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Fair value measurements - Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis (Details) - USD ($) $ in Thousands | Jun. 30, 2025 | Dec. 31, 2024 | Jun. 30, 2024
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Concentration risks, percentage [Abstract] |  |  |
Percentage in fixed-income and other investments | 56.00% | 58.00% | 55.00%
Percentage investment in common stock of large-cap companies | 18.00% | 17.00% | 19.00%
Percentage investment in target date investments | 10.00% | 8.00% | 10.00%
Percentage investment in common stock of mid-cap companies | 7.00% | 8.00% | 8.00%
Percentage investment in common stock of small-cap companies | 4.00% | 4.00% | 4.00%
Percentage investment in cash and cash equivalents | 4.00% | 4.00% | 3.00%
Percentage investment in international investments | 1.00% | 1.00% | 1.00%
Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | $ 84,740 | $ 83,739 | $ 79,275
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 | 84,740 | 83,739 | 79,275
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 | 8,965 | 12,879 | 10,904
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 | 8,965 | 12,879 | 10,904
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 | 64,141 | 59,282 | 56,899
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 | 64,141 | 59,282 | 56,899
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 | 8,167 | 7,554 | 7,612
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 | 8,167 | 7,554 | 7,612
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 | 3,467 | 4,024 | 3,860
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 | 3,467 | 4,024 | 3,860
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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