Fair value measurements - Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2025	Dec. 31, 2024	Mar. 31, 2024
Concentration risks, percentage [Abstract]
Percentage in fixed-income and other investments	58.00%	58.00%	57.00%
Percentage investment in common stock of large-cap companies	17.00%	17.00%	17.00%
Percentage investment in target date investments	9.00%	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%	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	$ 86,576	$ 83,739	$ 77,103
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	86,576	83,739	77,103
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	13,841	12,879	6,985
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	13,841	12,879	6,985
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	61,075	59,282	58,760
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	61,075	59,282	58,760
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	7,716	7,554	7,569
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	7,716	7,554	7,569
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	3,944	4,024	3,789
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	3,944	4,024	3,789
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