic benefit costs and the portion of these costs charged to expense (including administrative costs and excluding amounts capitalized as overhead construction or billed to electric plant participants) (dollars in thousands):
|  |  |  |  |  |  |  |  |  |  |  |  | Pension Plans |  | Other Benefits Plans
|  |  |  |  | Three Months Ended June 30, |  | Six Months Ended June 30, |  | Three Months Ended June 30, |  | Six Months Ended June 30,
|  |  |  |  |  |  |  |  |  |  |  |  | 2026 |  | 2025 |  | 2026 |  | 2025 |  | 2026 |  | 2025 |  | 2026 |  | 2025
Service cost-benefits earned during the period |  |  |  |  |  |  |  |  |  |  |  |  | $ | 11,684 |  |  | $ | 11,127 |  |  | $ | 23,783 |  |  | $ | 22,076 |  |  | $ | 2,140 |  |  | $ | 2,059 |  |  | $ | 4,377 |  |  | $ | 4,041 |
Non-service costs (credits): |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest cost on benefit obligation |  |  |  |  |  |  |  |  |  |  |  |  | 37,292 |  |  | 38,584 |  |  | 74,904 |  |  | 77,560 |  |  | 4,931 |  |  | 5,070 |  |  | 9,910 |  |  | 10,172 |
Expected return on plan assets |  |  |  |  |  |  |  |  |  |  |  |  | (44,365) |  |  | (44,849) |  |  | (88,461) |  |  | (89,396) |  |  | (12,719) |  |  | (12,142) |  |  | (25,438) |  |  | (24,284) |
Amortization of: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Prior service cost (credit) |  |  |  |  |  |  |  |  |  |  |  |  | 151 |  |  | — |  |  | 302 |  |  | — |  |  | — |  |  | — |  |  | — |  |  | (1,265) |
Net actuarial loss (gain) |  |  |  |  |  |  |  |  |  |  |  |  | 10,443 |  |  | 11,248 |  |  | 21,596 |  |  | 23,366 |  |  | (2,746) |  |  | (2,965) |  |  | (5,446) |  |  | (5,864) |
Net periodic benefit costs (credits) |  |  |  |  |  |  |  |  |  |  |  |  | $ | 15,205 |  |  | $ | 16,110 |  |  | $ | 32,124 |  |  | $ | 33,606 |  |  | $ | (8,394) |  |  | $ | (7,978) |  |  | $ | (16,597) |  |  | $ | (17,200) |
Portion of costs (credits) charged to expense |  |  |  |  |  |  |  |  |  |  |  |  | $ | 8,262 |  |  | $ | 9,365 |  |  | $ | 17,780 |  |  | $ | 19,826 |  |  | $ | (6,644) |  |  | $ | (6,399) |  |  | $ | (13,153) |  |  | $ | (13,272) |

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- Definition Tabular disclosure of the components of net benefit costs for pension plans and/or other employee benefit plans including service cost, interest cost, expected return on plan assets, gain (loss), prior service cost or credit, transition asset or obligation, and gain (loss) recognized due to settlements or curtailments. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Subparagraph (q) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-5 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 715 -SubTopic 20 -Subparagraph (h) -Name Accounting Standards Codification -Paragraph 1 -Section 50 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-1 + Details
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