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Fair value measurements (Details 3) - USD ($) $ in Thousands | Jun. 30, 2021 | Dec. 31, 2020 | Jun. 30, 2020
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Concentration risks, percentage [Abstract] |  |  |
Percentage in fixed-income and other investments | 53.00% | 57.00% | 38.00%
Percentage investment in common stock of large-cap companies | 20.00% | 18.00% | 23.00%
Percentage investment in common stock of mid-cap companies | 10.00% | 9.00% | 9.00%
Percentage investment in common stock of small-cap companies | 9.00% | 9.00% | 9.00%
Percentage investment in target date investments | 6.00% | 5.00% | 4.00%
Percentage investment in cash and cash equivalents | 2.00% | 2.00% | 17.00%
Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | $ 129,186 | $ 120,348 | $ 112,113
Fair value, inputs, level 2 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 129,186 | 120,348 | 112,113
Money market funds | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 12,234 | 8,917 | 8,478
Money market funds | Fair value, inputs, level 2 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 12,234 | 8,917 | 8,478
Insurance contracts* | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 105,684 | 100,104 | 92,413
Insurance contracts* | Fair value, inputs, level 2 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 105,684 | 100,104 | 92,413
Mortgage-backed securities | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 8,396 | 9,946 | 10,045
Mortgage-backed securities | Fair value, inputs, level 2 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 8,396 | 9,946 | 10,045
U.S. Treasury securities | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 2,872 | 1,381 | 1,177
U.S. Treasury securities | Fair value, inputs, level 2 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | $ 2,872 | $ 1,381 | $ 1,177

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