Commitment on loan agreement.

No definition available.

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

- Definition

Debt instrument maturity year and month range end.

+ References

No definition available.

+ Details

Name:	aa_DebtInstrumentMaturityYearAndMonthRangeEnd
Namespace Prefix:	aa_
Data Type:	xbrli:gYearMonthItemType
Balance Type:	na
Period Type:	duration

- Definition

Debt instrument maturity year and month range start.

+ References

No definition available.

+ Details

Name:	aa_DebtInstrumentMaturityYearAndMonthRangeStart
Namespace Prefix:	aa_
Data Type:	xbrli:gYearMonthItemType
Balance Type:	na
Period Type:	duration

- Definition

Early repayment of debt.

+ References

No definition available.

+ Details

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

- Definition

Interest rate margin.

+ References

No definition available.

+ Details

Name:	aa_InterestRateMargin
Namespace Prefix:	aa_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of subloan agreement.

+ References

No definition available.

+ Details

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

- Definition

Principal and interest.

+ References

No definition available.

+ Details

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

- Definition

Weighted average interest rate, outstanding borrowings.

+ References

No definition available.

+ Details

Name:	aa_WeightedAverageInterestRateOutstandingBorrowings
Namespace Prefix:	aa_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Weighted average margin on long term debt.

+ References

No definition available.

+ Details

Name:	aa_WeightedAverageMarginOnLongTermDebt
Namespace Prefix:	aa_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Description of the frequency of periodic payments (monthly, quarterly, annual).

+ References

+ Details

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

- Definition

Weighted average interest rate of debt outstanding.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtWeightedAverageInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of interest payable on debt, including, but not limited to, trade payables.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.15(5))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.15(a))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

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

- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

+ 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(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

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

- Definition

Amount of cash outflow for payment of an obligation from a lender, including but not limited to, letter of credit, standby letter of credit and revolving credit arrangements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3291-108585

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(f))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

+ Details

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

- Details

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

- Details

Name:	us-gaap_LongtermDebtTypeAxis=aa_LoanAgreementWithBndesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_LongtermDebtTypeAxis=aa_BndesLoansDueTwoThousandFourteenToTwoThousandTwentyNineMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Debt (BNDES) - Additional Information (Detail) - Alcoa Aluminio [Member] R$ in Millions, $ in Millions	1 Months Ended		4 Months Ended		12 Months Ended
	Aug. 31, 2017 USD ($)	Aug. 31, 2017 BRL (R$)	Dec. 31, 2018 USD ($)	Dec. 31, 2018 BRL (R$)	Dec. 31, 2018 USD ($) Agreement	Dec. 31, 2018 BRL (R$) Agreement	Dec. 31, 2017 USD ($)	Dec. 31, 2017 BRL (R$)	Dec. 31, 2018 BRL (R$)	Dec. 31, 2017 BRL (R$)
Debt Instrument [Line Items]
Commitment on loan agreement							$ 85			R$ 177
Debt instrument, frequency of periodic payment					monthly	monthly
Debt instrument maturity year and month range start							2013-01	2013-01
Debt instrument maturity year and month range end							2029-09	2029-09
Weighted-average interest rate							9.74%	9.74%
Early repayment of outstanding borrowings							$ 44	R$ 138
Interest rate margin							1.55%	1.55%
Early payment	$ 41	R$ 131	$ 122	R$ 500
Deferred interest							$ 25			R$ 82
Loan Agreement With Bndes [Member]
Debt Instrument [Line Items]
Long-term interest rate							7.00%			7.00%
Weighted-average margin on long term debt							2.74%	2.74%
Number of Subloan agreement \| Agreement					3	3
Loan Agreement With Bndes [Member] \| Three Of Subloans [Member]
Debt Instrument [Line Items]
Commitment on loan agreement			$ 397		$ 397				R$ 687
Loan Agreement With Bndes [Member] \| Two Of Subloans [Member]
Debt Instrument [Line Items]
Debt instrument, frequency of periodic payment					monthly	monthly
Debt instrument maturity year and month range start							2011-10	2011-10
Debt instrument maturity year and month range end							2029-09	2029-09
Principal and interest										R$ 667
Loan Agreement With Bndes [Member] \| Third Subloan [Member]
Debt Instrument [Line Items]
Debt instrument maturity year and month range start							2012-07	2012-07
Debt instrument maturity year and month range end							2018-06	2018-06
Principal and interest										20
BNDES Loans, Due 2014-2029 [Member]
Debt Instrument [Line Items]
Line of credit facility, outstanding borrowings							$ 137			R$ 454
Early repayment of outstanding borrowings					$ 132	R$ 539	$ 15	R$ 49