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Derivatives (Details) $ in Thousands | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2017 USD ($) MWh t | Jun. 30, 2016 USD ($) | Jun. 30, 2017 USD ($) MWh t | Jun. 30, 2016 USD ($)
Derivative [Line Items] |  |  |  |
Derivative asset | $ | $ 354 |  | $ 354 |
Derivative liability | $ | 7,539 |  | 7,539 |
Net gain (loss) on forward and derivative contracts | $ | $ 2,957 | $ 353 | $ (13,180) | $ 706
LME Swap [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Other forward delivery contracts to sell primary aluminum (in tonnes) | 36,090 |  | 36,090 |
Other forward delivery contracts to sell primary aluminum, settled monthly (in tonnes) | 33,000 |  | 33,000 |
Price Swap [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Open position to offset fixed prices |  |  | 7,386 |
Affiliated Entity [Member] | LME Swap [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Other forward delivery contracts to sell primary aluminum (in tonnes) | 33,390 |  | 33,390 |
Grundartangi [Member] | Power Price Swap [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Percentage of power purchases included in forward price contract (percent) |  |  | 4.00% |
Power available | MWh | 256,200 |  | 256,200 |

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