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Derivatives (Details) € in Millions, $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018 USD ($) MWh t	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) MWh t	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Jun. 30, 2018 EUR (€) MWh t
Derivative [Line Items]
Derivative asset	$ 3.2		$ 3.2		$ 1.7
Derivative liability	1.5		1.5		$ 1.2
Net gain (loss) on forward and derivative contracts	$ 1.2	$ 3.0	$ 1.9	$ (13.2)
LME Swap [Member]
Derivative [Line Items]
Other forward delivery contracts to sell primary aluminum (in tonnes) | t	3,090		3,090			3,090
Price Swap [Member]
Derivative [Line Items]
Open position to offset fixed prices | t			8,609
FX Swap [Member]
Derivative [Line Items]
Derivative asset | €						€ 2.8
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			256,200

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