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Derivatives (Details) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2018 USD ($) MWh t | Mar. 31, 2017 USD ($) | Dec. 31, 2017 USD ($)
Derivative [Line Items] |  |  |
Derivative asset | $ 2.0 |  | $ 1.7
Derivative liability | 1.1 |  | 1.2
Net gain (loss) on forward and derivative contracts | $ 0.7 | $ (16.1) | $ (16.1)
LME Swap [Member] |  |  |
Derivative [Line Items] |  |  |
Other forward delivery contracts to sell primary aluminum (in tonnes) | t | 3,090 |  |
Price Swap [Member] |  |  |
Derivative [Line Items] |  |  |
Open position to offset fixed prices | t | 11,337 |  |
Power Price Swap [Member] | Grundartangi [Member] |  |  |
Derivative [Line Items] |  |  |
Percentage of power purchases included in forward price contract (percent) | 4.00% |  |
Power available | MWh | 256,200 |  |

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