Derivatives, Hedging Programs and Other Financial Instruments - Summary of Derivative Positions (Details) mmlb in Millions	9 Months Ended
	Sep. 30, 2021 EUR (€) mmlb MMBTU MWh
Purchase | Aluminum
Derivative [Line Items]
Derivative non-monetary notional amount	97.8
Purchase | Aluminum | Minimum
Derivative [Line Items]
Derivative maturity period	2021-10
Purchase | Aluminum | Maximum
Derivative [Line Items]
Derivative maturity period	2023-12
Purchase | Midwest premium swap contracts
Derivative [Line Items]
Derivative non-monetary notional amount	81.1
Purchase | Midwest premium swap contracts | Minimum
Derivative [Line Items]
Derivative maturity period	2021-10
Purchase | Midwest premium swap contracts | Maximum
Derivative [Line Items]
Derivative maturity period	2023-12
Purchase | Alloying Metals
Derivative [Line Items]
Derivative non-monetary notional amount	9.4
Purchase | Alloying Metals | Minimum
Derivative [Line Items]
Derivative maturity period	2021-10
Purchase | Alloying Metals | Maximum
Derivative [Line Items]
Derivative maturity period	2022-12
Purchase | Natural Gas
Derivative [Line Items]
Derivative non-monetary notional amount | MMBTU	5,220,000
Purchase | Natural Gas | Minimum
Derivative [Line Items]
Derivative maturity period	2021-10
Purchase | Natural Gas | Maximum
Derivative [Line Items]
Derivative maturity period	2024-12
Purchase | Foreign Exchange Contract
Derivative [Line Items]
Derivative notional amount | €	€ 208,080
Purchase | Foreign Exchange Contract | Minimum
Derivative [Line Items]
Derivative maturity period	2021-12
Purchase | Foreign Exchange Contract | Maximum
Derivative [Line Items]
Derivative maturity period	2023-03
Purchase | Electricity
Derivative [Line Items]
Derivative non-monetary notional amount | MWh	285,270
Purchase | Electricity | Minimum
Derivative [Line Items]
Derivative maturity period	2021-10
Purchase | Electricity | Maximum
Derivative [Line Items]
Derivative maturity period	2022-12
Sales | Aluminum
Derivative [Line Items]
Derivative non-monetary notional amount	0.8
Sales | Aluminum | Minimum
Derivative [Line Items]
Derivative maturity period	2021-10
Sales | Aluminum | Maximum
Derivative [Line Items]
Derivative maturity period	2021-12
Sales | Alloying Metals
Derivative [Line Items]
Derivative non-monetary notional amount	1.8
Sales | Alloying Metals | Minimum
Derivative [Line Items]
Derivative maturity period	2021-10
Sales | Alloying Metals | Maximum
Derivative [Line Items]
Derivative maturity period	2021-12