Derivatives, Hedging Programs and Other Financial Instruments - Summary of Derivative Positions (Details) Mmlb in Millions	6 Months Ended
Jun. 30, 2025 EUR (€) Mmlb MMBTU
Purchase \| Aluminum
Derivative [Line Items]
Derivative non-monetary notional amount	61.7
Purchase \| Aluminum \| Minimum
Derivative [Line Items]
Derivative maturity period	Jul. 31, 2025
Purchase \| Aluminum \| Maximum
Derivative [Line Items]
Derivative maturity period	Nov. 30, 2026
Purchase \| Fixed Price Purchase Contracts for MWTP
Derivative [Line Items]
Derivative non-monetary notional amount	61.0
Purchase \| Fixed Price Purchase Contracts for MWTP \| Minimum
Derivative [Line Items]
Derivative maturity period	Jul. 31, 2025
Purchase \| Fixed Price Purchase Contracts for MWTP \| Maximum
Derivative [Line Items]
Derivative maturity period	Nov. 30, 2026
Purchase \| Alloying Metals
Derivative [Line Items]
Derivative non-monetary notional amount	5.8
Purchase \| Alloying Metals \| Minimum
Derivative [Line Items]
Derivative maturity period	Jul. 31, 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	3,360,000
Purchase \| Natural Gas \| Minimum
Derivative [Line Items]
Derivative maturity period	Jul. 31, 2025
Purchase \| Natural Gas \| Maximum
Derivative [Line Items]
Derivative maturity period	Dec. 31, 2027
Purchase \| Euro
Derivative [Line Items]
Derivative notional amount \| €	€ 3,200,000
Purchase \| Euro \| Minimum
Derivative [Line Items]
Derivative maturity period	Jul. 31, 2025
Purchase \| Euro \| Maximum
Derivative [Line Items]
Derivative maturity period	Jul. 31, 2027
Sales \| Aluminum
Derivative [Line Items]
Derivative non-monetary notional amount	17.9
Sales \| Aluminum \| Minimum
Derivative [Line Items]
Derivative maturity period	Jul. 31, 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	18.1
Sales \| Fixed Price Sale Contracts for MWTP \| Minimum
Derivative [Line Items]
Derivative maturity period	Jul. 31, 2025
Sales \| Fixed Price Sale Contracts for MWTP \| Maximum
Derivative [Line Items]
Derivative maturity period	Apr. 30, 2026

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