Deferred tax reconciliation (Details 6) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Change in net deferred income tax liability from the preceding table	$ (320,955)
Deferred taxes associated with other comprehensive loss	1,182
Deferred tax liabilities, regulatory assets	285,520
Other	8,830
Deferred income taxes	$ (25,423)	$ (2,049)	$ (25,356)