Comprehensive income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Accumulated other comprehensive income (loss) [Roll Forward]
Balance			$ 2,429,043	$ 2,316,244
Other comprehensive income	$ 516	$ 448	1,674	392
Balance	2,521,293	2,353,917	2,521,293	2,353,917
Net unrealized gain (loss) on derivative instruments qualifying as hedges
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(2,136)	(2,117)	(1,934)	(2,300)
Other comprehensive loss before reclassifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive loss	92	92	279	275
Amounts reclassified to accumulated other comprehensive loss from regulatory asset				0
Other comprehensive income	92	92	279	275
Balance	(2,044)	(2,025)	(2,044)	(2,025)
Postretirement liability adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(41,816)	(33,469)	(35,163)	(33,221)
Other comprehensive loss before reclassifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive loss	442	333	1,309	1,002
Amounts reclassified to accumulated other comprehensive loss from regulatory asset				(917)
Other comprehensive income	442	333	1,309	85
Balance	(41,374)	(33,136)	(41,374)	(33,136)
Foreign currency translation adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	0	(155)	(155)	(149)
Other comprehensive loss before reclassifications	0	15	(61)	9
Amounts reclassified from accumulated other comprehensive loss	0	0	249	0
Amounts reclassified to accumulated other comprehensive loss from regulatory asset				0
Other comprehensive income	0	15	188	9
Balance	0	(140)	0	(140)
Net unrealized gain (loss) on available-for-sale investments
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(183)	(48)	(82)	(63)
Other comprehensive loss before reclassifications	(51)	(19)	(199)	(70)
Amounts reclassified from accumulated other comprehensive loss	33	27	97	93
Amounts reclassified to accumulated other comprehensive loss from regulatory asset				0
Other comprehensive income	(18)	8	(102)	23
Balance	(201)	(40)	(201)	(40)
Total accumulated other comprehensive loss
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(44,135)	(35,789)	(37,334)	(35,733)
Other comprehensive loss before reclassifications	(51)	(4)	(260)	(61)
Amounts reclassified from accumulated other comprehensive loss	567	452	1,934	1,370
Amounts reclassified to accumulated other comprehensive loss from regulatory asset				(917)
Other comprehensive income	516	448	1,674	392
Balance	$ (43,619)	$ (35,341)	(43,619)	$ (35,341)
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 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)