Cash collections from previously sold receivables yet to be reinvested.

No definition available.

Name:	aa_CashCollectionsFromPreviouslySoldReceivablesYetToBeReinvested
Namespace Prefix:	aa_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Reflects the maximum aggregate revolving value of trade receivables which can be sold.

+ References

No definition available.

+ Details

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

- Definition

Reinvested collection from previously sold receivables.

+ References

No definition available.

+ Details

Name:	aa_ReinvestedCollectionFromPreviouslySoldReceivables
Namespace Prefix:	aa_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Sale of Gross Customer Receivables

+ References

No definition available.

+ Details

Name:	aa_SaleOfGrossCustomerReceivables
Namespace Prefix:	aa_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Unsold Customer Receivables As Collateral Sold Receivables.

+ References

No definition available.

+ Details

Name:	aa_UnsoldCustomerReceivablesAsCollateralSoldReceivables
Namespace Prefix:	aa_
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

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 310
-SubTopic 20
-Name Accounting Standards Codification
-Section 40
-Paragraph 7
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481628/310-20-40-7

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481990/310-10-45-2

+ Details

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

- Definition

The cash inflow from a long-term borrowing issued by the entity involved in financial services operations.

+ 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_ProceedsFromIssuanceOfFinancialServicesObligations
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_TypeOfArrangementAxis=aa_ReceivablesPurchaseAgreementMember
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:	us-gaap_ExtinguishmentOfDebtAxis=us-gaap_AccountsPayableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Receivables - Additional Information (Detail) - Receivables Purchase Agreement [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Nov. 14, 2024	Aug. 27, 2023
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unsold customer receivables as collateral sold receivables	$ 247	$ 104
Sale of gross customer receivables	1,186	591
Reinvested collections from previously sold receivables	1,170	477
Cash proceeds from financial institution	16	114
Maximum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Receivables previously secured by credit facility			$ 150	$ 130
Accounts Payable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Cash collections from previously sold receivables yet to be reinvested	$ 50	$ 99