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Fair value measurements (Details 3) - USD ($) $ in Thousands | Mar. 31, 2023 | Dec. 31, 2022 | Mar. 31, 2022
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Concentration risks, percentage [Abstract] |  |  |
Percentage in fixed-income and other investments | 61.00% | 63.00% | 61.00%
Percentage investment in common stock of large-cap companies | 15.00% | 15.00% | 17.00%
Percentage investment in common stock of mid-cap companies | 8.00% | 8.00% | 8.00%
Percentage investment in target date investments | 7.00% | 6.00% | 6.00%
Percentage investment in common stock of small-cap companies | 6.00% | 6.00% | 6.00%
Percentage investment in cash and cash equivalents | 2.00% | 2.00% | 2.00%
Percentage investment in international investments | 1.00% |  |
Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | $ 124,755 | $ 116,232 | $ 129,598
Fair value, inputs, level 2 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 124,755 | 116,232 | 129,598
Money market funds | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 7,919 | 7,361 | 13,131
Money market funds | Fair value, inputs, level 2 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 7,919 | 7,361 | 13,131
Insurance contracts* | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 105,857 | 98,041 | 105,371
Insurance contracts* | Fair value, inputs, level 2 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 105,857 | 98,041 | 105,371
Mortgage-backed securities | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 7,999 | 8,294 | 8,471
Mortgage-backed securities | Fair value, inputs, level 2 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 7,999 | 8,294 | 8,471
U.S. Treasury securities | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 2,980 | 2,536 | 2,625
U.S. Treasury securities | Fair value, inputs, level 2 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | $ 2,980 | $ 2,536 | $ 2,625

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