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Fair value measurements - Narrative (Details) $ in Thousands, shares in Millions | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2024 USD ($) | Mar. 31, 2024 USD ($) | Dec. 31, 2023 USD ($) | Sep. 30, 2023 USD ($) | Sep. 30, 2024 USD ($) | Sep. 30, 2023 USD ($) | May 31, 2023 shares
Available-for-sale securities [Line Items] |  |  |  |  |  |  |
Investments used to satisfy nonqualified benefit plans obligations | $ 63,700 |  | $ 66,200 | $ 62,700 | $ 63,700 | $ 62,700 |
Net unrealized gain (loss) on investments used to satisfy obligations under nonqualified benefit plans | 2,600 |  |  | (1,400) | 5,800 | 3,500 |
Investment withdrawal, reduction in cost basis |  | $ 9,000 | 20,000 |  |  |  |
Retained shares in Knife River | 0 |  | $ 0 | 276,213 | 0 | 276,213 |
Unrealized gain on retained shares in Knife River | $ 0 |  |  | 30,150 | $ 0 | 170,170 |
Discontinued Operations, Disposed of by Means Other than Sale, Spinoff | Knife River Corporation |  |  |  |  |  |  |
Available-for-sale securities [Line Items] |  |  |  |  |  |  |
Retained ownership percentage |  |  |  |  |  |  | 0.10
Number of retained shares (in shares) | shares |  |  |  |  |  |  | 5.7
Retained shares in Knife River |  |  |  | 276,200 |  | $ 276,200 |
Unrealized gain on retained shares in Knife River |  |  |  | $ 30,200 |  |  |

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- Definition This item represents the amount that could be realized under a life insurance contract or contracts owned by the Entity as of the date of the statement of financial position. Such Entity-owned life insurance policies are commonly known as corporate-owned life insurance (COLI) or bank-owned life insurance (BOLI). + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 210 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(12)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 325 -SubTopic 30 -Name Accounting Standards Codification -Section 25 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org/1943274/2147481612/325-30-25-1 + Details
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