Derivatives, Hedging Programs and Other Financial Instruments - Summary of Derivative Positions (Details) mmlb in Millions	12 Months Ended
	Dec. 31, 2020 mmlb MMBTU MWh
Purchase | Aluminum
Derivative [Line Items]
Derivative non-monetary notional amount	67.4
Purchase | Aluminum | Minimum
Derivative [Line Items]
Derivative maturity period	2021-01
Purchase | Aluminum | Maximum
Derivative [Line Items]
Derivative maturity period	2022-12
Purchase | Midwest premium swap contracts
Derivative [Line Items]
Derivative non-monetary notional amount	66.4
Purchase | Midwest premium swap contracts | Minimum
Derivative [Line Items]
Derivative maturity period	2021-01
Purchase | Midwest premium swap contracts | Maximum
Derivative [Line Items]
Derivative maturity period	2022-12
Purchase | Alloying Metals
Derivative [Line Items]
Derivative non-monetary notional amount	10.2
Purchase | Alloying Metals | Minimum
Derivative [Line Items]
Derivative maturity period	2021-01
Purchase | Alloying Metals | Maximum
Derivative [Line Items]
Derivative maturity period	2021-12
Purchase | Natural Gas
Derivative [Line Items]
Derivative non-monetary notional amount | MMBTU	6,630,000
Purchase | Natural Gas | Minimum
Derivative [Line Items]
Derivative maturity period	2021-01
Purchase | Natural Gas | Maximum
Derivative [Line Items]
Derivative maturity period	2024-12
Purchase | Electricity
Derivative [Line Items]
Derivative non-monetary notional amount | MWh	481,800
Purchase | Electricity | Minimum
Derivative [Line Items]
Derivative maturity period	2021-01
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	2021-01
Sales | Aluminum | Maximum
Derivative [Line Items]
Derivative maturity period	2021-11
Sales | Alloying Metals
Derivative [Line Items]
Derivative non-monetary notional amount	4.3
Sales | Alloying Metals | Minimum
Derivative [Line Items]
Derivative maturity period	2021-01
Sales | Alloying Metals | Maximum
Derivative [Line Items]
Derivative maturity period	2021-12