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Fair value measurements - Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis (Details) - USD ($) $ in Thousands | Jun. 30, 2024 | Dec. 31, 2023 | Jun. 30, 2023
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Concentration risks, percentage [Abstract] |  |  |
Percentage in fixed-income and other investments | 55.00% | 60.00% | 67.00%
Percentage investment in common stock of large-cap companies | 19.00% | 15.00% | 14.00%
Percentage investment in target date investments | 10.00% | 8.00% | 4.00%
Percentage investment in common stock of mid-cap companies | 8.00% | 7.00% | 7.00%
Percentage investment in common stock of small-cap companies | 4.00% | 5.00% | 6.00%
Percentage investment in cash and cash equivalents | 3.00% | 3.00% | 2.00%
Percentage investment in international investments | 1.00% | 1.00% |
Percentage investment in high-yield investments |  | 1.00% |
Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | $ 83,328 | $ 84,014 | $ 346,840
Fair Value, Inputs, Level 1 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 0 | 0 | 246,063
Fair value, inputs, level 2 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 83,328 | 84,014 | 100,777
Fair Value, Inputs, Level 3 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 0 | 0 | 0
Retained shares in Knife River | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure |  |  | 246,063
Retained shares in Knife River | Fair Value, Inputs, Level 1 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure |  |  | 246,063
Retained shares in Knife River | Fair value, inputs, level 2 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure |  |  | 0
Retained shares in Knife River | Fair Value, Inputs, Level 3 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure |  |  | 0
Money market funds | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 10,904 | 6,409 | 5,737
Money market funds | Fair Value, Inputs, Level 1 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 0 | 0 | 0
Money market funds | Fair value, inputs, level 2 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 10,904 | 6,409 | 5,737
Money market funds | Fair Value, Inputs, Level 3 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 0 | 0 | 0
Insurance contracts | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 60,952 | 66,283 | 84,099
Insurance contracts | Fair Value, Inputs, Level 1 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 0 | 0 | 0
Insurance contracts | Fair value, inputs, level 2 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 60,952 | 66,283 | 84,099
Insurance contracts | Fair Value, Inputs, Level 3 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 0 | 0 | 0
Mortgage-backed securities | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 7,612 | 7,781 | 7,891
Mortgage-backed securities | Fair Value, Inputs, Level 1 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 0 | 0 | 0
Mortgage-backed securities | Fair value, inputs, level 2 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 7,612 | 7,781 | 7,891
Mortgage-backed securities | Fair Value, Inputs, Level 3 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 0 | 0 | 0
U.S. Treasury securities | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 3,860 | 3,541 | 3,050
U.S. Treasury securities | Fair Value, Inputs, Level 1 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 0 | 0 | 0
U.S. Treasury securities | Fair value, inputs, level 2 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 3,860 | 3,541 | 3,050
U.S. Treasury securities | Fair Value, Inputs, Level 3 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | $ 0 | $ 0 | $ 0

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