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Fair value measurements - Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis (Details) - USD ($) $ in Thousands | Mar. 31, 2024 | Dec. 31, 2023 | Mar. 31, 2023
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Concentration risks, percentage [Abstract] |  |  |
Percentage in fixed-income and other investments | 57.00% | 60.00% | 61.00%
Percentage investment in common stock of large-cap companies | 17.00% | 15.00% | 15.00%
Percentage investment in target date investments | 10.00% | 8.00% | 7.00%
Percentage investment in common stock of mid-cap companies | 8.00% | 7.00% | 8.00%
Percentage investment in common stock of small-cap companies | 4.00% | 5.00% | 6.00%
Percentage investment in cash and cash equivalents | 3.00% | 3.00% | 2.00%
Percentage investment in international investments | 1.00% | 1.00% | 1.00%
Percentage investment in high-yield investments |  | 1.00% |
Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | $ 81,056 | $ 84,014 | $ 99,886
Fair Value, Inputs, Level 1 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 0 | 0 | 0
Fair value, inputs, level 2 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 81,056 | 84,014 | 99,886
Fair Value, Inputs, Level 3 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 0 | 0 | 0
Money market funds | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 6,985 | 6,409 | 5,447
Money market funds | Fair Value, Inputs, Level 1 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 0 | 0 | 0
Money market funds | Fair value, inputs, level 2 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 6,985 | 6,409 | 5,447
Money market funds | Fair Value, Inputs, Level 3 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 0 | 0 | 0
Insurance contracts | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 62,713 | 66,283 | 83,460
Insurance contracts | Fair Value, Inputs, Level 1 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 0 | 0 | 0
Insurance contracts | Fair value, inputs, level 2 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 62,713 | 66,283 | 83,460
Insurance contracts | Fair Value, Inputs, Level 3 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 0 | 0 | 0
Mortgage-backed securities | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 7,569 | 7,781 | 7,999
Mortgage-backed securities | Fair Value, Inputs, Level 1 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 0 | 0 | 0
Mortgage-backed securities | Fair value, inputs, level 2 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 7,569 | 7,781 | 7,999
Mortgage-backed securities | Fair Value, Inputs, Level 3 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 0 | 0 | 0
U.S. Treasury securities | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 3,789 | 3,541 | 2,980
U.S. Treasury securities | Fair Value, Inputs, Level 1 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 0 | 0 | 0
U.S. Treasury securities | Fair value, inputs, level 2 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 3,789 | 3,541 | 2,980
U.S. Treasury securities | Fair Value, Inputs, Level 3 | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | $ 0 | $ 0 | $ 0

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