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Employee and Retiree Benefit Plans (Tables) | 9 Months Ended
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Sep. 30, 2025
Retirement Benefits [Abstract] |
Schedule of Components of Net Periodic Benefit Expense | The table that follows provides the components of net periodic benefit expense for the three-month and nine-month periods ended September 30, 2025 and 2024.
| Three Months Ended September 30,
| Pension Benefits |  | Other Postretirement Benefits
(Thousands of dollars) | 2025 |  | 2024 |  | 2025 |  | 2024
Service cost | $ | 1,683 |  |  | $ | 1,706 |  |  | $ | 84 |  |  | $ | 135 |
Interest cost | 8,495 |  |  | 8,398 |  |  | 708 |  |  | 782 |
Expected return on plan assets | (8,967) |  |  | (8,366) |  |  | — |  |  | — |
Estimated defined contribution provision | 62 |  |  | 54 |  |  | — |  |  | — |
Amortization of prior service (credit) cost | 492 |  |  | 579 |  |  | (133) |  |  | (133) |
Recognized actuarial (gain) loss | 1,918 |  |  | 2,363 |  |  | (1,057) |  |  | (812) |
Total net periodic benefit expense | $ | 3,683 |  |  | $ | 4,734 |  |  | $ | (398) |  |  | $ | (28) |
| Nine Months Ended September 30,
| Pension Benefits |  | Other Postretirement Benefits
(Thousands of dollars) | 2025 |  | 2024 |  | 2025 |  | 2024
Service cost | $ | 5,049 |  |  | $ | 5,118 |  |  | $ | 252 |  |  | $ | 405 |
Interest cost | 25,375 |  |  | 25,182 |  |  | 2,124 |  |  | 2,346 |
Expected return on plan assets | (26,791) |  |  | (25,082) |  |  | — |  |  | — |
Estimated defined contribution provision | 184 |  |  | 163 |  |  | — |  |  | — |
Amortization of prior service (credit) cost | 1,475 |  |  | 1,737 |  |  | (399) |  |  | (399) |
Recognized actuarial (gain) loss | 5,723 |  |  | 7,084 |  |  | (3,170) |  |  | (2,436) |
Total net periodic benefit expense | $ | 11,015 |  |  | $ | 14,202 |  |  | $ | (1,193) |  |  | $ | (84) |

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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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