Derivatives, Hedging Programs and Other Financial Instruments - Summary of Derivative Positions (Details) mmlb in Millions	6 Months Ended
	Jun. 30, 2021 mmlb MMBTU MWh
Purchase | Aluminum
Derivative [Line Items]
Derivative non-monetary notional amount	107.2
Purchase | Aluminum | Minimum
Derivative [Line Items]
Derivative maturity period	2021-07
Purchase | Aluminum | Maximum
Derivative [Line Items]
Derivative maturity period	2023-12
Purchase | Midwest premium swap contracts
Derivative [Line Items]
Derivative non-monetary notional amount	80.0
Purchase | Midwest premium swap contracts | Minimum
Derivative [Line Items]
Derivative maturity period	2021-07
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	10.9
Purchase | Alloying Metals | Minimum
Derivative [Line Items]
Derivative maturity period	2021-07
Purchase | Alloying Metals | Maximum
Derivative [Line Items]
Derivative maturity period	2022-12
Purchase | Natural Gas
Derivative [Line Items]
Derivative non-monetary notional amount | MMBTU	5,630,000
Purchase | Natural Gas | Minimum
Derivative [Line Items]
Derivative maturity period	2021-07
Purchase | Natural Gas | Maximum
Derivative [Line Items]
Derivative maturity period	2024-12
Purchase | Electricity
Derivative [Line Items]
Derivative non-monetary notional amount | MWh	351,510
Purchase | Electricity | Minimum
Derivative [Line Items]
Derivative maturity period	2021-07
Purchase | Electricity | Maximum
Derivative [Line Items]
Derivative maturity period	2022-12
Sales | Aluminum
Derivative [Line Items]
Derivative non-monetary notional amount	1.0
Sales | Aluminum | Minimum
Derivative [Line Items]
Derivative maturity period	2021-07
Sales | Aluminum | Maximum
Derivative [Line Items]
Derivative maturity period	2021-12
Sales | Alloying Metals
Derivative [Line Items]
Derivative non-monetary notional amount	2.5
Sales | Alloying Metals | Minimum
Derivative [Line Items]
Derivative maturity period	2021-07
Sales | Alloying Metals | Maximum
Derivative [Line Items]
Derivative maturity period	2021-12