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Receivables - Additional Information (Detail) - USD ($) | 3 Months Ended |  |
--- | --- | --- | ---
Jun. 30, 2021 | Apr. 20, 2020 | Oct. 25, 2019
Accounts Notes And Loans Receivable [Line Items] |  |  |
Sale of customer receivables | $ 0 |  |
Receivables Purchase Agreement [Member] | Maximum [Member] |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |
Receivables previously secured by credit facility |  | $ 120,000,000 |
Revolving Credit Facility [Member] | Three-year Agreement [Member] |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |
Principal amount of debt |  |  | $ 120,000,000

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- 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

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- Definition Sale of customer receivables. + References No definition available. + Details
Name: | aa_SaleOfCustomerReceivables
Namespace Prefix: | aa_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- 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_AccountsNotesAndLoansReceivableLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility. + 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 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
Name: | us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Details
Name: | us-gaap_TypeOfArrangementAxis=aa_ReceivablesPurchaseAgreementMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_CreditFacilityAxis=us-gaap_RevolvingCreditFacilityMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_DebtInstrumentAxis=aa_ThreeYearCreditFacilityAgreementMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |