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Business Combinations, Goodwill and Intangible Assets - Schedule of Goodwill (Details) - USD ($) $ in Millions | Mar. 31, 2021 | Dec. 31, 2020
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Finite Lived Intangible Assets [Line Items] |  |
Gross Carrying Value | $ 64.6 | $ 37.2
Accumulated Impairment Loss | (18.4) | (18.4)
Net Carrying Value | 46.2 | $ 18.8
Alcoa Warrick LLC [Member] |  |
Finite Lived Intangible Assets [Line Items] |  |
Gross Carrying Value | 27.4 |
Net Carrying Value | $ 27.4 |

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- Definition Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(10)(1)) -URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=120320667&loc=SL49117168-202975 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=99380562&loc=d3e13770-109266 + Details
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