Accumulated other comprehensive income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	$ 2,897,687	$ 2,836,592	$ 2,847,246	$ 2,660,301	$ 2,605,692	$ 2,566,775	$ 2,847,246	$ 2,566,775
Other comprehensive income (loss)	560	586	707	412	478	774	1,853	1,664
Balance	3,012,765	2,897,687	2,836,592	2,795,606	2,660,301	2,605,692	3,012,765	2,795,606
Net unrealized gain (loss) on derivative instruments qualifying as hedges
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(1,207)	(1,319)	(1,430)	(1,653)	(1,764)	(2,161)	(1,430)	(2,161)
Other comprehensive income (loss) before reclassifications	0	0	0	0	0	0
Amounts reclassified (to) from accumulated other comprehensive loss	111	112	111	112	111	397
Other comprehensive income (loss)	111	112	111	112	111	397
Balance	(1,096)	(1,207)	(1,319)	(1,541)	(1,653)	(1,764)	(1,096)	(1,541)
Postretirement liability adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(39,792)	(40,272)	(40,734)	(35,483)	(35,759)	(36,069)	(40,734)	(36,069)
Other comprehensive income (loss) before reclassifications	0	0	0	0	0	0
Amounts reclassified (to) from accumulated other comprehensive loss	471	480	462	292	276	310
Other comprehensive income (loss)	471	480	462	292	276	310
Balance	(39,321)	(39,792)	(40,272)	(35,191)	(35,483)	(35,759)	(39,321)	(35,191)
Net unrealized gain (loss) on available-for-sale investments
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	190	196	62	46	(45)	(112)	62	(112)
Other comprehensive income (loss) before reclassifications	(50)	(13)	135	12	79	39
Amounts reclassified (to) from accumulated other comprehensive loss	28	7	(1)	(4)	12	28
Other comprehensive income (loss)	(22)	(6)	134	8	91	67
Balance	168	190	196	54	46	(45)	168	54
Total accumulated other comprehensive loss
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(40,809)	(41,395)	(42,102)	(37,090)	(37,568)	(38,342)	(42,102)	(38,342)
Other comprehensive income (loss) before reclassifications	(50)	(13)	135	12	79	39
Amounts reclassified (to) from accumulated other comprehensive loss	610	599	572	400	399	735
Other comprehensive income (loss)	560	586	707	412	478	774
Balance	$ (40,249)	$ (40,809)	$ (41,395)	$ (36,678)	$ (37,090)	$ (37,568)	$ (40,249)	$ (36,678)