Fair value measurements - Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023	Jun. 30, 2023
Concentration risks, percentage [Abstract]
Percentage in fixed-income and other investments	55.00%	60.00%	67.00%
Percentage investment in common stock of large-cap companies	19.00%	15.00%	14.00%
Percentage investment in target date investments	10.00%	8.00%	4.00%
Percentage investment in common stock of mid-cap companies	8.00%	7.00%	7.00%
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%
Percentage investment in high-yield investments		1.00%
Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	$ 83,328	$ 84,014	$ 346,840
Fair Value, Inputs, Level 1 | Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	0	0	246,063
Fair value, inputs, level 2 | Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	83,328	84,014	100,777
Fair Value, Inputs, Level 3 | Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	0	0	0
Retained shares in Knife River | Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure			246,063
Retained shares in Knife River | Fair Value, Inputs, Level 1 | Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure			246,063
Retained shares in Knife River | Fair value, inputs, level 2 | Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure			0
Retained shares in Knife River | Fair Value, Inputs, Level 3 | Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure			0
Money market funds | Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	10,904	6,409	5,737
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	10,904	6,409	5,737
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	60,952	66,283	84,099
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	60,952	66,283	84,099
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	7,612	7,781	7,891
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	7,612	7,781	7,891
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	3,860	3,541	3,050
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	3,860	3,541	3,050
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