Comprehensive income (loss) (Details 12) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	$ 2,316,244	$ 2,520,548	$ 3,249,784
Amounts reclassified from accumulated other comprehensive loss	(1,499)	(3,016)
Net current-period other comprehensive income (loss)	(1,601)	1,415	4,955
Balance	2,429,043	2,316,244	2,520,548
Net unrealized gain (loss) on derivative instruments qualifying as hedges
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(2,300)	(2,667)
Other comprehensive loss before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive loss	366	367
Amounts reclassified from accumulated other comprehensive loss to a regulatory asset	0
Net current-period other comprehensive income (loss)	366	367
Balance	(1,934)	(2,300)	(2,667)
Postretirement liability adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(33,221)	(34,257)
Other comprehensive loss before reclassifications	(1,812)	(1,470)
Amounts reclassified from accumulated other comprehensive loss	1,013	2,506
Amounts reclassified from accumulated other comprehensive loss to a regulatory asset	(1,143)
Net current-period other comprehensive income (loss)	(1,942)	1,036
Balance	(35,163)	(33,221)	(34,257)
Foreign currency translation adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(149)	(200)
Other comprehensive loss before reclassifications	(6)	51
Amounts reclassified from accumulated other comprehensive loss	0	0
Amounts reclassified from accumulated other comprehensive loss to a regulatory asset	0
Net current-period other comprehensive income (loss)	(6)	51
Balance	(155)	(149)	(200)
Net unrealizaed gasin (loss) on available-for-sale investments
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(63)	(24)
Other comprehensive loss before reclassifications	(139)	(182)
Amounts reclassified from accumulated other comprehensive loss	120	143
Amounts reclassified from accumulated other comprehensive loss to a regulatory asset	0
Net current-period other comprehensive income (loss)	(19)	(39)
Balance	(82)	(63)	(24)
Accumulated other comprehensive loss
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(35,733)	(37,148)	(42,103)
Other comprehensive loss before reclassifications	(1,957)	(1,601)
Amounts reclassified from accumulated other comprehensive loss	1,499	3,016
Amounts reclassified from accumulated other comprehensive loss to a regulatory asset	(1,143)
Net current-period other comprehensive income (loss)	(1,601)	1,415	4,955
Balance	$ (37,334)	$ (35,733)	$ (37,148)