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Debt - Short-Term Borrowings (Details) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019
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Debt Disclosure [Abstract] |  |
Short-term debt | $ 14 | $ 31

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- Definition Short-Term Debt, Excluding Current Maturities Of Long-Term Debt + References No definition available. + Details
Name: | arnc_ShortTermDebtExcludingCurrentMaturitiesOfLongtermDebt
Namespace Prefix: | arnc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- References No definition available. + Details
Name: | us-gaap_DebtDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration