Fair value measurements - Narrative (Details) $ in Thousands, shares in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2024 USD ($)	Mar. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)	Jun. 30, 2023 USD ($)	Jun. 30, 2024 USD ($)	Jun. 30, 2023 USD ($)	May 31, 2023 shares
Available-for-sale securities [Line Items]
Investments used to satisfy nonqualified benefit plans obligations	$ 61,000		$ 66,200	$ 84,100	$ 61,000	$ 84,100
Net unrealized gain (loss) on investments used to satisfy obligations under nonqualified benefit plans	362			1,900	3,200	4,900
Investment withdrawal, reduction in cost basis		$ 9,000	20,000
Retained shares in Knife River	$ 0		$ 0	246,063	0	246,063
Unrealized (gains) losses on investments					$ 3,235	4,885
Discontinued Operations, Disposed of by Means Other than Sale, Spinoff | Knife River Corporation
Available-for-sale securities [Line Items]
Retained ownership percentage							0.10
Number of retained shares (in shares) | shares							5.7
Retained shares in Knife River				$ 246,100		246,100
Unrealized (gains) losses on investments						$ 140,000