Accumulated other comprehensive loss (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Sep. 30, 2023	Sep. 30, 2022
Accumulated other comprehensive loss [Roll Forward]
Balance	$ 2,706,193	$ 3,575,382	$ 3,587,129	$ 3,386,810	$ 3,360,718	$ 3,382,874	$ 3,587,129	$ 3,382,874
Other Comprehensive (Income) Loss, Reclassification Adjustment to AOCI, Postretirement Liability Adjustment From Regulatory Asset , Net of Tax	0			0			0	(3,265)
Other comprehensive income (loss)	(62)	363	217	285	(2,796)	269	518	(2,242)
Balance	2,757,434	2,706,193	3,575,382	3,491,858	3,386,810	3,360,718	2,757,434	3,491,858
Net unrealized gain (loss) on derivative instruments qualifying as hedges
Accumulated other comprehensive loss [Roll Forward]
Balance	0	(91)	(125)	(315)	(426)	(538)	(125)	(538)
Other comprehensive loss before reclassifications	0	0	0	0	0	0
Other Comprehensive (Income) Loss, Reclassification Adjustment to AOCI, Postretirement Liability Adjustment From Regulatory Asset , Net of Tax					0
Amounts reclassified from accumulated other comprehensive loss	0	47	34	111	111	112
Other comprehensive income (loss)	0	47	34	111	111	112
Amounts reclassified related to the separation of Knife River		44
Balance	0	0	(91)	(204)	(315)	(426)	0	(204)
Postretirement liability adjustment
Accumulated other comprehensive loss [Roll Forward]
Balance	(17,151)	(29,800)	(29,900)	(42,820)	(40,016)	(40,461)	(29,900)	(40,461)
Other comprehensive loss before reclassifications	0	0	0	0	0	0
Other Comprehensive (Income) Loss, Reclassification Adjustment to AOCI, Postretirement Liability Adjustment From Regulatory Asset , Net of Tax					(3,265)
Amounts reclassified from accumulated other comprehensive loss	(6)	387	100	461	461	445
Other comprehensive income (loss)	(6)	387	100	461	(2,804)	445
Amounts reclassified related to the separation of Knife River		12,262
Balance	(17,157)	(17,151)	(29,800)	(42,359)	(42,820)	(40,016)	(17,157)	(42,359)
Net unrealized gain (loss) on available-for-sale investments
Accumulated other comprehensive loss [Roll Forward]
Balance	(546)	(475)	(558)	(396)	(293)	(5)	(558)	(5)
Other comprehensive loss before reclassifications	(66)	(84)	70	329	(128)	(320)
Other Comprehensive (Income) Loss, Reclassification Adjustment to AOCI, Postretirement Liability Adjustment From Regulatory Asset , Net of Tax	269				0
Amounts reclassified from accumulated other comprehensive loss	10	13	13	42	25	32
Other comprehensive income (loss)	(56)	(71)	83	(287)	(103)	(288)
Amounts reclassified related to the separation of Knife River		0
Balance	(602)	(546)	(475)	(683)	(396)	(293)	(602)	(683)
Total accumulated other comprehensive loss
Accumulated other comprehensive loss [Roll Forward]
Balance	(17,697)	(30,366)	(30,583)	(43,531)	(40,735)	(41,004)	(30,583)	(41,004)
Other comprehensive loss before reclassifications	(66)	(84)	70	329	(128)	(320)
Other Comprehensive (Income) Loss, Reclassification Adjustment to AOCI, Postretirement Liability Adjustment From Regulatory Asset , Net of Tax					(3,265)
Amounts reclassified from accumulated other comprehensive loss	4	447	147	614	597	589
Other comprehensive income (loss)	(62)	363	217	285	(2,796)	269
Amounts reclassified related to the separation of Knife River		12,306
Balance	$ (17,759)	$ (17,697)	$ (30,366)	$ (43,246)	$ (43,531)	$ (40,735)	$ (17,759)	$ (43,246)