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Derivatives, Hedging Programs and Other Financial Instruments - Summary of Derivative Positions (Details) - 6 months ended Jun. 30, 2024 Mmlb in Millions	EUR (€) MWh Mmlb MMBTU	GBP (£) MWh Mmlb MMBTU
Purchase \| Aluminum
Derivative [Line Items]
Derivative non-monetary notional amount	62.4	62.4
Purchase \| Aluminum \| Minimum
Derivative [Line Items]
Derivative maturity period	Jul. 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	48.9	48.9
Purchase \| Fixed Price Purchase Contracts for MWTP \| Minimum
Derivative [Line Items]
Derivative maturity period	Jul. 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	10.0	10.0
Purchase \| Alloying Metals \| Minimum
Derivative [Line Items]
Derivative maturity period	Jul. 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	3,240,000	3,240,000
Purchase \| Natural Gas \| Minimum
Derivative [Line Items]
Derivative maturity period	Jul. 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	110,425	110,425
Purchase \| Electricity \| Minimum
Derivative [Line Items]
Derivative maturity period	Jul. 31, 2024
Purchase \| Electricity \| Maximum
Derivative [Line Items]
Derivative maturity period	Dec. 31, 2024
Purchase \| Euro
Derivative [Line Items]
Derivative notional amount \| €	€ 10,856,964
Purchase \| Euro \| Minimum
Derivative [Line Items]
Derivative maturity period	Aug. 31, 2024
Purchase \| Euro \| Maximum
Derivative [Line Items]
Derivative maturity period	Jan. 31, 2026
Purchase \| British Pounds
Derivative [Line Items]
Derivative maturity period	Jul. 31, 2024
Derivative notional amount \| £		£ 72,266
Sales \| Fixed Price Sale Contracts for MWTP
Derivative [Line Items]
Derivative non-monetary notional amount	15.5	15.5
Sales \| Fixed Price Sale Contracts for MWTP \| Minimum
Derivative [Line Items]
Derivative maturity period	Jul. 31, 2024
Sales \| Fixed Price Sale Contracts for MWTP \| Maximum
Derivative [Line Items]
Derivative maturity period	Sep. 30, 2024