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Fair value measurements Assets (Details 4) - USD ($) $ in Thousands | Jun. 30, 2023 | Dec. 31, 2022 | Jun. 30, 2022
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Concentration risks, percentage [Abstract] |  |  |
Percentage in fixed-income and other investments | 67.00% | 63.00% | 64.00%
Percentage investment in common stock of large-cap companies | 14.00% | 15.00% | 15.00%
Percentage investment in common stock of mid-cap companies | 7.00% | 8.00% | 7.00%
Percentage investment in common stock of small-cap companies | 6.00% | 6.00% | 6.00%
Percentage investment in target date investments | 4.00% | 6.00% | 6.00%
Percentage investment in cash and cash equivalents | 2.00% | 2.00% | 2.00%
Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | $ 346,840 | $ 93,701 | $ 94,256
Fair Value, Inputs, Level 1 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 246,063 |  |
Fair value, inputs, level 2 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 100,777 | 93,701 | 94,256
Investment in Knife River | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 246,063 |  |
Investment in Knife River | Fair Value, Inputs, Level 1 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 246,063 |  |
Money market funds | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 5,737 | 4,913 | 5,976
Money market funds | Fair value, inputs, level 2 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 5,737 | 4,913 | 5,976
Insurance contracts* | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 84,099 | 77,958 | 77,529
Insurance contracts* | Fair value, inputs, level 2 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 84,099 | 77,958 | 77,529
Mortgage-backed securities | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 7,891 | 8,294 | 7,709
Mortgage-backed securities | Fair value, inputs, level 2 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 7,891 | 8,294 | 7,709
U.S. Treasury securities | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 3,050 | 2,536 | 3,042
U.S. Treasury securities | Fair value, inputs, level 2 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | $ 3,050 | $ 2,536 | $ 3,042

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