Accumulated other comprehensive income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	$ 2,847,246	$ 2,566,775
Other comprehensive income	707	774
Balance	2,836,592	2,605,692
Net unrealized gain (loss) on derivative instruments qualifying as hedges
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(1,430)	(2,161)
Other comprehensive income before reclassifications	0	0
Amounts reclassified from (to) accumulated other comprehensive loss	111	397
Other comprehensive income	111	397
Balance	(1,319)	(1,764)
Postretirement liability adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(40,734)	(36,069)
Other comprehensive income before reclassifications	0	0
Amounts reclassified from (to) accumulated other comprehensive loss	462	310
Other comprehensive income	462	310
Balance	(40,272)	(35,759)
Net unrealized gain (loss) on available-for-sale investments
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	62	(112)
Other comprehensive income before reclassifications	135	39
Amounts reclassified from (to) accumulated other comprehensive loss	(1)	28
Other comprehensive income	134	67
Balance	196	(45)
Total accumulated other comprehensive loss
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(42,102)	(38,342)
Other comprehensive income before reclassifications	135	39
Amounts reclassified from (to) accumulated other comprehensive loss	572	735
Other comprehensive income	707	774
Balance	$ (41,395)	$ (37,568)