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Derivatives (Details) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($) MWh t	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Derivative [Line Items]
Derivative asset	$ 2.0		$ 1.7
Derivative liability	1.1		1.2
Net gain (loss) on forward and derivative contracts	$ 0.7	$ (16.1)	$ (16.1)
LME Swap [Member]
Derivative [Line Items]
Other forward delivery contracts to sell primary aluminum (in tonnes) | t	3,090
Price Swap [Member]
Derivative [Line Items]
Open position to offset fixed prices | t	11,337
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

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- Definition

Derivative, Financial Contract, Percentage

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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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Underlying, Derivative Energy, Available

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Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Nominal mass used to calculate payments on a derivative instrument.

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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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