Accumulated other comprehensive income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	$ 3,079,105	$ 2,847,246
Other comprehensive income (loss)	568	707
Balance	3,100,306	2,836,592
Net unrealized gain (loss) on derivative instruments qualifying as hedges
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(984)	(1,430)
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified (to) from accumulated other comprehensive loss	111	111
Other comprehensive income (loss)	111	111
Balance	(873)	(1,319)
Postretirement liability adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(47,207)	(40,734)
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified (to) from accumulated other comprehensive loss	466	462
Other comprehensive income (loss)	466	462
Balance	(46,741)	(40,272)
Net unrealized gain (loss) on available-for-sale investments
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	113	62
Other comprehensive income (loss) before reclassifications	(44)	135
Amounts reclassified (to) from accumulated other comprehensive loss	35	(1)
Other comprehensive income (loss)	(9)	134
Balance	104	196
Total accumulated other comprehensive loss
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(48,078)	(42,102)
Other comprehensive income (loss) before reclassifications	(44)	135
Amounts reclassified (to) from accumulated other comprehensive loss	612	572
Other comprehensive income (loss)	568	707
Balance	$ (47,510)	$ (41,395)