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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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