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Derivatives, Hedging Programs and Other Financial Instruments - Summary of Derivative Positions (Details) mmlb in Millions | 12 Months Ended
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Dec. 31, 2020 mmlb MMBTU MWh
Purchase | Aluminum |
Derivative [Line Items] |
Derivative non-monetary notional amount | 67.4
Purchase | Aluminum | Minimum |
Derivative [Line Items] |
Derivative maturity period | 2021-01
Purchase | Aluminum | Maximum |
Derivative [Line Items] |
Derivative maturity period | 2022-12
Purchase | Midwest premium swap contracts |
Derivative [Line Items] |
Derivative non-monetary notional amount | 66.4
Purchase | Midwest premium swap contracts | Minimum |
Derivative [Line Items] |
Derivative maturity period | 2021-01
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 | 10.2
Purchase | Alloying Metals | Minimum |
Derivative [Line Items] |
Derivative maturity period | 2021-01
Purchase | Alloying Metals | Maximum |
Derivative [Line Items] |
Derivative maturity period | 2021-12
Purchase | Natural Gas |
Derivative [Line Items] |
Derivative non-monetary notional amount | MMBTU | 6,630,000
Purchase | Natural Gas | Minimum |
Derivative [Line Items] |
Derivative maturity period | 2021-01
Purchase | Natural Gas | Maximum |
Derivative [Line Items] |
Derivative maturity period | 2024-12
Purchase | Electricity |
Derivative [Line Items] |
Derivative non-monetary notional amount | MWh | 481,800
Purchase | Electricity | Minimum |
Derivative [Line Items] |
Derivative maturity period | 2021-01
Purchase | Electricity | Maximum |
Derivative [Line Items] |
Derivative maturity period | 2022-12
Sales | Aluminum |
Derivative [Line Items] |
Derivative non-monetary notional amount | 0.8
Sales | Aluminum | Minimum |
Derivative [Line Items] |
Derivative maturity period | 2021-01
Sales | Aluminum | Maximum |
Derivative [Line Items] |
Derivative maturity period | 2021-11
Sales | Alloying Metals |
Derivative [Line Items] |
Derivative non-monetary notional amount | 4.3
Sales | Alloying Metals | Minimum |
Derivative [Line Items] |
Derivative maturity period | 2021-01
Sales | Alloying Metals | Maximum |
Derivative [Line Items] |
Derivative maturity period | 2021-12

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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 + Details
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