Accumulated Other Comprehensive Loss - Schedule of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 2,905,233	$ 3,587,129
Other comprehensive income (loss) before reclassifications	1,134	(473)
Amounts reclassified from accumulated other comprehensive loss	452	366
Other comprehensive income (loss)	1,586	(107)
Ending balance	2,690,574	2,905,233
Discontinued Operations, Disposed of by Means Other than Sale, Spinoff | Knife River Corporation
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from accumulated other comprehensive loss		12,306
Total Accumulated Other Comprehensive Loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(18,384)	(30,583)
Ending balance	(16,798)	(18,384)
Net Unrealized Loss on Derivative Instruments Qualifying as Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	0	(125)
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive loss	0	81
Other comprehensive income (loss)	0	81
Ending balance	0	0
Net Unrealized Loss on Derivative Instruments Qualifying as Hedges | Discontinued Operations, Disposed of by Means Other than Sale, Spinoff | Knife River Corporation
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from accumulated other comprehensive loss		44
Postretirement Liability Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(18,042)	(29,900)
Other comprehensive income (loss) before reclassifications	1,049	(646)
Amounts reclassified from accumulated other comprehensive loss	432	242
Other comprehensive income (loss)	1,481	(404)
Ending balance	(16,561)	(18,042)
Postretirement Liability Adjustment | Discontinued Operations, Disposed of by Means Other than Sale, Spinoff | Knife River Corporation
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from accumulated other comprehensive loss		12,262
Net Unrealized Gain (Loss) on Available- for-sale Investments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(342)	(558)
Other comprehensive income (loss) before reclassifications	85	173
Amounts reclassified from accumulated other comprehensive loss	20	43
Other comprehensive income (loss)	105	216
Ending balance	$ (237)	(342)
Net Unrealized Gain (Loss) on Available- for-sale Investments | Discontinued Operations, Disposed of by Means Other than Sale, Spinoff | Knife River Corporation
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from accumulated other comprehensive loss		$ 0