Comprehensive income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	$ 2,429,043	$ 2,316,244
Amounts reclassified from accumulated other comprehensive loss	(540)	(483)
Net current-period other comprehensive income (loss)	433	(452)
Balance	2,426,111	2,313,905
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	92	91
Net current-period other comprehensive income (loss)	92	91
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset		0
Balance	(2,231)	(2,209)
Postretirement liability adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(35,163)	(33,221)
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive loss	418	357
Net current-period other comprehensive income (loss)	418	(560)
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset		(917)
Balance	(42,265)	(33,781)
Foreign currency translation adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(155)	(149)
Other comprehensive income (loss) before reclassifications	(2)	9
Amounts reclassified from accumulated other comprehensive loss	0	0
Net current-period other comprehensive income (loss)	(2)	9
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset		0
Balance	(190)	(140)
Net unrealized gain (loss) on available-for-sale investments
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(82)	(63)
Other comprehensive income (loss) before reclassifications	(105)	(27)
Amounts reclassified from accumulated other comprehensive loss	30	35
Net current-period other comprehensive income (loss)	(75)	8
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset		0
Balance	(174)	(55)
Total accumulated other comprehensive loss
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(37,334)	(35,733)
Other comprehensive income (loss) before reclassifications	(107)	(18)
Amounts reclassified from accumulated other comprehensive loss	540	483
Net current-period other comprehensive income (loss)	433	(452)
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset		(917)
Balance	(44,860)	$ (36,185)
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)
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)
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)
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)
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)