Accumulated other comprehensive loss (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2022	Mar. 31, 2022	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2022	Jun. 30, 2021
Accumulated other comprehensive loss [Roll Forward]
Balance	$ 3,360,718	$ 3,382,874	$ 3,100,306	$ 3,079,105	$ 3,382,874	$ 3,079,105
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset	(3,265)		0		(3,265)	0
Other comprehensive income (loss)	(2,796)	269	542	568	(2,527)	1,110
Balance	3,386,810	3,360,718	3,198,802	3,100,306	3,386,810	3,198,802
Net unrealized gain (loss) on derivative instruments qualifying as hedges
Accumulated other comprehensive loss [Roll Forward]
Balance	(426)	(538)	(873)	(984)	(538)	(984)
Other comprehensive loss before reclassifications	0	0	0	0
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset	0
Amounts reclassified from accumulated other comprehensive loss	111	112	112	111
Other comprehensive income (loss)	111	112	112	111
Balance	(315)	(426)	(761)	(873)	(315)	(761)
Postretirement liability adjustment
Accumulated other comprehensive loss [Roll Forward]
Balance	(40,016)	(40,461)	(46,741)	(47,207)	(40,461)	(47,207)
Other comprehensive loss before reclassifications	0	0	0	0
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset	(3,265)
Amounts reclassified from accumulated other comprehensive loss	461	445	457	466
Other comprehensive income (loss)	(2,804)	445	457	466
Balance	(42,820)	(40,016)	(46,284)	(46,741)	(42,820)	(46,284)
Net unrealized gain (loss) on available-for-sale investments
Accumulated other comprehensive loss [Roll Forward]
Balance	(293)	(5)	104	113	(5)	113
Other comprehensive loss before reclassifications	(128)	(320)	(52)	(44)
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset	0
Amounts reclassified from accumulated other comprehensive loss	25	32	25	35
Other comprehensive income (loss)	(103)	(288)	(27)	(9)
Balance	(396)	(293)	77	104	(396)	77
Total accumulated other comprehensive loss
Accumulated other comprehensive loss [Roll Forward]
Balance	(40,735)	(41,004)	(47,510)	(48,078)	(41,004)	(48,078)
Other comprehensive loss before reclassifications	(128)	(320)	(52)	(44)
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset	(3,265)
Amounts reclassified from accumulated other comprehensive loss	597	589	594	612
Other comprehensive income (loss)	(2,796)	269	542	568
Balance	$ (43,531)	$ (40,735)	$ (46,968)	$ (47,510)	$ (43,531)	$ (46,968)