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Subsequent Events - Narrative (Detail) - Subsequent Event $ in Millions |  | 3 Months Ended
--- | --- | ---
Apr. 13, 2018 employee | Jun. 30, 2018 USD ($)
Subsequent Event [Line Items] |  |
Labor agreement, duration | 5 years |
Labor agreement, number of employees | 1,300 |
Defined benefit plan, labor agreement, employees subject to retirement benefit increase | 300 |
Curtailments | $ |  | $ 10

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- Definition Defined Benefit Plan, Labor Agreement, Employees Subject To Retirement Benefit Increase + References No definition available. + Details
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Namespace Prefix: | arnc_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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- Definition Labor Agreement, Duration + References No definition available. + Details
Name: | arnc_LaborAgreementDuration
Namespace Prefix: | arnc_
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Balance Type: | na
Period Type: | duration

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- Definition Labor Agreement, Number Of Employees + References No definition available. + Details
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Namespace Prefix: | arnc_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount of gain (loss) recognized in net periodic benefit (cost) credit from event reducing expected years of future service of present employees or eliminating accrual of defined benefits for some or all future services of present employees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(7) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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Period Type: | duration

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- Definition Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |