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Fair value measurements (Details 3) - USD ($) $ in Thousands | Sep. 30, 2022 | Dec. 31, 2021 | Sep. 30, 2021
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Concentration risks, percentage [Abstract] |  |  |
Percentage in fixed-income and other investments | 65.00% | 61.00% | 63.00%
Percentage investment in common stock of large-cap companies | 14.00% | 17.00% | 16.00%
Percentage investment in common stock of mid-cap companies | 7.00% | 8.00% | 8.00%
Percentage investment in common stock of small-cap companies | 6.00% | 7.00% | 7.00%
Percentage investment in target date investments | 6.00% | 5.00% | 5.00%
Percentage investment in cash and cash equivalents | 2.00% | 2.00% | 1.00%
Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | $ 112,568 | $ 130,895 | $ 127,610
Fair value, inputs, level 2 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 112,568 | 130,895 | 127,610
Money market funds | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 7,309 | 10,190 | 9,907
Money market funds | Fair value, inputs, level 2 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 7,309 | 10,190 | 9,907
Insurance contracts* | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 95,170 | 109,603 | 106,371
Insurance contracts* | Fair value, inputs, level 2 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 95,170 | 109,603 | 106,371
Mortgage-backed securities | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 7,703 | 8,706 | 8,679
Mortgage-backed securities | Fair value, inputs, level 2 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 7,703 | 8,706 | 8,679
U.S. Treasury securities | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 2,386 | 2,396 | 2,653
U.S. Treasury securities | Fair value, inputs, level 2 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | $ 2,386 | $ 2,396 | $ 2,653

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