Derivatives, Hedging Programs and Other Financial Instruments - Summary of Derivative Positions (Details) Mmlb in Millions	9 Months Ended
	Sep. 30, 2023 GBP (£) MMBTU Mmlb EUR (€) MWh
Purchase | Euro
Derivative [Line Items]
Derivative non-monetary notional amount | €	3,047,291
Purchase | British Pounds
Derivative [Line Items]
Derivative non-monetary notional amount | £	393,693
Purchase | Minimum | Euro
Derivative [Line Items]
Derivative maturity period	Oct. 31, 2023
Purchase | Minimum | British Pounds
Derivative [Line Items]
Derivative maturity period	Nov. 30, 2023
Purchase | Maximum | Euro
Derivative [Line Items]
Derivative maturity period	Jun. 30, 2024
Purchase | Maximum | British Pounds
Derivative [Line Items]
Derivative maturity period	May 31, 2024
Purchase | Natural Gas
Derivative [Line Items]
Derivative non-monetary notional amount | MMBTU	3,130,000
Purchase | Natural Gas | Minimum
Derivative [Line Items]
Derivative maturity period	Oct. 31, 2023
Purchase | Natural Gas | Maximum
Derivative [Line Items]
Derivative maturity period	Dec. 31, 2026
Purchase | Aluminum
Derivative [Line Items]
Derivative non-monetary notional amount	101.4
Purchase | Aluminum | Minimum
Derivative [Line Items]
Derivative maturity period	Oct. 31, 2023
Purchase | Aluminum | Maximum
Derivative [Line Items]
Derivative maturity period	Dec. 31, 2025
Purchase | Alloying Metals
Derivative [Line Items]
Derivative non-monetary notional amount	3.7
Purchase | Alloying Metals | Minimum
Derivative [Line Items]
Derivative maturity period	Oct. 31, 2023
Purchase | Alloying Metals | Maximum
Derivative [Line Items]
Derivative maturity period	Dec. 31, 2025
Purchase | Fixed Price Purchase Contracts for Midwest Premium
Derivative [Line Items]
Derivative non-monetary notional amount	85.4
Purchase | Fixed Price Purchase Contracts for Midwest Premium | Minimum
Derivative [Line Items]
Derivative maturity period	Oct. 31, 2023
Purchase | Fixed Price Purchase Contracts for Midwest Premium | Maximum
Derivative [Line Items]
Derivative maturity period	Dec. 31, 2025
Purchase | Electricity
Derivative [Line Items]
Derivative non-monetary notional amount | MWh	35,136
Purchase | Electricity | Minimum
Derivative [Line Items]
Derivative maturity period	Jan. 31, 2024
Purchase | Electricity | Maximum
Derivative [Line Items]
Derivative maturity period	Dec. 31, 2024
Sales | Aluminum
Derivative [Line Items]
Derivative non-monetary notional amount	29.7
Sales | Aluminum | Minimum
Derivative [Line Items]
Derivative maturity period	Oct. 31, 2023
Sales | Aluminum | Maximum
Derivative [Line Items]
Derivative maturity period	Apr. 30, 2024
Sales | Fixed Price Sale Contracts for Midwest Premium
Derivative [Line Items]
Derivative non-monetary notional amount	48.9
Sales | Fixed Price Sale Contracts for Midwest Premium | Minimum
Derivative [Line Items]
Derivative maturity period	Oct. 31, 2023
Sales | Fixed Price Sale Contracts for Midwest Premium | Maximum
Derivative [Line Items]
Derivative maturity period	May 31, 2024