Non-operating income (expense), after tax.

No definition available.

Name:	arnc_NonoperatingIncomeExpenseAfterTax
Namespace Prefix:	arnc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Preferred Stock Conversion Price

+ References

No definition available.

+ Details

Name:	arnc_PreferredStockConversionPrice
Namespace Prefix:	arnc_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Preferred Stock Conversion Rate Per Share

+ References

No definition available.

+ Details

Name:	arnc_PreferredStockConversionRatePerShare
Namespace Prefix:	arnc_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

The number of shares converted in a noncash (or part noncash) transaction. Noncash is defined as transactions during a period that do not result in cash receipts or cash payments in the period. "Part noncash" refers to that portion of the transaction not resulting in cash receipts or cash payments in the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4332-108586

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4304-108586

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4313-108586

+ Details

Name:	us-gaap_ConversionOfStockSharesConverted1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=82913245&loc=d3e12069-110248

+ Details

Name:	us-gaap_GuaranteeObligationsMaximumExposure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The aggregate amount of income or expense from ancillary business-related activities (that is to say, excluding major activities considered part of the normal operations of the business).

+ 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 210.5-03.7)
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

+ Details

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

- Definition

The number of shares issued or sold by the subsidiary or equity method investee per stock transaction.

+ References

No definition available.

+ Details

Name:	us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- 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

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

- Details

Name:	us-gaap_StatementScenarioAxis=us-gaap_ScenarioForecastMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ScheduleOfSharesSubjectToMandatoryRedemptionBySettlementTermsAxis=arnc_MandatoryConvertiblePreferredStockMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LossContingenciesByNatureOfContingencyAxis=arnc_ElectricityContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Subsequent Events - Additional Information (Detail) $ / shares in Units, $ in Millions		1 Months Ended	3 Months Ended				9 Months Ended
	Oct. 02, 2017 $ / shares shares	Feb. 28, 2017 shares	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 shares	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Oct. 13, 2017 USD ($)
Subsequent Event [Line Items]
Depository shares		23,353,000			12,958,767
Non-operating income \| $				$ 1		$ 11	$ 526	$ 40
Scenario, Forecast [Member] \| Alcoa Corporation [Member]
Subsequent Event [Line Items]
Non-operating income \| $			$ 25
Non cash non-operating income, after-tax \| $			$ 16
Subsequent Event [Member]
Subsequent Event [Line Items]
Depository shares	24,022
Preferred stock converted to common stock	31,428
Subsequent Event [Member] \| Mandatory Convertible Preferred Stock [Member]
Subsequent Event [Line Items]
Depository shares	24,975,978
Interest in preferred stock	0.1
Preferred stock conversion price \| $ / shares	$ 1.56996
Preferred stock converted to common stock	39,211,286
Subsequent Event [Member] \| Alcoa Corporation [Member] \| Electricity Contract [Member]
Subsequent Event [Line Items]
Guarantees of third party related to project financing \| $									$ 485