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Business Combinations, Goodwill and Intangible Assets - Schedule of Goodwill (Details) - USD ($) $ in Millions | Sep. 30, 2021 | Mar. 31, 2021 | Dec. 31, 2020
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Finite Lived Intangible Assets [Line Items] |  |  |
Gross Carrying Value | $ 57.7 |  | $ 37.2
Accumulated Impairment Loss | (18.4) |  | (18.4)
Net Carrying Value | 39.3 |  | $ 18.8
Alcoa Warrick LLC [Member] |  |  |
Finite Lived Intangible Assets [Line Items] |  |  |
Gross Carrying Value | 20.5 |  |
Net Carrying Value | $ 20.5 | $ 27.4 |

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