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Derivatives, Hedging Programs and Other Financial Instruments - Summary of Derivative Positions (Details) - 3 months ended Mar. 31, 2025 Mmlb in Millions	EUR (€) Mmlb MMBTU	GBP (£) Mmlb MMBTU
Purchase \| Aluminum
Derivative [Line Items]
Derivative non-monetary notional amount	61.0	61.0
Purchase \| Aluminum \| Minimum
Derivative [Line Items]
Derivative maturity period	Apr. 30, 2025
Purchase \| Aluminum \| Maximum
Derivative [Line Items]
Derivative maturity period	Aug. 31, 2026
Purchase \| Fixed Price Purchase Contracts for MWTP
Derivative [Line Items]
Derivative non-monetary notional amount	56.5	56.5
Purchase \| Fixed Price Purchase Contracts for MWTP \| Minimum
Derivative [Line Items]
Derivative maturity period	Apr. 30, 2025
Purchase \| Fixed Price Purchase Contracts for MWTP \| Maximum
Derivative [Line Items]
Derivative maturity period	Aug. 31, 2026
Purchase \| Alloying Metals
Derivative [Line Items]
Derivative non-monetary notional amount	7.4	7.4
Purchase \| Alloying Metals \| Minimum
Derivative [Line Items]
Derivative maturity period	Apr. 30, 2025
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	2,700,000	2,700,000
Purchase \| Natural Gas \| Minimum
Derivative [Line Items]
Derivative maturity period	Apr. 30, 2025
Purchase \| Natural Gas \| Maximum
Derivative [Line Items]
Derivative maturity period	Dec. 31, 2027
Purchase \| Euro
Derivative [Line Items]
Derivative notional amount \| €	€ 5,812,634
Purchase \| Euro \| Minimum
Derivative [Line Items]
Derivative maturity period	Apr. 30, 2025
Purchase \| Euro \| Maximum
Derivative [Line Items]
Derivative maturity period	Jul. 31, 2027
Purchase \| British Pounds
Derivative [Line Items]
Derivative notional amount \| £		£ 20,000
Purchase \| British Pounds \| Minimum
Derivative [Line Items]
Derivative maturity period	Apr. 30, 2025
Purchase \| British Pounds \| Maximum
Derivative [Line Items]
Derivative maturity period	May 31, 2025
Sales \| Aluminum
Derivative [Line Items]
Derivative non-monetary notional amount	11.0	11.0
Sales \| Aluminum \| Minimum
Derivative [Line Items]
Derivative maturity period	Apr. 30, 2025
Sales \| Aluminum \| Maximum
Derivative [Line Items]
Derivative maturity period	Dec. 31, 2025
Sales \| Fixed Price Sale Contracts for MWTP
Derivative [Line Items]
Derivative non-monetary notional amount	10.9	10.9
Derivative maturity period	Apr. 30, 2025