Accumulated other comprehensive income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	$ 2,605,692	$ 2,566,775	$ 2,426,111	$ 2,429,043	$ 2,566,775	$ 2,429,043
Other comprehensive income	478	774	725	433	1,252	1,158
Balance	2,660,301	2,605,692	2,451,078	2,426,111	2,660,301	2,451,078
Net unrealized gain (loss) on derivative instruments qualifying as hedges
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(1,764)	(2,161)	(2,231)	(1,934)	(2,161)	(1,934)
Other comprehensive income before reclassifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive loss	111	397	95	92
Other comprehensive income	111	397	95	92
Balance	(1,653)	(1,764)	(2,136)	(2,231)	(1,653)	(2,136)
Postretirement liability adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(35,759)	(36,069)	(42,265)	(35,163)	(36,069)	(35,163)
Other comprehensive income before reclassifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive loss	276	310	449	418
Other comprehensive income	276	310	449	418
Balance	(35,483)	(35,759)	(41,816)	(42,265)	(35,483)	(41,816)
Foreign currency translation adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance			(190)	(155)		(155)
Other comprehensive income before reclassifications			(59)	(2)
Amounts reclassified from accumulated other comprehensive loss			249	0
Other comprehensive income			190	(2)
Balance			0	(190)		0
Net unrealized gain (loss) on available-for-sale investments
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(45)	(112)	(174)	(82)	(112)	(82)
Other comprehensive income before reclassifications	79	39	(43)	(105)
Amounts reclassified from accumulated other comprehensive loss	12	28	34	30
Other comprehensive income	91	67	(9)	(75)
Balance	46	(45)	(183)	(174)	46	(183)
Total accumulated other comprehensive loss
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(37,568)	(38,342)	(44,860)	(37,334)	(38,342)	(37,334)
Other comprehensive income before reclassifications	79	39	(102)	(107)
Amounts reclassified from accumulated other comprehensive loss	399	735	827	540
Other comprehensive income	478	774	725	433
Balance	$ (37,090)	$ (37,568)	$ (44,135)	(44,860)	$ (37,090)	$ (44,135)
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)