Accumulated other comprehensive income (loss) (Details 9) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	$ 2,566,775
Net current-period other comprehensive income (loss)	(3,760)	$ 6,951	$ (1,601)
Balance	2,847,246	2,566,775
Net Unrealized Loss on Derivative Instruments Qualifying as Hedges
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(2,161)	(1,934)
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive loss	731	162
Net current-period other comprehensive income (loss)	731	162
Balance	(1,430)	(2,161)	(1,934)
Post- retirement Liability Adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(36,069)	(35,163)
Other comprehensive income (loss) before reclassifications	(6,151)	4,441
Amounts reclassified from accumulated other comprehensive loss	1,486	2,173
Net current-period other comprehensive income (loss)	(4,665)	6,614
Balance	(40,734)	(36,069)	(35,163)
Foreign Currency Translation Adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	0	(155)
Other comprehensive income (loss) before reclassifications	0	(61)
Amounts reclassified from accumulated other comprehensive loss	0	249
Net current-period other comprehensive income (loss)	0	188
Balance	0	0	(155)
Net Unrealized Gain (Loss) on Available- for-sale Investments
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(112)	(82)
Other comprehensive income (loss) before reclassifications	134	(144)
Amounts reclassified from accumulated other comprehensive loss	40	131
Net current-period other comprehensive income (loss)	174	(13)
Balance	62	(112)	(82)
Total Accumulated Other Comprehensive Loss
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(38,342)	(37,334)
Other comprehensive income (loss) before reclassifications	(6,017)	4,236
Amounts reclassified from accumulated other comprehensive loss	2,257	2,715
Net current-period other comprehensive income (loss)	(3,760)	6,951	(1,601)
Balance	$ (42,102)	(38,342)	$ (37,334)
Adjustments for New Accounting Principle, Early Adoption | Tax Cuts and Jobs Act | Net Unrealized 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)
Adjustments for New Accounting Principle, Early Adoption | Tax Cuts and Jobs Act | Post- retirement 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)
Adjustments for New Accounting Principle, Early Adoption | Tax Cuts and Jobs Act | 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)
Adjustments for New Accounting Principle, Early Adoption | Tax Cuts and Jobs Act | 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)
Adjustments for New Accounting Principle, Early Adoption | Tax Cuts and Jobs Act | 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)