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Fair value measurements - Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis (Details) - USD ($) $ in Thousands | Sep. 30, 2025 | Dec. 31, 2024 | Sep. 30, 2024
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Concentration risks, percentage [Abstract] |  |  |
Percentage in fixed-income and other investments | 57.00% | 58.00% | 55.00%
Percentage investment in common stock of large-cap companies | 18.00% | 17.00% | 18.00%
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 | 3.00% | 4.00% | 4.00%
Percentage investment in international investments | 1.00% | 1.00% | 1.00%
Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | $ 87,748 | $ 83,739 | $ 82,047
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 0 | 0 | 0
Significant Other Observable Inputs (Level 2) | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 87,748 | 83,739 | 82,047
Significant Unobservable 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 | 9,743 | 12,879 | 11,033
Money market funds | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 0 | 0 | 0
Money market funds | Significant Other Observable Inputs (Level 2) | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 9,743 | 12,879 | 11,033
Money market funds | Significant Unobservable 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 | 66,229 | 59,282 | 59,210
Insurance contracts | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 0 | 0 | 0
Insurance contracts | Significant Other Observable Inputs (Level 2) | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 66,229 | 59,282 | 59,210
Insurance contracts | Significant Unobservable 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 | 8,260 | 7,554 | 7,833
Mortgage-backed securities | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 0 | 0 | 0
Mortgage-backed securities | Significant Other Observable Inputs (Level 2) | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 8,260 | 7,554 | 7,833
Mortgage-backed securities | Significant Unobservable 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 | 3,516 | 4,024 | 3,971
U.S. Treasury securities | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 0 | 0 | 0
U.S. Treasury securities | Significant Other Observable Inputs (Level 2) | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 3,516 | 4,024 | 3,971
U.S. Treasury securities | Significant Unobservable Inputs (Level 3) | Fair value, measurements, recurring |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | $ 0 | $ 0 | $ 0

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