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Related Party Transactions - Additional Information (Detail)	Nov. 01, 2016
Alcoa Corporation [Member] | Toll Processing and Services Agreement [Member]
Related Party Transaction [Line Items]
Tolling arrangement expiry date	Dec. 31, 2018

X

- Definition

Date of maturity or expiration of arrangements with a related party (for example, but not limited to, leasing and debt arrangements between related parties), in CCYY-MM-DD format.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
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X

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=arnc_AlcoaCorporationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_RelatedPartyTransactionAxis=arnc_TollProcessingAndServicesAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: