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Derivatives (Details) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2017 USD ($) MWh t | Mar. 31, 2016 USD ($) | Dec. 31, 2016 USD ($) t | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($)
Derivative [Line Items] |  |  |  |  |
Derivative asset | $ | $ 1,610 |  |  |  |
Derivative liability | $ | 14,915 |  |  |  |
Recognized loss on derivative instruments | $ | $ 16,137 | $ (353) | $ (3,487) | $ (1,600) | $ (179)
LME Swap [Member] |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Other forward delivery contracts to sell primary aluminum (in tonnes) | 52,590 |  |  |  |
Other forward delivery contracts to sell primary aluminum, settled monthly (in tonnes) | 49,500 |  |  |  |
Price Swap [Member] |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Open position to offset fixed prices | 7,931 |  |  |  |
Glencore [Member] |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Other forward delivery contracts to sell primary aluminum (in tonnes) |  |  | 60,600 |  |
Glencore [Member] | LME Swap [Member] |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Other forward delivery contracts to sell primary aluminum (in tonnes) | 48,540 |  |  |  |
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 |  |  |  |

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