Comprehensive income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Accumulated other comprehensive income (loss) [Roll Forward]
Balance				$ 2,520,548
Amounts reclassified from accumulated other comprehensive loss	$ (452)	$ (361)	$ (1,370)	730
Net current-period other comprehensive income (loss)	448	315	392	(743)
Balance		2,285,061		2,285,061
Net unrealized gain (loss) on derivative instruments qualifying as hedges
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(2,117)	(2,484)	(2,300)	(2,667)
Other comprehensive loss before reclassifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive loss	92	92	275	275
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset			0
Net current-period other comprehensive income (loss)	92	92	275	275
Balance	(2,025)	(2,392)	(2,025)	(2,392)
Postretirement liability adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(33,469)	(35,604)	(33,221)	(34,257)
Other comprehensive loss before reclassifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive loss	333	236	1,002	(1,111)
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset			(917)
Net current-period other comprehensive income (loss)	333	236	85	(1,111)
Balance	(33,136)	(35,368)	(33,136)	(35,368)
Foreign currency translation adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(155)	(144)	(149)	(200)
Other comprehensive loss before reclassifications	15	(4)	9	52
Amounts reclassified from accumulated other comprehensive loss	0	0	0	0
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset			0
Net current-period other comprehensive income (loss)	15	(4)	9	52
Balance	(140)	(148)	(140)	(148)
Net unrealized gain (loss) on available-for-sale investments
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(48)	26	(63)	(24)
Other comprehensive loss before reclassifications	(19)	(42)	(70)	(65)
Amounts reclassified from accumulated other comprehensive loss	27	33	93	106
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset			0
Net current-period other comprehensive income (loss)	8	(9)	23	41
Balance	(40)	17	(40)	17
Total accumulated other comprehensive loss
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(35,789)	(38,206)	(35,733)	(37,148)
Other comprehensive loss before reclassifications	(4)	(46)	(61)	(13)
Amounts reclassified from accumulated other comprehensive loss	452	361	1,370	(730)
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset			(917)
Net current-period other comprehensive income (loss)	448	315	392	(743)
Balance	$ (35,341)	$ (37,891)	$ (35,341)	$ (37,891)