Accumulated Other Comprehensive Loss - Schedule of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 3,587,129
Other comprehensive income (loss) before reclassifications	(473)	$ 11,340
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset		(3,265)
Amounts reclassified from accumulated other comprehensive loss	366	2,346
Other comprehensive income (loss)	(107)	10,421
Ending balance	2,905,233	3,587,129
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	(30,583)	(41,004)
Ending balance	(18,384)	(30,583)
Net Unrealized Loss on Derivative Instruments Qualifying as Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(125)	(538)
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset		0
Amounts reclassified from accumulated other comprehensive loss	81	413
Other comprehensive income (loss)	81	413
Ending balance	0	(125)
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	(29,900)	(40,461)
Other comprehensive income (loss) before reclassifications	(646)	12,007
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset		(3,265)
Amounts reclassified from accumulated other comprehensive loss	242	1,819
Other comprehensive income (loss)	(404)	10,561
Ending balance	(18,042)	(29,900)
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	(558)	(5)
Other comprehensive income (loss) before reclassifications	173	(667)
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset		0
Amounts reclassified from accumulated other comprehensive loss	43	114
Other comprehensive income (loss)	216	(553)
Ending balance	(342)	$ (558)
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