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Fair value measurements Assets (Details 4) - USD ($) $ in Thousands | Sep. 30, 2023 | Dec. 31, 2022 | Sep. 30, 2022
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Concentration risks, percentage [Abstract] |  |  |
Percentage in fixed-income and other investments | 50.00% | 63.00% | 65.00%
Percentage investment in common stock of large-cap companies | 19.00% | 15.00% | 14.00%
Percentage investment in target date investments | 10.00% | 6.00% | 6.00%
Percentage investment in common stock of mid-cap companies | 10.00% | 8.00% | 7.00%
Percentage investment in common stock of small-cap companies | 6.00% | 6.00% | 6.00%
Percentage investment in cash and cash equivalents | 3.00% | 2.00% | 2.00%
Percentage investment in international investments | 1.00% |  |
Percentage Investment in High-Yield Investments | 1.00% |  |
Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | $ 375,699 | $ 93,701 | $ 90,690
Fair Value, Inputs, Level 1 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 276,213 |  |
Fair value, inputs, level 2 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 99,486 | 93,701 | 90,690
Investment in Knife River | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 276,213 |  |
Investment in Knife River | Fair Value, Inputs, Level 1 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 276,213 |  |
Money market funds | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 25,818 | 4,913 | 4,878
Money market funds | Fair value, inputs, level 2 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 25,818 | 4,913 | 4,878
Insurance contracts* | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 62,736 | 77,958 | 75,723
Insurance contracts* | Fair value, inputs, level 2 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 62,736 | 77,958 | 75,723
Mortgage-backed securities | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 7,637 | 8,294 | 7,703
Mortgage-backed securities | Fair value, inputs, level 2 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 7,637 | 8,294 | 7,703
U.S. Treasury securities | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 3,295 | 2,536 | 2,386
U.S. Treasury securities | Fair value, inputs, level 2 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | $ 3,295 | $ 2,536 | $ 2,386

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