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Derivatives (Details) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) MWh t	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Supply Commitment [Line Items]
Derivative liability		$ 1,200
Derivative asset		1,724
Loss on derivative instruments, net, pretax	$ 16,137	$ 16,549	$ (3,487)	$ (1,600)
Fixed to Floating Swap [Member]
Supply Commitment [Line Items]
Open position to offset fixed prices | t		8,964
Fixed to Variable London Metals Exchange Swap [Member]
Supply Commitment [Line Items]
Other forward delivery contracts to sell primary aluminum (in tonnes) | t		3,090
Grundartangi [Member] | Power Price Swap [Member]
Supply Commitment [Line Items]
Derivative, financial contract, percentage		4.00%
Underlying, derivative energy, available | MWh		256,200

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Total amount of primary aluminum committed to be supplied under existing contracts or agreements outside of major long-term supply commitments.

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Aggregate net gain (loss) on all derivative instruments recognized in earnings during the period, before tax effects.

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