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Derivatives (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($) MWh t	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) MWh t	Sep. 30, 2016 USD ($)
Derivative [Line Items]
Derivative asset | $	$ 1,469		$ 1,469
Derivative liability | $	7,715		7,715
Net (loss) on forward and derivative contracts | $	$ (3,888)	$ 1,275	$ (17,068)	$ 2,998
LME Swap [Member]
Derivative [Line Items]
Other forward delivery contracts to sell primary aluminum (in tonnes)	19,590		19,590
Other forward delivery contracts to sell primary aluminum, settled monthly (in tonnes)	16,500		16,500
LME Swap [Member] | Affiliated Entity [Member]
Derivative [Line Items]
Other forward delivery contracts to sell primary aluminum (in tonnes)	18,240		18,240
Price Swap [Member]
Derivative [Line Items]
Open position to offset fixed prices			6,207
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		256,200

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