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Derivatives (Details) MMBTU in Thousands, € in Millions, $ in Millions | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2020 USD ($) MMBTU MWh t | Dec. 31, 2020 EUR (€) MMBTU MWh t | Dec. 31, 2019 USD ($)
Supply Commitment [Line Items] |  |  |
Derivative asset | $ | $ 15.1 |  | $ 19.7
LME Swap [Member] |  |  |
Supply Commitment [Line Items] |  |  |
Other forward delivery contracts to sell primary aluminum (in tonnes) | 194,116 | 194,116 |
Midwest Premium (MWP) [Member] |  |  |
Supply Commitment [Line Items] |  |  |
Open position to offset fixed prices | 318,000 |  |
Fixed to Floating Swap [Member] |  |  |
Supply Commitment [Line Items] |  |  |
Open position to offset fixed prices | 9,694 |  |
NYMEX Henry Hub Natural Gas Price Swaps [Member] |  |  |
Supply Commitment [Line Items] |  |  |
Underlying, derivative energy, available | MMBTU | 2,355 | 2,355 |
FX Swap [Member] |  |  |
Supply Commitment [Line Items] |  |  |
Derivative asset | € |  | € 40.3 |
Grundartangi [Member] | Nordpool Power Price Swap [Member] |  |  |
Supply Commitment [Line Items] |  |  |
Underlying, derivative energy, available | MWh | 1,830,840 | 1,830,840 |

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