Discontinued operations - Schedule of Assets and Liabilities of Discontinued Operations (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Sep. 30, 2022
Current assets:
Cash and cash equivalents			$ 7,200
Noncurrent assets:
Total noncurrent assets of discontinued operations	$ 0	$ 0	1,687,611
Current liabilities:
Total current liabilities of discontinued operations	0	0	448,715
Noncurrent liabilities:
Total noncurrent liabilities of discontinued operations	$ 0	$ 0	$ 897,085
Knife River Corporation | Discontinued Operations, Disposed of by Means Other than Sale, Spinoff
Current assets:
Cash and cash equivalents				$ 7,218
Receivables, net				205,564
Inventories				373,215
Prepayments and other current assets				34,899
Total current assets of discontinued operations				620,896
Noncurrent assets:
Net property, plant and equipment				1,317,368
Goodwill				274,540
Other intangible assets, net				12,763
Investments				35,542
Operating lease right-of-use assets				43,995
Other				3,403
Total noncurrent assets of discontinued operations				1,687,611
Total assets of discontinued operations				2,308,507
Current liabilities:
Short-term borrowings				208,000
Long-term debt due within one year				30,211
Accounts payable				118,175
Taxes payable				(1,402)
Accrued compensation				14,010
Operating lease liabilities due within one year				12,981
Other accrued liabilities				66,740
Total current liabilities of discontinued operations				448,715
Noncurrent liabilities:
Long-term debt				577,138
Deferred income taxes				175,062
Asset retirement obligations				33,462
Operating lease liabilities				31,014
Other				80,409
Total noncurrent liabilities of discontinued operations				897,085
Total liabilities of discontinued operations				$ 1,345,800