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
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