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Tax reform (Details 3) - USD ($) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2017 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
TCJA revaluation related to accumulated other comprehensive loss balance | $ (39,500,000) | $ 7,735,000 | $ 0 | $ 0
Deferred tax liabilities, regulatory assets |  | 285,520,000 |  |
Effective income tax rate reconciliation, repatriation of foreign earnings, amount | 447,000 | $ 447,000 |  |
Other comprehensive income (loss) [Member] |  |  |  |
TCJA revaluation related to accumulated other comprehensive loss balance | 7,700,000 |  |  |
Discontinued operations, held-for-sale [Member] |  |  |  |
TCJA revaluation related to accumulated other comprehensive loss balance | 1,000,000.0 |  |  |
Stranded costs [Member] |  |  |  |
Deferred tax liabilities, regulatory assets | $ (285,500,000) |  |  |

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