Accumulated other comprehensive loss (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2023	Mar. 31, 2023	Jun. 30, 2022	Mar. 31, 2022	Jun. 30, 2023	Jun. 30, 2022
Accumulated other comprehensive loss [Roll Forward]
Balance	$ 3,575,382	$ 3,587,129	$ 3,360,718	$ 3,382,874	$ 3,587,129	$ 3,382,874
Other comprehensive income (loss)	363	217	(2,796)	269	580	(2,527)
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset, net of tax	0		(3,265)		0	(3,265)
Balance	2,706,193	3,575,382	3,386,810	3,360,718	2,706,193	3,386,810
Net unrealized gain (loss) on derivative instruments qualifying as hedges
Accumulated other comprehensive loss [Roll Forward]
Balance	(91)	(125)	(426)	(538)	(125)	(538)
Other comprehensive loss before reclassifications	0	0	0	0
Other comprehensive income (loss)	47	34	111	112
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset, net of tax			0
Amounts reclassified from accumulated other comprehensive loss		34	111	112
Amounts reclassified related to the separation of Knife River	(44)
Balance	0	(91)	(315)	(426)	0	(315)
Postretirement liability adjustment
Accumulated other comprehensive loss [Roll Forward]
Balance	(29,800)	(29,900)	(40,016)	(40,461)	(29,900)	(40,461)
Other comprehensive loss before reclassifications	0	0	0	0
Other comprehensive income (loss)	387	100	(2,804)	445
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset, net of tax			(3,265)
Amounts reclassified from accumulated other comprehensive loss		100	461	445
Amounts reclassified related to the separation of Knife River	(12,262)
Balance	(17,151)	(29,800)	(42,820)	(40,016)	(17,151)	(42,820)
Net unrealized gain (loss) on available-for-sale investments
Accumulated other comprehensive loss [Roll Forward]
Balance	(475)	(558)	(293)	(5)	(558)	(5)
Other comprehensive loss before reclassifications		70	(128)	(320)
Other comprehensive income (loss)	(71)	83	(103)	(288)
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset, net of tax			0
Amounts reclassified from accumulated other comprehensive loss		13	25	32
Amounts reclassified related to the separation of Knife River	0
Balance	(546)	(475)	(396)	(293)	(546)	(396)
Total accumulated other comprehensive loss
Accumulated other comprehensive loss [Roll Forward]
Balance	(30,366)	(30,583)	(40,735)	(41,004)	(30,583)	(41,004)
Other comprehensive loss before reclassifications	(84)	70	(128)	(320)
Other comprehensive income (loss)	363	217	(2,796)	269
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset, net of tax			(3,265)
Amounts reclassified from accumulated other comprehensive loss	447	147	597	589
Amounts reclassified related to the separation of Knife River	(12,306)
Balance	$ (17,697)	$ (30,366)	$ (43,531)	$ (40,735)	$ (17,697)	$ (43,531)