Business combination aggregate payable cash contingent value.

No definition available.

Name:	aa_BusinessCombinationAggregatePayableCashContingentValue
Namespace Prefix:	aa_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Business combination successive annual periods.

+ References

No definition available.

+ Details

Name:	aa_BusinessCombinationSuccessiveAnnualPeriods
Namespace Prefix:	aa_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Business disposition.

+ References

No definition available.

+ Details

Name:	aa_BusinessDispositionLineItems
Namespace Prefix:	aa_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Foreign currency translation adjustments reclassified to accumulated other comprehensive loss.

+ References

No definition available.

+ Details

Name:	aa_ForeignCurrencyTranslationAdjustmentsReclassifiedToAccumulatedOtherComprehensiveLoss
Namespace Prefix:	aa_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Joint venture consideration amount.

+ References

No definition available.

+ Details

Name:	aa_JointVentureConsiderationAmount
Namespace Prefix:	aa_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Minimum holding period.

+ References

No definition available.

+ Details

Name:	aa_MinimumHoldingPeriod
Namespace Prefix:	aa_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Net assets included in joint venture formation.

+ References

No definition available.

+ Details

Name:	aa_NetAssetsIncludedInJointVentureFormation
Namespace Prefix:	aa_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Ownership interests in joint venture.

+ References

No definition available.

+ Details

Name:	aa_OwnershipInterestsInJointVenture
Namespace Prefix:	aa_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage of shares transferable.

+ References

No definition available.

+ Details

Name:	aa_PercentageOfSharesTransferable
Namespace Prefix:	aa_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Purchase percentage interest of put option.

+ References

No definition available.

+ Details

Name:	aa_PurchasePercentageInterestOfPutOption
Namespace Prefix:	aa_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Purchase price for remaining ownership amount.

+ References

No definition available.

+ Details

Name:	aa_RemainingOwnershipInterestAmount
Namespace Prefix:	aa_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Sale of ownership interest under agreement exchange for cash.

+ References

No definition available.

+ Details

Name:	aa_SaleOfOwnershipInterestUnderAgreementExchangeForCash
Namespace Prefix:	aa_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Sale of ownership interest under agreement exchange for issuance per share.

+ References

No definition available.

+ Details

Name:	aa_SaleOfOwnershipInterestUnderAgreementExchangeForIssuancePerShare
Namespace Prefix:	aa_
Data Type:	dtr-types:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

Sale of ownership interest under agreement exchange for issuance shares.

+ References

No definition available.

+ Details

Name:	aa_SaleOfOwnershipInterestUnderAgreementExchangeForIssuanceShares
Namespace Prefix:	aa_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Sale of ownership interest under agreement exchange for issuance shares value.

+ References

No definition available.

+ Details

Name:	aa_SaleOfOwnershipInterestUnderAgreementExchangeForIssuanceSharesValue
Namespace Prefix:	aa_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Ticking fee percentage paid per annum.

+ References

No definition available.

+ Details

Name:	aa_TickingFeePercentagePaidPerAnnum
Namespace Prefix:	aa_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of shares of equity interest issued or issuable by acquirer as part of consideration transferred in business combination.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 805
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 8
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479328/805-10-50-8

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Topic 805
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 41
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479303/805-10-55-41

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Topic 805
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479581/805-30-50-2

Reference 4: http://www.xbrl.org/2003/role/exampleRef
-Topic 805
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (b)(4)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479581/805-30-50-1

Reference 5: http://www.xbrl.org/2003/role/exampleRef
-Topic 805
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 42
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479303/805-10-55-42

+ Details

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

- Definition

Amount of expense for acquisition-related cost incurred to effect business combination. Includes, but is not limited to, finder's fee; advisory, legal, accounting, valuation, and other professional and consulting fees; and general administrative cost, including cost of maintaining internal acquisition department.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 805
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 7
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479328/805-10-50-7

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 805
-SubTopic 10
-Name Accounting Standards Codification
-Section 25
-Paragraph 23
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479405/805-10-25-23

+ Details

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

- Definition

Amount of consideration transferred, consisting of acquisition-date fair value of assets transferred by the acquirer, liabilities incurred by the acquirer, and equity interest issued by the acquirer.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 805
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 8
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479328/805-10-50-8

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Topic 805
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 41
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479303/805-10-55-41

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 805
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479581/805-30-50-2

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 805
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479581/805-30-50-1

+ Details

Name:	us-gaap_BusinessCombinationConsiderationTransferred1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Face amount or stated value per share of common stock.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(29))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

+ Details

Name:	us-gaap_CommonStockParOrStatedValuePerShare
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 835
-SubTopic 30
-Name Accounting Standards Codification
-Section 55
-Paragraph 8
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482949/835-30-55-8

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1B
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481139/470-20-50-1B

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 69B
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481568/470-20-55-69B

Reference 4: http://www.xbrl.org/2003/role/exampleRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 69C
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481568/470-20-55-69C

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 835
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482900/835-30-50-1

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 835
-SubTopic 30
-Name Accounting Standards Codification
-Section 45
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482925/835-30-45-2

+ Details

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

- Definition

The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 825
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 28
-Subparagraph (f)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482907/825-10-50-28

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 323
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481687/323-10-50-3

+ Details

Name:	us-gaap_EquityMethodInvestmentOwnershipPercentage
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Fair value of joint venture at its formation date.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 805
-SubTopic 60
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147476180/805-60-50-2

+ Details

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

- Definition

Amount of unrealized gain (loss) on investment in marketable security.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 220
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(7)(c))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483621/220-10-S99-2

+ Details

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

- Definition

The equity interest of noncontrolling shareholders, partners or other equity holders in consolidated entity.

+ References

No definition available.

+ Details

Name:	us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The cash outflow associated with the acquisition of business during the period. The cash portion only of the acquisition price.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 805
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 8
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479328/805-10-50-8

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Topic 805
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 41
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479303/805-10-55-41

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Topic 805
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479581/805-30-50-2

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 805
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (b)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479581/805-30-50-1

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 13
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482740/230-10-45-13

+ Details

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

- Definition

The cash outflow associated with the investment in or advances to an entity in which the reporting entity shares control of the entity with another party or group.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 13
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482740/230-10-45-13

+ Details

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

- Definition

The cash inflow from a borrowing having initial term of repayment within one year or the normal operating cycle, if longer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482740/230-10-45-14

+ Details

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

- Definition

Price of a single share of a number of saleable stocks of a company.

+ References

No definition available.

+ Details

Name:	us-gaap_SharePrice
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:perShareItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=aa_TrentoEquityMember
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:	srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=aa_SanCiprianOperationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementScenarioAxis=srt_ScenarioForecastMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_OwnershipAxis=aa_SouthThirtyTwoLimitedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=aa_AligroupAcquisitionMember
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:	srt_OwnershipAxis=aa_CHESSDepositaryInterestsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_DebtInstrumentAxis=us-gaap_ConvertibleDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	srt_OwnershipAxis=aa_TrentoEquityHoldingsSLMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=aa_AlcoaSpecialPurposeVehicleMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Acquisitions, Divestitures, and Joint Ventures - Additional Information (Detail) $ / shares in Units, € in Millions						3 Months Ended				6 Months Ended
	Aug. 01, 2026 USD ($)	Aug. 01, 2026 EUR (€)	Jun. 30, 2026 USD ($) Period $ / shares shares	Jul. 01, 2025 USD ($) shares	Jul. 01, 2025 ر.س / shares	Jun. 30, 2026 USD ($) $ / shares	Jun. 30, 2025 USD ($)	Mar. 31, 2025 USD ($)	Mar. 31, 2025 EUR (€)	Jun. 30, 2026 USD ($) $ / shares	Jun. 30, 2025 USD ($)	Jul. 01, 2026 USD ($)	Jul. 01, 2026 EUR (€)	Jun. 30, 2026 ر.س / shares	Jun. 26, 2026 $ / shares	Dec. 31, 2025 $ / shares	Dec. 31, 2025 ر.س / shares
Business Disposition [Line Items]
Mark-to-market losses						$ 123,000,000	$ 0			$ 35,000,000	$ 0
Trento EQT [Member] \| Subsequent Event [Member]
Business Disposition [Line Items]
Purchase price for remaining ownership amount	$ 28,000,000	€ 25
San Ciprian Operations [Member] \| Forecast [Member]
Business Disposition [Line Items]
Percentage of ownership held	100.00%	100.00%
South32 Limited [Member]
Business Disposition [Line Items]
Business combination, cash amount			$ 3,100,000,000
Aligroup Acquisition [Member]
Business Disposition [Line Items]
Business combination, cash amount			$ 3,100,000,000
Business combination successive annual periods \| Period			4
Proceeds from bridge financing			$ 3,100,000,000
Transaction-related costs						$ 11,000,000				$ 13,000,000
Business acquisition, shares issued \| shares			17,000,000
Aligroup Acquisition [Member] \| Maximum [Member]
Business Disposition [Line Items]
Business combination aggregate payable cash contingent value			$ 750,000,000
Aligroup Acquisition [Member] \| CHESS Depositary Interests [Member]
Business Disposition [Line Items]
Weighted average price per share \| $ / shares															$ 58.79
Aggregate purchase consideration			$ 1,000,000,000
Aligroup Acquisition [Member] \| South32 Limited [Member]
Business Disposition [Line Items]
Ticking fee percentage paid per annum			5.00%
Aligroup Acquisition [Member] \| South32 Limited [Member] \| Mineracao Rio Do Norte [Member]
Business Disposition [Line Items]
Non-controlling interest, ownership percentage			33.00%			33.00%				33.00%				33.00%
Aligroup Acquisition [Member] \| South32 Limited [Member] \| Alumar Smelter [Member]
Business Disposition [Line Items]
Non-controlling interest, ownership percentage			40.00%			40.00%				40.00%				40.00%
Aligroup Acquisition [Member] \| South32 Limited [Member] \| Australia [Member] \| Boddington Bauxite Mine [Member]
Business Disposition [Line Items]
Non-controlling interest, ownership percentage			86.00%			86.00%				86.00%				86.00%
Aligroup Acquisition [Member] \| South32 Limited [Member] \| Australia [Member] \| Worsley Alumina [Member]
Business Disposition [Line Items]
Non-controlling interest, ownership percentage			86.00%			86.00%				86.00%				86.00%
Aligroup Acquisition [Member] \| South32 Limited [Member] \| South Africa [Member] \| Hillside Aluminum Smelter [Member]
Business Disposition [Line Items]
Non-controlling interest, ownership percentage			100.00%			100.00%				100.00%				100.00%
Aligroup Acquisition [Member] \| South32 Limited [Member] \| South Africa [Member] \| Idled Bayside Smelter Property [Member]
Business Disposition [Line Items]
Non-controlling interest, ownership percentage			100.00%			100.00%				100.00%				100.00%
Aligroup Acquisition [Member] \| South32 Limited [Member] \| Brazil [Member] \| Alumar Refinery [Member]
Business Disposition [Line Items]
Non-controlling interest, ownership percentage			36.00%			36.00%				36.00%				36.00%
Saudi Arabia Joint Venture [Member]
Business Disposition [Line Items]
Ownership interest in joint venture				25.10%	25.10%
Minimum holding period				3 years
Percentage of shares transferable				0.3333%
Total consideration				$ 1,350,000,000
Value per share \| (per share)			$ 1,360			$ 1,360				$ 1,360				ر.س 59.4		$ 1,397	ر.س 60.95
Sale of ownership interest under agreement exchange for issuance shares \| shares				85,977,547
Sale of ownership interest under agreement exchange for issuance shares value				$ 1,200,000,000
Sale of ownership interest under agreement exchange for issuance per share \| ر.س / shares					ر.س 52.35
Sale of ownership interest under agreement exchange for cash				$ 150,000,000
Mark-to-market losses						$ 123,000,000				$ 35,000,000
San Ciprian Joint Venture [Member] \| Alcoa Corporation [Member]
Business Disposition [Line Items]
Ownership interest in joint venture								75.00%
Contribution to joint venture								$ 81,000,000	€ 75
San Ciprian Joint Venture [Member] \| Convertible Debt \| Subsequent Event [Member] \| ANH Holdings One B V [Member]
Business Disposition [Line Items]
Principal amount of debt												$ 153,000,000	€ 130
San Ciprian Joint Venture [Member] \| Trento EQT [Member]
Business Disposition [Line Items]
Foreign currency translation adjustments reclassified to AOCI								31,000,000
Net assets included in joint venture formation								103,000,000
Contribution to joint venture								27,000,000	€ 25
Contribution to form the joint venture								$ 108,000,000
Purchase percentage of interest								25.00%	25.00%	25.00%
San Ciprian Joint Venture [Member] \| Trento Equity Holdings SL [Member] \| Trento EQT [Member]
Business Disposition [Line Items]
Non-controlling interest, ownership percentage								25.00%
Gallium Joint Venture [Member] \| Alcoa Special Purpose Vehicle [Member] \| Alcoa Corporation [Member]
Business Disposition [Line Items]
Contribution to joint venture						$ 24,000,000