Fair value measurements - Narrative (Details) $ in Thousands, shares in Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2024 USD ($)	Mar. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)	Sep. 30, 2023 USD ($)	Sep. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)	May 31, 2023 shares
Available-for-sale securities [Line Items]
Investments used to satisfy nonqualified benefit plans obligations	$ 63,700		$ 66,200	$ 62,700	$ 63,700	$ 62,700
Net unrealized gain (loss) on investments used to satisfy obligations under nonqualified benefit plans	2,600			(1,400)	5,800	3,500
Investment withdrawal, reduction in cost basis		$ 9,000	20,000
Retained shares in Knife River	0		$ 0	276,213	0	276,213
Unrealized gain on retained shares in Knife River	$ 0			30,150	$ 0	170,170
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				276,200		$ 276,200
Unrealized gain on retained shares in Knife River				$ 30,200