Commitment to invest in joint venture.

No definition available.

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

- Definition

This element represents the aggregate cost of investments accounted for under the equity method of accounting.

+ 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 210.5-02.12)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Definition

Percentage investment held by the managing member or general partner of the limited liability company (LLC) or limited partnership (LP).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 272
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6373374&loc=d3e70478-108055

+ Details

Name:	us-gaap_LimitedLiabilityCompanyLLCOrLimitedPartnershipLPManagingMemberOrGeneralPartnerOwnershipInterest
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage investment held by members or limited partners of limited liability company (LLC) or limited partnership (LP).

+ References

+ Details

Name:	us-gaap_LimitedLiabilityCompanyLLCOrLimitedPartnershipLPMembersOrLimitedPartnersOwnershipInterest
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 13
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3213-108585

+ Details

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

- Details

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

- Details

Name:	srt_CounterpartyNameAxis=aa_RioTintoPlcMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=aa_QuebecCanadaMember
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:	dei_LegalEntityAxis=aa_AlcoaCorporationAndRioTintoPlcMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Investments - Additional Information (Detail) $ in Millions, $ in Millions	3 Months Ended		12 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2018 CAD ($)	Dec. 31, 2017 USD ($)	Jun. 30, 2018 CAD ($)
Elysis Limited Partnership [Member]
Schedule of Equity Method Investments [Line Items]
Combined investment in joint venture	$ 145			$ 188
Elysis Limited Partnership [Member] \| Rio Tinto Plc [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest percentage	48.235%	48.235%
Elysis Limited Partnership [Member] \| Quebec Provincial Government [Member]
Schedule of Equity Method Investments [Line Items]
Limited partner ownership interest percentage	3.53%	3.53%
Alcoa Corporation [Member]
Schedule of Equity Method Investments [Line Items]
Contribution to joint venture			$ 66
Alcoa Corporation [Member] \| Elysis Limited Partnership [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest percentage	48.235%	48.235%
Contribution to joint venture	$ 5	$ 6
Alcoa Corporation and Rio Tinto plc [Member] \| Elysis Limited Partnership [Member]
Schedule of Equity Method Investments [Line Items]
Commitment to invest in joint venture	$ 44			$ 55