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Derivatives (Details) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2019 t	Dec. 31, 2019 USD ($) MWh t	Dec. 31, 2017	Dec. 31, 2018 USD ($)
Supply Commitment [Line Items]
Derivative asset | $		$ 19.7		$ 8.2
Fixed to Variable London Metals Exchange Swap [Member]
Supply Commitment [Line Items]
Other forward delivery contracts to sell primary aluminum (in tonnes)		96,065
Midwest Premium (MWP) [Member]
Supply Commitment [Line Items]
Open position to offset fixed prices		150,000
Hawesville L4 Power Price Swaps [Member]
Supply Commitment [Line Items]
Underlying, derivative energy, available | MWh		790,560
Fixed to Floating Swap [Member]
Supply Commitment [Line Items]
Open position to offset fixed prices	12,611
Grundartangi [Member]
Supply Commitment [Line Items]
Derivative asset | $		$ 4.8
Grundartangi [Member] | Power Price Swap [Member]
Supply Commitment [Line Items]
Underlying, derivative energy, available | MWh		219,600
Derivative, financial contract, percentage			4.00%

X

- Definition

Derivative, Financial Contract, Percentage

+ References

No definition available.

+ Details

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- Definition

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

+ References

No definition available.

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X

- Definition

Underlying, Derivative Energy, Available

+ References

No definition available.

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X

- Definition

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=118260190&loc=d3e13433-108611

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

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X

- Definition

Nominal mass used to calculate payments on a derivative instrument.

+ References

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- Definition

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.

+ References

No definition available.

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