Fair value measurements - Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2024	Dec. 31, 2023	Sep. 30, 2023
Concentration risks, percentage [Abstract]
Percentage in fixed-income and other investments	55.00%	60.00%	50.00%
Percentage investment in common stock of large-cap companies	18.00%	15.00%	19.00%
Percentage investment in target date investments	10.00%	8.00%	10.00%
Percentage investment in common stock of mid-cap companies	8.00%	7.00%	10.00%
Percentage investment in common stock of small-cap companies	4.00%	5.00%	6.00%
Percentage investment in cash and cash equivalents	4.00%	3.00%	3.00%
Percentage investment in international investments	1.00%	1.00%	1.00%
Percentage investment in high-yield investments		1.00%	1.00%
Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	$ 86,495	$ 84,014	$ 375,699
Fair Value, Inputs, Level 1 | Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	0	0	276,213
Fair value, inputs, level 2 | Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	86,495	84,014	99,486
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			276,213
Retained shares in Knife River | Fair Value, Inputs, Level 1 | Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure			276,213
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	11,033	6,409	25,818
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	11,033	6,409	25,818
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	63,658	66,283	62,736
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	63,658	66,283	62,736
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	7,833	7,781	7,637
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	7,833	7,781	7,637
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	3,971	3,541	3,295
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	3,971	3,541	3,295
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