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 ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024	Dec. 31, 2024	Nov. 14, 2024	Aug. 27, 2023
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unsold customer receivables as collateral sold receivables			$ 355,000,000		$ 247,000,000
Sale of gross customer receivables	$ 248,000,000	$ 293,000,000	447,000,000	$ 600,000,000
Reinvested collections from previously sold receivables	248,000,000	293,000,000	447,000,000	584,000,000
Cash proceeds from financial institution	0	$ 0	0	$ 16,000,000
Maximum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Receivables previously secured by credit facility						$ 150,000,000	$ 130,000,000
Accounts Payable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Cash collections from previously sold receivables yet to be reinvested	$ 58,000,000		$ 58,000,000		$ 50,000,000