Accumulated other comprehensive income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	$ 2,836,592	$ 2,847,246	$ 2,605,692	$ 2,566,775	$ 2,847,246	$ 2,566,775
Other comprehensive income	586	707	478	774	1,293	1,252
Balance	2,897,687	2,836,592	2,660,301	2,605,692	2,897,687	2,660,301
Net unrealized gain (loss) on derivative instruments qualifying as hedges
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(1,319)	(1,430)	(1,764)	(2,161)	(1,430)	(2,161)
Other comprehensive income (loss) before reclassifications	0	0	0	0
Amounts reclassified from (to) accumulated other comprehensive loss	112	111	111	397
Other comprehensive income	112	111	111	397
Balance	(1,207)	(1,319)	(1,653)	(1,764)	(1,207)	(1,653)
Postretirement liability adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(40,272)	(40,734)	(35,759)	(36,069)	(40,734)	(36,069)
Other comprehensive income (loss) before reclassifications	0	0	0	0
Amounts reclassified from (to) accumulated other comprehensive loss	480	462	276	310
Other comprehensive income	480	462	276	310
Balance	(39,792)	(40,272)	(35,483)	(35,759)	(39,792)	(35,483)
Net unrealized gain (loss) on available-for-sale investments
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	196	62	(45)	(112)	62	(112)
Other comprehensive income (loss) before reclassifications	(13)	135	79	39
Amounts reclassified from (to) accumulated other comprehensive loss	7	(1)	12	28
Other comprehensive income	(6)	134	91	67
Balance	190	196	46	(45)	190	46
Total accumulated other comprehensive loss
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(41,395)	(42,102)	(37,568)	(38,342)	(42,102)	(38,342)
Other comprehensive income (loss) before reclassifications	(13)	135	79	39
Amounts reclassified from (to) accumulated other comprehensive loss	599	572	399	735
Other comprehensive income	586	707	478	774
Balance	$ (40,809)	$ (41,395)	$ (37,090)	$ (37,568)	$ (40,809)	$ (37,090)