Derivatives, Hedging Programs and Other Financial Instruments - Summary of Derivative Positions (Details) Mmlb in Millions	3 Months Ended
	Mar. 31, 2026 EUR (€) Mmlb MWh MMBTU
Purchase \| Aluminum
Derivative [Line Items]
Derivative non-monetary notional amount	72.5
Purchase \| Aluminum \| Minimum
Derivative [Line Items]
Derivative maturity period	Apr. 30, 2026
Purchase \| Aluminum \| Maximum
Derivative [Line Items]
Derivative maturity period	Jul. 31, 2027
Purchase \| Fixed Price Purchase Contracts for MWTP
Derivative [Line Items]
Derivative non-monetary notional amount	72.6
Purchase \| Fixed Price Purchase Contracts for MWTP \| Minimum
Derivative [Line Items]
Derivative maturity period	Apr. 30, 2026
Purchase \| Fixed Price Purchase Contracts for MWTP \| Maximum
Derivative [Line Items]
Derivative maturity period	Jul. 31, 2027
Purchase \| Alloying Metals
Derivative [Line Items]
Derivative non-monetary notional amount	6.2
Purchase \| Alloying Metals \| Minimum
Derivative [Line Items]
Derivative maturity period	Apr. 30, 2026
Purchase \| Alloying Metals \| Maximum
Derivative [Line Items]
Derivative maturity period	Dec. 31, 2027
Purchase \| Natural Gas
Derivative [Line Items]
Derivative non-monetary notional amount \| MMBTU	3,450,000
Purchase \| Natural Gas \| Minimum
Derivative [Line Items]
Derivative maturity period	Apr. 30, 2026
Purchase \| Natural Gas \| Maximum
Derivative [Line Items]
Derivative maturity period	Dec. 31, 2028
Purchase \| Electricity
Derivative [Line Items]
Derivative non-monetary notional amount \| MWh	316,228
Purchase \| Electricity \| Minimum
Derivative [Line Items]
Derivative maturity period	Apr. 30, 2026
Purchase \| Electricity \| Maximum
Derivative [Line Items]
Derivative maturity period	Dec. 31, 2027
Purchase \| Euro
Derivative [Line Items]
Derivative notional amount \| €	€ 600,000
Purchase \| Euro \| Minimum
Derivative [Line Items]
Derivative maturity period	Apr. 30, 2026
Purchase \| Euro \| Maximum
Derivative [Line Items]
Derivative maturity period	Jul. 31, 2027
Sales \| Aluminum
Derivative [Line Items]
Derivative non-monetary notional amount	10.1
Sales \| Aluminum \| Minimum
Derivative [Line Items]
Derivative maturity period	Apr. 30, 2026
Sales \| Aluminum \| Maximum
Derivative [Line Items]
Derivative maturity period	Apr. 30, 2027
Sales \| Fixed Price Sale Contracts for MWTP
Derivative [Line Items]
Derivative non-monetary notional amount	10.2
Sales \| Fixed Price Sale Contracts for MWTP \| Minimum
Derivative [Line Items]
Derivative maturity period	Apr. 30, 2026
Sales \| Fixed Price Sale Contracts for MWTP \| Maximum
Derivative [Line Items]
Derivative maturity period	Apr. 30, 2027