Cash expense related to divestiture of businesses.

No definition available.

Name:	arnc_CashExpenseRelatedToDivestitureOfBusinesses
Namespace Prefix:	arnc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Guarantee payments to third party.

+ References

No definition available.

+ Details

Name:	arnc_GuaranteePaymentsToThirdParty
Namespace Prefix:	arnc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Number of employees.

+ References

No definition available.

+ Details

Name:	arnc_NumberOfEmployees
Namespace Prefix:	arnc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Proceeds from divestiture of businesses net of transaction costs.

+ References

No definition available.

+ Details

Name:	arnc_ProceedsFromDivestitureOfBusinessesNetOfTransactionCosts
Namespace Prefix:	arnc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- 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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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Gain (loss) from sale or disposal of an organization or integrated set of activities (for example, but not limited to, a partnership or corporation) engaged in providing a product or service in a commercial, industrial, or professional environment.

+ References

No definition available.

+ Details

Name:	us-gaap_GainLossOnSaleOfBusiness
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The cash inflow associated with the amount received from the sale of a portion of the company's business, for example a segment, division, branch or other business, during the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 12
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3179-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Investing Activities
-URI http://asc.fasb.org/extlink&oid=6516133

+ Details

Name:	us-gaap_ProceedsFromDivestitureOfBusinesses
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Aggregate revenue during the period from the sale of goods in the normal course of business, after deducting returns, allowances and discounts.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1(a))
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_IncomeStatementLocationAxis=arnc_RestructuringAndOtherChargesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=arnc_RTIMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Acquisitions and Divestitures - Additional Information (Detail) $ in Millions	1 Months Ended		3 Months Ended
	Mar. 31, 2017 USD ($)	Apr. 30, 2016 USD ($)	Mar. 31, 2017 USD ($) Employees	Mar. 31, 2016 USD ($) Employees
Business Acquisition [Line Items]
Third-party sale generated in last annual period prior to divestiture			$ 3,192	$ 3,055
LISI MEDICAL [Member]
Business Acquisition [Line Items]
Cash received on sale of operations		$ 102
Sale of Remmele Medical business, net of transaction costs		$ 99
Slim Aluminium [Member]
Business Acquisition [Line Items]
Third-party sale generated in last annual period prior to divestiture			$ 37	38
Number of employees \| Employees			312
Cash expense related to sale of rolling mill	$ 10
Third party guarantee with a fair value related to environmental remediation	$ 5
Slim Aluminium [Member] \| Restructuring and Other Charges [Member]
Business Acquisition [Line Items]
Loss on sale of rolling mill			$ 60
RTI [Member]
Business Acquisition [Line Items]
Third-party sale generated in last annual period prior to divestiture				$ 16
Number of employees \| Employees				330