Accumulated other comprehensive loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Accumulated other comprehensive loss [Roll Forward]
Balance	$ 3,079,105
Other comprehensive income (loss)	7,074	$ (5,976)	$ (3,760)
Balance	3,382,874	3,079,105
Net unrealized gain (loss) on derivative instruments qualifying as hedges
Accumulated other comprehensive loss [Roll Forward]
Balance	(984)	(1,430)
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive loss	446	446
Other comprehensive income (loss)	446	446
Balance	(538)	(984)	(1,430)
Postretirement liability adjustment
Accumulated other comprehensive loss [Roll Forward]
Balance	(47,207)	(40,734)
Other comprehensive income (loss) before reclassifications	4,876	(8,395)
Amounts reclassified from accumulated other comprehensive loss	1,870	1,922
Other comprehensive income (loss)	6,746	(6,473)
Balance	(40,461)	(47,207)	(40,734)
Net unrealized gain (loss) on available-for-sale investments
Accumulated other comprehensive loss [Roll Forward]
Balance	113	62
Other comprehensive income (loss) before reclassifications	(252)	(1)
Amounts reclassified from accumulated other comprehensive loss	134	52
Other comprehensive income (loss)	(118)	51
Balance	(5)	113	62
Total accumulated other comprehensive loss
Accumulated other comprehensive loss [Roll Forward]
Balance	(48,078)	(42,102)
Other comprehensive income (loss) before reclassifications	4,624	(8,396)
Amounts reclassified from accumulated other comprehensive loss	2,450	2,420
Other comprehensive income (loss)	7,074	(5,976)	(3,760)
Balance	$ (41,004)	$ (48,078)	$ (42,102)