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Derivatives, Hedging Programs and Other Financial Instruments - Summary of Derivative Positions (Details) - 12 months ended Dec. 31, 2023 Mmlb in Millions	EUR (€) Mmlb MMBTU MW	GBP (£) Mmlb MMBTU MW
Purchase \| Aluminum
Derivative [Line Items]
Derivative non-monetary notional amount	69.8	69.8
Purchase \| Aluminum \| Minimum
Derivative [Line Items]
Derivative maturity period	Jan. 31, 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	54.1	54.1
Purchase \| Fixed Price Purchase Contracts for MWTP \| Minimum
Derivative [Line Items]
Derivative maturity period	Jan. 31, 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	8.4	8.4
Purchase \| Alloying Metals \| Minimum
Derivative [Line Items]
Derivative maturity period	Jan. 31, 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	3,390,000	3,390,000
Purchase \| Natural Gas \| Minimum
Derivative [Line Items]
Derivative maturity period	Jan. 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 \| MW	52,704	52,704
Purchase \| Electricity \| Minimum
Derivative [Line Items]
Derivative maturity period	Jan. 31, 2024
Purchase \| Electricity \| Maximum
Derivative [Line Items]
Derivative maturity period	Dec. 31, 2024
Purchase \| Euro
Derivative [Line Items]
Derivative notional amount \| €	€ 16,745,648
Purchase \| Euro \| Minimum
Derivative [Line Items]
Derivative maturity period	Jan. 31, 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	6.3	6.3
Sales \| Aluminum \| Minimum
Derivative [Line Items]
Derivative maturity period	Jan. 31, 2024
Sales \| Aluminum \| Maximum
Derivative [Line Items]
Derivative maturity period	Apr. 30, 2024
Sales \| Fixed Price Sale Contracts for MWTP
Derivative [Line Items]
Derivative non-monetary notional amount	26.6	26.6
Sales \| Fixed Price Sale Contracts for MWTP \| Minimum
Derivative [Line Items]
Derivative maturity period	Jan. 31, 2024
Sales \| Fixed Price Sale Contracts for MWTP \| Maximum
Derivative [Line Items]
Derivative maturity period	May 31, 2024