Earnings Per Share - Additional Information (Detail) - USD ($) shares in Millions, $ in Millions	12 Months Ended
Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Undistributed earnings	$ 3
Preferred stock dividends paid	$ 1
Stock Awards and Stock Options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common shares equivalents that would have been included in diluted average shares outstanding		3	3
Number of anti-dilutive securities		3	5

Shares that would have been included in diluted earnings per share calculation.

No definition available.

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

- Definition

Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 260
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482662/260-10-50-1

+ Details

Name:	us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
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_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of paid and unpaid preferred stock dividends declared with the form of settlement in cash, stock and payment-in-kind (PIK).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480008/505-10-S99-1

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 45
-Paragraph 2
-SubTopic 405
-Topic 942
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477787/942-405-45-2

+ Details

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

- Definition

Amount of undistributed earnings (loss) allocated to participating securities for the basic earnings (loss) per share or per unit calculation under the two-class method.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 260
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 65
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482689/260-10-45-65

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 260
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 66
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482689/260-10-45-66

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 260
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 60B
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482689/260-10-45-60B

+ Details

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

- Details

Name:	us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis=aa_StockAwardsAndStockOptionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: