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Quarterly information (Unaudited) - Financial Results Overview (Details) - USD ($) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2015 | Sep. 30, 2015 | Jun. 30, 2015 | Mar. 31, 2015 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Effect of Fourth Quarter Events [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Helguvik impairment |  |  |  |  |  |  |  |  |  | $ 152,220,000 | $ 0 | $ 0
Charge related to discrete tax items |  | $ 6,900,000 |  |  |  |  |  |  |  |  |  |
Impairment charge |  |  |  |  |  |  |  |  |  | 3,830,000 | 30,850,000 | 0
Ravenswood retiree legal settlement |  |  | $ 23,000,000 |  |  |  |  |  |  | 23,000,000 | 0 | 0
Lower cost or market inventory adjustment | $ (3,880,000) |  |  |  | $ (5,784,000) |  | $ 5,324,000 |  |  | (660,000) | 7,539,000 | (1,247,000)
Favorable lower of cost or market inventory adjustment |  |  |  |  | $ 5,800,000 | $ 23,474,000 |  |  |  |  |  |
Pension charges |  |  |  |  |  | 3,400,000 |  |  |  |  |  |
Depreciation |  |  |  |  |  |  |  |  |  | 84,780,000 | 80,117,000 | 70,827,000
Impairment |  |  |  |  |  |  |  |  |  | 0 | 11,584,000 | 0
Labor disruption costs |  |  |  |  |  |  | 1,400,000 | $ 11,700,000 |  |  |  |
Gain on fair value of contingent consideration |  |  |  |  |  |  |  |  | $ 6,527,000 | 0 | $ 18,337,000 | $ 7,943,000
Severance charge |  |  |  |  |  |  |  |  | 1,000,000 |  |  |
Hawesville and Mt. Holly [Member] |  |  |  |  |  |  |  |  |  |  |  |
Effect of Fourth Quarter Events [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Curtailment expenses |  |  |  |  |  | 3,500,000 |  |  |  |  |  |
Hawesville [Member] |  |  |  |  |  |  |  |  |  |  |  |
Effect of Fourth Quarter Events [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Curtailment expenses |  |  |  |  |  |  | $ 2,900,000 |  |  |  |  |
Ravenswood Aluminum Smelter [Member] |  |  |  |  |  |  |  |  |  |  |  |
Effect of Fourth Quarter Events [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Impairment charge |  |  | $ 3,800,000 |  |  |  |  | 30,850,000 |  | $ 3,830,000 |  |
Lower cost or market inventory adjustment |  |  |  | $ 4,900,000 |  |  |  | 25,689,000 |  |  |  |
Grundartangi, Iceland [Member] |  |  |  |  |  |  |  |  |  |  |  |
Effect of Fourth Quarter Events [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Signing bonuses, amount |  |  |  |  |  |  |  |  | $ 1,570,000 |  |  |
Mt. Holly Smelter [Member] |  |  |  |  |  |  |  |  |  |  |  |
Effect of Fourth Quarter Events [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Depreciation |  |  |  |  |  | 5,000,000 |  |  |  |  |  |
Gain on fair value of contingent consideration |  |  |  |  |  |  |  | $ 10,287,000 |  |  |  |
BHH [Member] |  |  |  |  |  |  |  |  |  |  |  |
Effect of Fourth Quarter Events [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Impairment |  |  |  |  |  | $ 11,584,000 |  |  |  |  |  |
Helguvik [Member] |  |  |  |  |  |  |  |  |  |  |  |
Effect of Fourth Quarter Events [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Helguvik impairment |  | $ 152,200,000 |  |  |  |  |  |  |  |  |  |

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