Accumulated other comprehensive loss (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Accumulated other comprehensive loss [Roll Forward]
Balance	$ 3,382,874	$ 3,079,105
Other comprehensive income (loss)	269	568
Balance	3,360,718	3,100,306
Net unrealized gain (loss) on derivative instruments qualifying as hedges
Accumulated other comprehensive loss [Roll Forward]
Balance	(538)	(984)
Other comprehensive loss before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive loss	112	111
Other comprehensive income (loss)	112	111
Balance	(426)	(873)
Postretirement liability adjustment
Accumulated other comprehensive loss [Roll Forward]
Balance	(40,461)	(47,207)
Other comprehensive loss before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive loss	445	466
Other comprehensive income (loss)	445	466
Balance	(40,016)	(46,741)
Net unrealized gain (loss) on available-for-sale investments
Accumulated other comprehensive loss [Roll Forward]
Balance	(5)	113
Other comprehensive loss before reclassifications	(320)	(44)
Amounts reclassified from accumulated other comprehensive loss	32	35
Other comprehensive income (loss)	(288)	(9)
Balance	(293)	104
Total accumulated other comprehensive loss
Accumulated other comprehensive loss [Roll Forward]
Balance	(41,004)	(48,078)
Other comprehensive loss before reclassifications	(320)	(44)
Amounts reclassified from accumulated other comprehensive loss	589	612
Other comprehensive income (loss)	269	568
Balance	$ (40,735)	$ (47,510)