Comprehensive income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	$ 2,566,775	$ 2,429,043
Other comprehensive income	774	433
Balance	2,605,692	2,426,111
Net unrealized gain (loss) on derivative instruments qualifying as hedges
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(2,161)	(1,934)
Other comprehensive income before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive loss	397	92
Other comprehensive income	397	92
Balance	(1,764)	(2,231)
Postretirement liability adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(36,069)	(35,163)
Other comprehensive income before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive loss	310	418
Other comprehensive income	310	418
Balance	(35,759)	(42,265)
Foreign currency translation adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance		(155)
Other comprehensive income before reclassifications		(2)
Amounts reclassified from accumulated other comprehensive loss		0
Other comprehensive income		(2)
Balance		(190)
Net unrealized gain (loss) on available-for-sale investments
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(112)	(82)
Other comprehensive income before reclassifications	39	(105)
Amounts reclassified from accumulated other comprehensive loss	28	30
Other comprehensive income	67	(75)
Balance	(45)	(174)
Total accumulated other comprehensive loss
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(38,342)	(37,334)
Other comprehensive income before reclassifications	39	(107)
Amounts reclassified from accumulated other comprehensive loss	735	540
Other comprehensive income	774	433
Balance	$ (37,568)	(44,860)
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)