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Deferred tax reconciliation (Details 6) - USD ($) $ in Thousands | 12 Months Ended
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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)

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