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Quarterly information (Unaudited) - Financial Results Overview (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Effect of Fourth Quarter Events [Line Items] |  |  |  |  |  |  |
Gain (Loss) on Derivative Instruments, Net, Pretax |  | $ (16,137) |  |  | $ (16,549) | $ 3,487 | $ 1,600
Helguvik (gains) losses |  |  |  |  |  | (152,220) |
Ravenswood retiree legal settlement |  |  |  |  | $ 5,000 | $ (23,000) | $ 0
Helguvik [Member] |  |  |  |  |  |  |
Effect of Fourth Quarter Events [Line Items] |  |  |  |  |  |  |
Gain on extinguishment of liability | $ (7,310) |  |  |  |  |  |
Helguvik (gains) losses |  |  | $ (152,200) |  |  |  |
Ravenswood Retiree Medical Benefits Changes [Member] |  |  |  |  |  |  |
Effect of Fourth Quarter Events [Line Items] |  |  |  |  |  |  |
Ravenswood retiree legal settlement |  |  |  | $ (23,000) |  |  |

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