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Debt - Short-Term Borrowings (Details) - USD ($) $ in Millions | Dec. 31, 2018 | Dec. 31, 2017
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Debt Disclosure [Abstract] |  |
Short-term debt | $ 29 | $ 38
Accounts payable settlement arrangements | $ 29 | $ 33

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- Definition Accounts payable settlement arrangements. + References No definition available. + Details
Name: | arnc_AccountsPayableSettlementArrangements
Namespace Prefix: | arnc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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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