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Curtailment of Operations - Hawesville (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Restructuring Cost and Reserve [Line Items] |  |  |
Asset impairment | $ 0.0 | $ 159.4 | $ 0.0
OPEB curtailment gain, net | 0.0 | (8.9) | $ 0.0
Curtailment Of Operations |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |
Severance costs | 16.6 |  |
Gain (loss) on material sales | 1.7 |  |
Curtailment Of Operations | Temporary Facility Closing, Excess Capacity |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |
Excess capacity charges | $ 9.0 |  |
Worker Adjustment and Retraining Notification Act |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |
Severance costs |  | 18.1 |
Hawesville |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |
Asset impairment |  | 159.4 |
OPEB curtailment gain, net |  | $ 8.9 |

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