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Derivatives (Details) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2017 USD ($) | Dec. 31, 2017 USD ($) MWh t | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($)
Supply Commitment [Line Items] |  |  |  |
Derivative liability |  | $ 1,200 |  |
Derivative asset |  | 1,724 |  |
Loss on derivative instruments, net, pretax | $ 16,137 | $ 16,549 | $ (3,487) | $ (1,600)
Fixed to Floating Swap [Member] |  |  |  |
Supply Commitment [Line Items] |  |  |  |
Open position to offset fixed prices | t |  | 8,964 |  |
Fixed to Variable London Metals Exchange Swap [Member] |  |  |  |
Supply Commitment [Line Items] |  |  |  |
Other forward delivery contracts to sell primary aluminum (in tonnes) | t |  | 3,090 |  |
Grundartangi [Member] | Power Price Swap [Member] |  |  |  |
Supply Commitment [Line Items] |  |  |  |
Derivative, financial contract, percentage |  | 4.00% |  |
Underlying, derivative energy, available | MWh |  | 256,200 |  |

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