Derivatives, Hedging Programs and Other Financial Instruments - Summary of Derivative Positions (Details) mmlb in Millions	3 Months Ended
	Mar. 31, 2021 mmlb MMBTU MWh
Purchase | Aluminum
Derivative [Line Items]
Derivative non-monetary notional amount	113.5
Purchase | Aluminum | Minimum
Derivative [Line Items]
Derivative maturity period	2021-04
Purchase | Aluminum | Maximum
Derivative [Line Items]
Derivative maturity period	2023-12
Purchase | Midwest premium swap contracts
Derivative [Line Items]
Derivative non-monetary notional amount	69.1
Purchase | Midwest premium swap contracts | Minimum
Derivative [Line Items]
Derivative maturity period	2021-04
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	9.1
Purchase | Alloying Metals | Minimum
Derivative [Line Items]
Derivative maturity period	2021-04
Purchase | Alloying Metals | Maximum
Derivative [Line Items]
Derivative maturity period	2022-12
Purchase | Natural Gas
Derivative [Line Items]
Derivative non-monetary notional amount | MMBTU	6,060,000
Purchase | Natural Gas | Minimum
Derivative [Line Items]
Derivative maturity period	2021-04
Purchase | Natural Gas | Maximum
Derivative [Line Items]
Derivative maturity period	2024-12
Purchase | Electricity
Derivative [Line Items]
Derivative non-monetary notional amount | MWh	417,030
Purchase | Electricity | Minimum
Derivative [Line Items]
Derivative maturity period	2021-04
Purchase | Electricity | Maximum
Derivative [Line Items]
Derivative maturity period	2022-12
Sales | Aluminum
Derivative [Line Items]
Derivative non-monetary notional amount	0.9
Sales | Aluminum | Minimum
Derivative [Line Items]
Derivative maturity period	2021-04
Sales | Aluminum | Maximum
Derivative [Line Items]
Derivative maturity period	2021-11
Sales | Alloying Metals
Derivative [Line Items]
Derivative non-monetary notional amount	3.2
Sales | Alloying Metals | Minimum
Derivative [Line Items]
Derivative maturity period	2021-04
Sales | Alloying Metals | Maximum
Derivative [Line Items]
Derivative maturity period	2021-12