Comprehensive income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	$ 2,316,244	$ 2,520,548
Amounts reclassified from accumulated other comprehensive loss	(483)	1,467
Net current-period other comprehensive income (loss)	(452)	(1,434)
Balance	2,313,905	2,495,384
Net unrealized gain (loss) on derivative instruments qualifying as hedges
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(2,300)	(2,667)
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive loss	91	92
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset	0
Net current-period other comprehensive income (loss)	91	92
Balance	(2,209)	(2,575)
Postretirement liability adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(33,221)	(34,257)
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive loss	357	(1,595)
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset	(917)
Net current-period other comprehensive income (loss)	(560)	(1,595)
Balance	(33,781)	(35,852)
Foreign currency translation adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(149)	(200)
Other comprehensive income (loss) before reclassifications	9	25
Amounts reclassified from accumulated other comprehensive loss	0	0
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset	0
Net current-period other comprehensive income (loss)	9	25
Balance	(140)	(175)
Net unrealized gain (loss) on available-for-sale investments
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(63)	(24)
Other comprehensive income (loss) before reclassifications	(27)	8
Amounts reclassified from accumulated other comprehensive loss	35	36
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset	0
Net current-period other comprehensive income (loss)	8	44
Balance	(55)	20
Total accumulated other comprehensive loss
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(35,733)	(37,148)
Other comprehensive income (loss) before reclassifications	(18)	33
Amounts reclassified from accumulated other comprehensive loss	483	(1,467)
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset	(917)
Net current-period other comprehensive income (loss)	(452)	(1,434)
Balance	$ (36,185)	$ (38,582)