Derivatives, Hedging Programs and Other Financial Instruments - Summary of Derivative Positions (Details) - Designated as Hedging Instrument mmlb in Millions	6 Months Ended
	Jun. 30, 2020 mmlb MMBTU MWh euro
Purchase | Aluminum
Derivative [Line Items]
Derivative non-monetary notional amount	92.4
Purchase | Aluminum | Minimum
Derivative [Line Items]
Derivative maturity period	2020-07
Purchase | Aluminum | Maximum
Derivative [Line Items]
Derivative maturity period	2022-12
Purchase | Midwest premium swap contracts
Derivative [Line Items]
Derivative non-monetary notional amount	91.5
Purchase | Midwest premium swap contracts | Minimum
Derivative [Line Items]
Derivative maturity period	2020-07
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	14.5
Purchase | Alloying Metals | Minimum
Derivative [Line Items]
Derivative maturity period	2020-07
Purchase | Alloying Metals | Maximum
Derivative [Line Items]
Derivative maturity period	2021-12
Purchase | Natural Gas
Derivative [Line Items]
Derivative non-monetary notional amount | MMBTU	7,490,000
Purchase | Natural Gas | Minimum
Derivative [Line Items]
Derivative maturity period	2020-07
Purchase | Natural Gas | Maximum
Derivative [Line Items]
Derivative maturity period	2024-12
Purchase | Foreign Exchange Contract
Derivative [Line Items]
Derivative non-monetary notional amount | euro	711,324
Purchase | Foreign Exchange Contract | Minimum
Derivative [Line Items]
Derivative maturity period	2020-09
Purchase | Foreign Exchange Contract | Maximum
Derivative [Line Items]
Derivative maturity period	2021-01
Purchase | Electricity
Derivative [Line Items]
Derivative non-monetary notional amount | MWh	570,140
Purchase | Electricity | Minimum
Derivative [Line Items]
Derivative maturity period	2020-07
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	2020-07
Sales | Aluminum | Maximum
Derivative [Line Items]
Derivative maturity period	2021-11
Sales | Alloying Metals
Derivative [Line Items]
Derivative non-monetary notional amount	5.8
Sales | Alloying Metals | Minimum
Derivative [Line Items]
Derivative maturity period	2020-07
Sales | Alloying Metals | Maximum
Derivative [Line Items]
Derivative maturity period	2021-12