Comprehensive income (loss) (Details 13) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	$ 2,520,548	$ 3,249,784	$ 2,855,902
Amounts reclassified from accumulated other comprehensive loss	(3,016)	(3,131)
Net current-period other comprehensive income (loss)	1,415	4,955	(3,898)
Balance	2,316,244	2,520,548	3,249,784
Net unrealized gain (loss) on derivative instruments qualifying as hedges
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(2,667)	(3,071)
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive loss	367	404
Amounts reclassified from accumulated other comprehensive loss to a regulatory asset		0
Net current-period other comprehensive income (loss)	367	404
Balance	(2,300)	(2,667)	(3,071)
Postretirement liability adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(34,257)	(38,218)
Other comprehensive income (loss) before reclassifications	(1,470)	(88)
Amounts reclassified from accumulated other comprehensive loss	2,506	1,794
Amounts reclassified from accumulated other comprehensive loss to a regulatory asset		2,255
Net current-period other comprehensive income (loss)	1,036	3,961
Balance	(33,221)	(34,257)	(38,218)
Foreign currency translation adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(200)	(829)
Other comprehensive income (loss) before reclassifications	51	(173)
Amounts reclassified from accumulated other comprehensive loss	0	802
Amounts reclassified from accumulated other comprehensive loss to a regulatory asset		0
Net current-period other comprehensive income (loss)	51	629
Balance	(149)	(200)	(829)
Net unrealizaed gasin (loss) on available-for-sale investments
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(24)	15
Other comprehensive income (loss) before reclassifications	(182)	(170)
Amounts reclassified from accumulated other comprehensive loss	143	131
Amounts reclassified from accumulated other comprehensive loss to a regulatory asset		0
Net current-period other comprehensive income (loss)	(39)	(39)
Balance	(63)	(24)	15
Accumulated other comprehensive loss
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(37,148)	(42,103)	(38,205)
Other comprehensive income (loss) before reclassifications	(1,601)	(431)
Amounts reclassified from accumulated other comprehensive loss	3,016	3,131
Amounts reclassified from accumulated other comprehensive loss to a regulatory asset		2,255
Net current-period other comprehensive income (loss)	1,415	4,955	(3,898)
Balance	$ (35,733)	$ (37,148)	$ (42,103)