Comprehensive income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Accumulated other comprehensive income (loss) [Roll Forward]
Balance			$ 2,429,043	$ 2,316,244
Amounts reclassified from accumulated other comprehensive loss	$ (827)	$ (435)	(1,367)	(918)
Other comprehensive income (loss)	725	396	1,158	(56)
Balance	2,451,078	2,302,952	2,451,078	2,302,952
Net unrealized gain (loss) on derivative instruments qualifying as hedges
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(2,231)	(2,209)	(1,934)	(2,300)
Other comprehensive loss before reclassifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive loss	95	92	187	183
Amounts reclassified to accumulated other comprehensive loss from regulatory asset				0
Other comprehensive income (loss)	95	92	187	183
Balance	(2,136)	(2,117)	(2,136)	(2,117)
Postretirement liability adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(42,265)	(33,781)	(35,163)	(33,221)
Other comprehensive loss before reclassifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive loss	449	312	867	669
Amounts reclassified to accumulated other comprehensive loss from regulatory asset				(917)
Other comprehensive income (loss)	449	312	867	(248)
Balance	(41,816)	(33,469)	(41,816)	(33,469)
Foreign currency translation adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(190)	(140)	(155)	(149)
Other comprehensive loss before reclassifications	(59)	(15)	(61)	(6)
Amounts reclassified from accumulated other comprehensive loss	249	0	249	0
Amounts reclassified to accumulated other comprehensive loss from regulatory asset				0
Other comprehensive income (loss)	190	(15)	188	(6)
Balance	0	(155)	0	(155)
Net unrealized gain (loss) on available-for-sale investments
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(174)	(55)	(82)	(63)
Other comprehensive loss before reclassifications	(43)	(24)	(148)	(51)
Amounts reclassified from accumulated other comprehensive loss	34	31	64	66
Amounts reclassified to accumulated other comprehensive loss from regulatory asset				0
Other comprehensive income (loss)	(9)	7	(84)	15
Balance	(183)	(48)	(183)	(48)
Total accumulated other comprehensive loss
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(44,860)	(36,185)	(37,334)	(35,733)
Other comprehensive loss before reclassifications	(102)	(39)	(209)	(57)
Amounts reclassified from accumulated other comprehensive loss	827	435	1,367	918
Amounts reclassified to accumulated other comprehensive loss from regulatory asset				(917)
Other comprehensive income (loss)	725	396	1,158	(56)
Balance	$ (44,135)	$ (35,789)	(44,135)	$ (35,789)
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)