Comprehensive income (loss) (Details 9) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	$ 2,429,043	$ 2,316,244	$ 2,520,548
Net current-period other comprehensive income (loss)	6,951	(1,601)	1,415
Balance	2,566,775	2,429,043	2,316,244
Net unrealized gain (loss) on derivative instruments qualifying as hedges
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(1,934)	(2,300)
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive loss	162	366
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset		0
Net current-period other comprehensive income (loss)	162	366
Balance	(2,161)	(1,934)	(2,300)
Postretirement liability adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(35,163)	(33,221)
Other comprehensive income (loss) before reclassifications	4,441	(1,812)
Amounts reclassified from accumulated other comprehensive loss	2,173	1,013
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset		(1,143)
Net current-period other comprehensive income (loss)	6,614	(1,942)
Balance	(36,069)	(35,163)	(33,221)
Foreign currency translation adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(155)	(149)
Other comprehensive income (loss) before reclassifications	(61)	(6)
Amounts reclassified from accumulated other comprehensive loss	249	0
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset		0
Net current-period other comprehensive income (loss)	188	(6)
Balance	0	(155)	(149)
Net unrealizaed gasin (loss) on available-for-sale investments
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(82)	(63)
Other comprehensive income (loss) before reclassifications	(144)	(139)
Amounts reclassified from accumulated other comprehensive loss	131	120
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset		0
Net current-period other comprehensive income (loss)	(13)	(19)
Balance	(112)	(82)	(63)
Accumulated other comprehensive loss
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(37,334)	(35,733)	(37,148)
Other comprehensive income (loss) before reclassifications	4,236	(1,957)
Amounts reclassified from accumulated other comprehensive loss	2,715	1,499
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset		(1,143)
Net current-period other comprehensive income (loss)	6,951	(1,601)	1,415
Balance	(38,342)	$ (37,334)	$ (35,733)
Adjustments for New Accounting Principle, Early Adoption | Tax Cuts and Jobs Act | 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)
Adjustments for New Accounting Principle, Early Adoption | Tax Cuts and Jobs Act | 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)
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 unrealizaed gasin (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 | 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)