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Derivatives, Hedging Programs and Other Financial Instruments - Summary of Derivative Positions (Details) - 9 months ended Sep. 30, 2024 Mmlb in Millions	EUR (€) MWh MMBTU Mmlb	GBP (£) MWh MMBTU Mmlb
Purchase \| Aluminum
Derivative [Line Items]
Derivative non-monetary notional amount	61.2	61.2
Purchase \| Aluminum \| Minimum
Derivative [Line Items]
Derivative maturity period	Oct. 31, 2024
Purchase \| Aluminum \| Maximum
Derivative [Line Items]
Derivative maturity period	Feb. 28, 2026
Purchase \| Fixed Price Purchase Contracts for MWTP
Derivative [Line Items]
Derivative non-monetary notional amount	50.2	50.2
Purchase \| Fixed Price Purchase Contracts for MWTP \| Minimum
Derivative [Line Items]
Derivative maturity period	Oct. 31, 2024
Purchase \| Fixed Price Purchase Contracts for MWTP \| Maximum
Derivative [Line Items]
Derivative maturity period	Feb. 28, 2026
Purchase \| Alloying Metals
Derivative [Line Items]
Derivative non-monetary notional amount	9.7	9.7
Purchase \| Alloying Metals \| Minimum
Derivative [Line Items]
Derivative maturity period	Oct. 31, 2024
Purchase \| Alloying Metals \| Maximum
Derivative [Line Items]
Derivative maturity period	Dec. 31, 2026
Purchase \| Natural Gas
Derivative [Line Items]
Derivative non-monetary notional amount \| MMBTU	3,080,000	3,080,000
Purchase \| Natural Gas \| Minimum
Derivative [Line Items]
Derivative maturity period	Oct. 31, 2024
Purchase \| Natural Gas \| Maximum
Derivative [Line Items]
Derivative maturity period	Dec. 31, 2026
Purchase \| Electricity
Derivative [Line Items]
Derivative non-monetary notional amount \| MWh	55,225	55,225
Purchase \| Electricity \| Minimum
Derivative [Line Items]
Derivative maturity period	Oct. 31, 2024
Purchase \| Electricity \| Maximum
Derivative [Line Items]
Derivative maturity period	Dec. 31, 2024
Purchase \| Euro
Derivative [Line Items]
Derivative notional amount \| €	€ 10,482,824
Purchase \| Euro \| Minimum
Derivative [Line Items]
Derivative maturity period	Oct. 31, 2024
Purchase \| Euro \| Maximum
Derivative [Line Items]
Derivative maturity period	Jul. 31, 2027
Purchase \| British Pounds
Derivative [Line Items]
Derivative maturity period	Oct. 31, 2024
Derivative notional amount \| £		£ 72,266
Sales \| Aluminum
Derivative [Line Items]
Derivative non-monetary notional amount	9.4	9.4
Derivative maturity period	Oct. 31, 2024
Sales \| Fixed Price Sale Contracts for MWTP
Derivative [Line Items]
Derivative non-monetary notional amount	9.3	9.3
Derivative maturity period	Oct. 31, 2024
Sales \| Electricity
Derivative [Line Items]
Derivative non-monetary notional amount \| MWh	28,717	28,717
Sales \| Electricity \| Minimum
Derivative [Line Items]
Derivative maturity period	Oct. 31, 2024
Sales \| Electricity \| Maximum
Derivative [Line Items]
Derivative maturity period	Dec. 31, 2024