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EMPLOYEE BENEFIT PLANS (Tables) | 6 Months Ended
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Jun. 30, 2026
Employee Benefit Plans [Line Items] |
Schedule of Net Benefit Costs [Table Text Block] | The following tables set forth the components of net periodic benefit cost (credit) for our pension, inclusive of our defined benefit pension plan, supplemental executive retirement plan, and other postemployment benefit plans for the periods indicated:
| Pension Benefits |
| Three Months Ended |  | Six Months Ended |
| June 30, | June 30, |
| 2026 |  | 2025 |  | 2026 |  | 2025 |  |
| (Thousands of dollars) |
Components of net periodic benefit cost (credit) |  |  |  |  |  |  |  |  |
Service cost | $ | 1,291 |  |  | $ | 1,337 |  |  | $ | 2,582 |  |  | $ | 2,674 |  |  |
Interest cost (a) | 9,525 |  |  | 10,042 |  |  | 19,050 |  |  | 20,084 |  |  |
Expected return on assets (a) | (10,489) |  |  | (11,910) |  |  | (20,979) |  |  | (23,820) |  |  |
Amortization of unrecognized prior service cost (credit) (a) | 93 |  |  | 93 |  |  | 186 |  |  | 186 |  |  |
Amortization of net loss (gain) (a) | 4,220 |  |  | 1,865 |  |  | 8,440 |  |  | 3,730 |  |  |
Net periodic benefit cost (credit) | $ | 4,640 |  |  | $ | 1,427 |  |  | $ | 9,279 |  |  | $ | 2,854 |  |  |
(a) These amounts, net of any amounts capitalized as a regulatory asset, have been recognized as other (income) expense, net in the consolidated statement of income. See Note 11 for additional detail. |
| Other Postemployment Benefits |
| Three Months Ended |  | Six Months Ended |
| June 30, | June 30, |
| 2026 |  | 2025 |  | 2026 |  | 2025 |  |
| (Thousands of dollars) |
Components of net periodic benefit cost (credit) |  |  |  |  |  |  |  |  |
Service cost | $ | 112 |  |  | $ | 123 |  |  | $ | 222 |  |  | $ | 246 |  |  |
Interest cost (a) | 2,020 |  |  | 2,069 |  |  | 4,040 |  |  | 4,138 |  |  |
Expected return on assets (a) | (2,194) |  |  | (2,212) |  |  | (4,388) |  |  | (4,424) |  |  |
Amortization of net loss (gain) (a) | (8) |  |  | (31) |  |  | (16) |  |  | (62) |  |  |
Net periodic benefit cost (credit) | $ | (70) |  |  | $ | (51) |  |  | $ | (142) |  |  | $ | (102) |  |  |
(a) These amounts, net of any amounts capitalized as a regulatory asset, have been recognized as other (income) expense, net in the consolidated statement of income. See Note 11 for additional detail. |

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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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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