Accumulated other comprehensive income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	$ 2,660,301	$ 2,605,692	$ 2,566,775	$ 2,451,078	$ 2,426,111	$ 2,429,043	$ 2,566,775	$ 2,429,043
Other comprehensive income	412	478	774	516	725	433	1,664	1,674
Balance	2,795,606	2,660,301	2,605,692	2,521,293	2,451,078	2,426,111	2,795,606	2,521,293
Net unrealized gain (loss) on derivative instruments qualifying as hedges
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(1,653)	(1,764)	(2,161)	(2,136)	(2,231)	(1,934)	(2,161)	(1,934)
Other comprehensive income before reclassifications	0	0	0	0	0	0
Amounts reclassified (to) from accumulated other comprehensive loss	112	111	397	92	95	92
Other comprehensive income	112	111	397	92	95	92
Balance	(1,541)	(1,653)	(1,764)	(2,044)	(2,136)	(2,231)	(1,541)	(2,044)
Postretirement liability adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(35,483)	(35,759)	(36,069)	(41,816)	(42,265)	(35,163)	(36,069)	(35,163)
Other comprehensive income before reclassifications	0	0	0	0	0	0
Amounts reclassified (to) from accumulated other comprehensive loss	292	276	310	442	449	418
Other comprehensive income	292	276	310	442	449	418
Balance	(35,191)	(35,483)	(35,759)	(41,374)	(41,816)	(42,265)	(35,191)	(41,374)
Foreign currency translation adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance				0	(190)	(155)		(155)
Other comprehensive income before reclassifications				0	(59)	(2)
Amounts reclassified (to) from accumulated other comprehensive loss				0	249	0
Other comprehensive income				0	190	(2)
Balance				0	0	(190)		0
Net unrealized gain (loss) on available-for-sale investments
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	46	(45)	(112)	(183)	(174)	(82)	(112)	(82)
Other comprehensive income before reclassifications	12	79	39	(51)	(43)	(105)
Amounts reclassified (to) from accumulated other comprehensive loss	(4)	12	28	33	34	30
Other comprehensive income	8	91	67	(18)	(9)	(75)
Balance	54	46	(45)	(201)	(183)	(174)	54	(201)
Total accumulated other comprehensive loss
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(37,090)	(37,568)	(38,342)	(44,135)	(44,860)	(37,334)	(38,342)	(37,334)
Other comprehensive income before reclassifications	12	79	39	(51)	(102)	(107)
Amounts reclassified (to) from accumulated other comprehensive loss	400	399	735	567	827	540
Other comprehensive income	412	478	774	516	725	433
Balance	$ (36,678)	$ (37,090)	$ (37,568)	$ (43,619)	$ (44,135)	(44,860)	$ (36,678)	$ (43,619)
Tax Cuts and Jobs Act [Member] | Adjustments for New Accounting Principle, Early Adoption [Member] | Net unrealized gain (loss) on derivative instruments qualifying as hedges
Accumulated other comprehensive income (loss) [Roll Forward]
Reclassification adjustment of prior period tax effects related to TCJA included in accumulated other comprehensive loss						(389)
Tax Cuts and Jobs Act [Member] | Adjustments for New Accounting Principle, Early Adoption [Member] | Postretirement liability adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Reclassification adjustment of prior period tax effects related to TCJA included in accumulated other comprehensive loss						(7,520)
Tax Cuts and Jobs Act [Member] | Adjustments for New Accounting Principle, Early Adoption [Member] | Foreign currency translation adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Reclassification adjustment of prior period tax effects related to TCJA included in accumulated other comprehensive loss						(33)
Tax Cuts and Jobs Act [Member] | Adjustments for New Accounting Principle, Early Adoption [Member] | Net unrealized gain (loss) on available-for-sale investments
Accumulated other comprehensive income (loss) [Roll Forward]
Reclassification adjustment of prior period tax effects related to TCJA included in accumulated other comprehensive loss						(17)
Tax Cuts and Jobs Act [Member] | Adjustments for New Accounting Principle, Early Adoption [Member] | Total accumulated other comprehensive loss
Accumulated other comprehensive income (loss) [Roll Forward]
Reclassification adjustment of prior period tax effects related to TCJA included in accumulated other comprehensive loss						$ (7,959)