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Derivatives, Hedging Programs and Other Financial Instruments - Summary of Derivative Positions (Details) - 3 months ended Mar. 31, 2024 Mmlb in Millions	EUR (€) Mmlb MWh MMBTU	GBP (£) Mmlb MWh MMBTU
Purchase \| Aluminum
Derivative [Line Items]
Derivative non-monetary notional amount	74.5	74.5
Purchase \| Aluminum \| Minimum
Derivative [Line Items]
Derivative maturity period	Apr. 30, 2024
Purchase \| Aluminum \| Maximum
Derivative [Line Items]
Derivative maturity period	Dec. 31, 2025
Purchase \| Fixed Price Purchase Contracts for MWTP
Derivative [Line Items]
Derivative non-monetary notional amount	60.5	60.5
Purchase \| Fixed Price Purchase Contracts for MWTP \| Minimum
Derivative [Line Items]
Derivative maturity period	Apr. 30, 2024
Purchase \| Fixed Price Purchase Contracts for MWTP \| Maximum
Derivative [Line Items]
Derivative maturity period	Dec. 31, 2025
Purchase \| Alloying Metals
Derivative [Line Items]
Derivative non-monetary notional amount	10.0	10.0
Purchase \| Alloying Metals \| Minimum
Derivative [Line Items]
Derivative maturity period	Apr. 30, 2024
Purchase \| Alloying Metals \| Maximum
Derivative [Line Items]
Derivative maturity period	Dec. 31, 2025
Purchase \| Natural Gas
Derivative [Line Items]
Derivative non-monetary notional amount \| MMBTU	3,880,000	3,880,000
Purchase \| Natural Gas \| Minimum
Derivative [Line Items]
Derivative maturity period	Apr. 30, 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	39,606	39,606
Purchase \| Electricity \| Minimum
Derivative [Line Items]
Derivative maturity period	Apr. 30, 2024
Purchase \| Electricity \| Maximum
Derivative [Line Items]
Derivative maturity period	Dec. 31, 2024
Purchase \| Euro
Derivative [Line Items]
Derivative notional amount \| €	€ 17,485,138
Purchase \| Euro \| Minimum
Derivative [Line Items]
Derivative maturity period	Apr. 30, 2024
Purchase \| Euro \| Maximum
Derivative [Line Items]
Derivative maturity period	Jan. 31, 2026
Purchase \| British Pounds
Derivative [Line Items]
Derivative notional amount \| £		£ 216,799
Purchase \| British Pounds \| Minimum
Derivative [Line Items]
Derivative maturity period	May 31, 2024
Purchase \| British Pounds \| Maximum
Derivative [Line Items]
Derivative maturity period	Jul. 31, 2024
Sales \| Aluminum
Derivative [Line Items]
Derivative non-monetary notional amount	7.0	7.0
Derivative maturity period	Apr. 30, 2024
Sales \| Fixed Price Sale Contracts for MWTP
Derivative [Line Items]
Derivative non-monetary notional amount	26.7	26.7
Sales \| Fixed Price Sale Contracts for MWTP \| Minimum
Derivative [Line Items]
Derivative maturity period	Apr. 30, 2024
Sales \| Fixed Price Sale Contracts for MWTP \| Maximum
Derivative [Line Items]
Derivative maturity period	Sep. 30, 2024