Derivatives, Hedging Programs and Other Financial Instruments - Summary of Derivative Positions (Details) - Designated as Hedging Instrument mmlb in Millions	9 Months Ended
Sep. 30, 2020 EUR (€) mmlb MMBTU MWh
Purchase \| Aluminum
Derivative [Line Items]
Derivative non-monetary notional amount	80.1
Purchase \| Aluminum \| Minimum
Derivative [Line Items]
Derivative maturity period	2020-10
Purchase \| Aluminum \| Maximum
Derivative [Line Items]
Derivative maturity period	2022-12
Purchase \| Midwest premium swap contracts
Derivative [Line Items]
Derivative non-monetary notional amount	79.1
Purchase \| Midwest premium swap contracts \| Minimum
Derivative [Line Items]
Derivative maturity period	2020-10
Purchase \| Midwest premium swap contracts \| Maximum
Derivative [Line Items]
Derivative maturity period	2022-12
Purchase \| Alloying Metals
Derivative [Line Items]
Derivative non-monetary notional amount	12.0
Purchase \| Alloying Metals \| Minimum
Derivative [Line Items]
Derivative maturity period	2020-10
Purchase \| Alloying Metals \| Maximum
Derivative [Line Items]
Derivative maturity period	2021-12
Purchase \| Natural Gas
Derivative [Line Items]
Derivative non-monetary notional amount \| MMBTU	7,070,000
Purchase \| Natural Gas \| Minimum
Derivative [Line Items]
Derivative maturity period	2020-10
Purchase \| Natural Gas \| Maximum
Derivative [Line Items]
Derivative maturity period	2024-12
Purchase \| Foreign Exchange Contract
Derivative [Line Items]
Derivative monetary notional amount \| €	€ 355,662
Purchase \| Foreign Exchange Contract \| Minimum
Derivative [Line Items]
Derivative maturity period	2020-12
Purchase \| Foreign Exchange Contract \| Maximum
Derivative [Line Items]
Derivative maturity period	2021-01
Purchase \| Electricity
Derivative [Line Items]
Derivative non-monetary notional amount \| MWh	525,980
Purchase \| Electricity \| Minimum
Derivative [Line Items]
Derivative maturity period	2020-10
Purchase \| Electricity \| Maximum
Derivative [Line Items]
Derivative maturity period	2022-12
Sales \| Aluminum
Derivative [Line Items]
Derivative non-monetary notional amount	1.0
Sales \| Aluminum \| Minimum
Derivative [Line Items]
Derivative maturity period	2020-10
Sales \| Aluminum \| Maximum
Derivative [Line Items]
Derivative maturity period	2021-11
Sales \| Alloying Metals
Derivative [Line Items]
Derivative non-monetary notional amount	5.0
Sales \| Alloying Metals \| Minimum
Derivative [Line Items]
Derivative maturity period	2020-10
Sales \| Alloying Metals \| Maximum
Derivative [Line Items]
Derivative maturity period	2021-12

Derivative maturity period.

No definition available.

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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.

+ Details

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- Definition

Nominal number of units used to calculate payment on derivative.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5580258-113959

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5579245-113959

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- Definition

Nominal or face amount used to calculate payment on derivative.

+ References

+ Details

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