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Curtailment of Operations - Hawesville (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2023 | Sep. 30, 2022 | Sep. 30, 2023 | Sep. 30, 2022
Restructuring Cost and Reserve [Line Items] |  |  |  |
OPEB curtailment gain, net | $ 0.0 | $ (8.0) | $ 0.0 | $ (8.0)
Hawesville |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |
OPEB curtailment gain, net |  | 8.0 |  | 8.0
Curtailment Of Operations |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |
Severance costs | 2.0 |  | 13.4 |
Gain (loss) on material sales | 0.0 |  | 1.2 |
Curtailment Of Operations | Temporary Facility Closing, Excess Capacity |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |
Excess capacity charges | $ 0.0 |  | $ 9.0 |
WARN Notice |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |
Severance costs |  | $ 4.5 |  | $ 12.6

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