Actual remediation fieldwork period.

No definition available.

Name:	arnc_ActualRemediationFieldworkPeriod
Namespace Prefix:	arnc_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Income tax examination assessment outstanding.

+ References

No definition available.

+ Details

Name:	arnc_IncomeTaxExaminationAssessmentOutstanding
Namespace Prefix:	arnc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Number of cleanup locations.

+ References

No definition available.

+ Details

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

- Definition

Percentage of cost of goods sold.

+ References

No definition available.

+ Details

Name:	arnc_PercentageOfCostOfGoodsSold
Namespace Prefix:	arnc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Tax Agreement, Indemnification of Ultimate Liability, Percent

+ References

No definition available.

+ Details

Name:	arnc_TaxAgreementIndemnificationofUltimateLiabilityPercent
Namespace Prefix:	arnc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Total costs accrued as of the balance sheet date for environmental loss contingencies.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82911808&loc=d3e14326-108349

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 25
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6395460&loc=d3e13647-108346

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 30
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13207-110859

+ Details

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

- Definition

Amount of cash outflow to settle environmental loss contingencies.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section S99
-Paragraph 1
-Subparagraph (SAB TOPIC 5.Y.Q1)
-URI http://asc.fasb.org/extlink&oid=27011672&loc=d3e149879-122751

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 17
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3367-108585

+ Details

Name:	us-gaap_AccrualForEnvironmentalLossContingenciesPayments1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Carrying value of the obligation (known or estimated) arising from requirements to perform activities to remediate one or more sites, payable in twelve months or in the next operating cycle if longer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.20)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 30
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13207-110859

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 30
-Section 50
-Paragraph 10
-Subparagraph (d)(1)
-URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13237-110859

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82911808&loc=d3e14326-108349

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section 45
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=82887183&loc=d3e6911-107765

+ Details

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

- Definition

Carrying amount as of the balance sheet date of income taxes previously overpaid to tax authorities (such as U.S. Federal, state and local tax authorities) representing refunds of overpayments or recoveries based on agreed-upon resolutions of disputes. Also called income tax refund receivable.

+ References

+ Details

Name:	us-gaap_IncomeTaxReceivable
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The change in the reserve for commissions, expenses and taxes recorded on the balance sheet, which is needed to adjust net income to arrive at net cash flows provided by or used in operations.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585

+ Details

Name:	us-gaap_IncreaseDecreaseInReserveForCommissionsExpenseAndTaxes
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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

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

- Details

Name:	srt_StatementGeographicalAxis=arnc_MassenaNyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=country_ES
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=arnc_AlcoaCorporationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Contingencies and Commitments - Contingencies (Details) € in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) location	Dec. 31, 2018 EUR (€) location	Dec. 31, 2017 USD ($)	Dec. 31, 2018 EUR (€)
Loss Contingencies [Line Items]
Number of cleanup locations (more than) \| location	100	100
Remediation reserve balance	$ 266		$ 294
Remediation reserve balance, classified as a current liability	81		41
Payments related to remediation expenses applied against the reserve	$ 32		26
Tax agreement, indemnification of ultimate liability, percent	49.00%			49.00%
Total combined assessments	$ 174			€ 152
Income tax reserve	60	€ 52
Massena West, NY
Loss Contingencies [Line Items]
Remediation reserve balance	$ 198		$ 215
Actual remediation fieldwork period	4 years	4 years
Spain
Loss Contingencies [Line Items]
Income taxes receivable	$ 29			€ 25
Recurring Costs of Managing Hazardous Substances and Environmental Programs \| Maximum
Loss Contingencies [Line Items]
Percentage of cost of goods sold	1.00%	1.00%
Alcoa Corporation
Loss Contingencies [Line Items]
Tax agreement, indemnification of ultimate liability, percent	49.00%			49.00%