Accumulated other comprehensive loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Accumulated other comprehensive loss [Roll Forward]
Balance	$ 3,382,874
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset	(3,265)	$ 0	$ 0
Other comprehensive income (loss)	10,421	7,074	(5,976)
Balance	3,587,129	3,382,874
Net unrealized gain (loss) on derivative instruments qualifying as hedges
Accumulated other comprehensive loss [Roll Forward]
Balance	(538)	(984)
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	413	446
Other comprehensive income (loss)	413	446
Balance	(125)	(538)	(984)
Postretirement liability adjustment
Accumulated other comprehensive loss [Roll Forward]
Balance	(40,461)	(47,207)
Other comprehensive income (loss) before reclassifications	12,007	4,876
Amounts reclassified from accumulated other comprehensive loss	1,819	1,870
Other comprehensive income (loss)	10,561	6,746
Balance	(29,900)	(40,461)	(47,207)
Net unrealized gain (loss) on available-for-sale investments
Accumulated other comprehensive loss [Roll Forward]
Balance	(5)	113
Other comprehensive income (loss) before reclassifications	(667)	(252)
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset	0
Amounts reclassified from accumulated other comprehensive loss	114	134
Other comprehensive income (loss)	(553)	(118)
Balance	(558)	(5)	113
Total accumulated other comprehensive loss
Accumulated other comprehensive loss [Roll Forward]
Balance	(41,004)	(48,078)
Other comprehensive income (loss) before reclassifications	11,340	4,624
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset	(3,265)
Amounts reclassified from accumulated other comprehensive loss	2,346	2,450
Other comprehensive income (loss)	10,421	7,074	(5,976)
Balance	$ (30,583)	$ (41,004)	$ (48,078)