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Derivatives (Details) MMBTU in Thousands, € in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) MMBTU MWh t	Dec. 31, 2020 EUR (€) MMBTU MWh t	Dec. 31, 2019 USD ($)
Supply Commitment [Line Items]
Derivative asset | $	$ 15.1		$ 19.7
LME Swap [Member]
Supply Commitment [Line Items]
Other forward delivery contracts to sell primary aluminum (in tonnes)	194,116	194,116
Midwest Premium (MWP) [Member]
Supply Commitment [Line Items]
Open position to offset fixed prices	318,000
Fixed to Floating Swap [Member]
Supply Commitment [Line Items]
Open position to offset fixed prices	9,694
NYMEX Henry Hub Natural Gas Price Swaps [Member]
Supply Commitment [Line Items]
Underlying, derivative energy, available | MMBTU	2,355	2,355
FX Swap [Member]
Supply Commitment [Line Items]
Derivative asset | €		€ 40.3
Grundartangi [Member] | Nordpool Power Price Swap [Member]
Supply Commitment [Line Items]
Underlying, derivative energy, available | MWh	1,830,840	1,830,840

X

- Definition

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

+ References

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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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=121572278&loc=d3e13433-108611

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- Definition

Nominal mass used to calculate payments on a derivative instrument.

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