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Helguvik and Ravenswood gains and losses (Details) - USD ($) $ in Thousands | 1 Months Ended | 12 Months Ended
--- | --- | ---
Jan. 31, 2017 | Jun. 30, 2015 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Impaired Assets to be Disposed of by Method Other than Sale [Line Items] |  |  |  |  |
Helguvik (gains) losses |  |  |  | $ 152,220 |
Gain project termination |  |  | $ 7,310 | (152,220) | $ 0
Ravenswood Aluminum Smelter [Member] |  |  |  |  |
Impaired Assets to be Disposed of by Method Other than Sale [Line Items] |  |  |  |  |
Ravenswood charges |  | $ 30,850 |  | $ 3,830 |
Proceeds from sale of Ravenswood facility and assets | $ 13,585 |  |  |  |

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