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Debt (Public Debt) - Additional Information (Detail) (USD $)
In Millions, except Per Share data, unless otherwise specified
 1 Months Ended 9 Months Ended 12 Months Ended 3 Months Ended
May 31, 2013
Sep. 30, 2014
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Mar. 31, 2014
Debt Instrument [Line Items]            
Principal amount of convertible notes   $ 1,250us-gaap_LongTermDebt $ 8,798us-gaap_LongTermDebt $ 8,262us-gaap_LongTermDebt    
Number of shares, option exercised     18.1us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised      
Par value of issued shares     $ 1us-gaap_CommonStockParOrStatedValuePerShare      
Proceeds from issuance of public debt offering   1,238us-gaap_ProceedsFromIssuanceInitialPublicOffering        
Debt instrument redemption price percentage   101.00%us-gaap_DebtInstrumentRedemptionPricePercentage        
Payments on long-term debt     1,723us-gaap_RepaymentsOfLongTermDebt 2,317us-gaap_RepaymentsOfLongTermDebt 1,489us-gaap_RepaymentsOfLongTermDebt  
Purchase premiums repaid recorded as interest expense 1us-gaap_InterestExpenseDebt          
5.25% Convertible Notes Due 2014 [Member]            
Debt Instrument [Line Items]            
Principal amount of convertible notes     575us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= aa_FivePointTwoFivePercentConvertibleNotesDueTwoThousandFourteenMember		     575us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= aa_FivePointTwoFivePercentConvertibleNotesDueTwoThousandFourteenMember
Convertible notes, interest rate     5.25%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= aa_FivePointTwoFivePercentConvertibleNotesDueTwoThousandFourteenMember		     5.25%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= aa_FivePointTwoFivePercentConvertibleNotesDueTwoThousandFourteenMember
Convertible notes due date     Mar. 15, 2014     Mar. 15, 2014
Number of shares, option exercised     89.0us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised
/ us-gaap_DebtInstrumentAxis
= aa_FivePointTwoFivePercentConvertibleNotesDueTwoThousandFourteenMember		     89.0us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised
/ us-gaap_DebtInstrumentAxis
= aa_FivePointTwoFivePercentConvertibleNotesDueTwoThousandFourteenMember
Conversion rate, numerator           155.4908us-gaap_DebtInstrumentConvertibleConversionRatio1
/ us-gaap_DebtInstrumentAxis
= aa_FivePointTwoFivePercentConvertibleNotesDueTwoThousandFourteenMember
Conversion rate, denominator           $ 1,000aa_DebtInstrumentConvertibleConversionPricePerShare
/ us-gaap_DebtInstrumentAxis
= aa_FivePointTwoFivePercentConvertibleNotesDueTwoThousandFourteenMember
Convertible notes, conversion price           $ 6.43us-gaap_DebtInstrumentConvertibleConversionPrice1
/ us-gaap_DebtInstrumentAxis
= aa_FivePointTwoFivePercentConvertibleNotesDueTwoThousandFourteenMember
Par value of issued shares           $ 89us-gaap_CommonStockParOrStatedValuePerShare
/ us-gaap_DebtInstrumentAxis
= aa_FivePointTwoFivePercentConvertibleNotesDueTwoThousandFourteenMember
6.00% Notes, Due 2013 [Member]            
Debt Instrument [Line Items]            
Convertible notes, interest rate 6.00%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= aa_SixPercentNotesDueTwoThousandThirteenMember		          
Early repayment of outstanding notes 422us-gaap_DebtInstrumentCarryingAmount
/ us-gaap_DebtInstrumentAxis
= aa_SixPercentNotesDueTwoThousandThirteenMember		          
Payments on long-term debt 435us-gaap_RepaymentsOfLongTermDebt
/ us-gaap_DebtInstrumentAxis
= aa_SixPercentNotesDueTwoThousandThirteenMember		          
Debt Accrued and Unpaid Interest [Member]            
Debt Instrument [Line Items]            
Payments on long-term debt $ 12us-gaap_RepaymentsOfLongTermDebt
/ us-gaap_DebtInstrumentAxis
= aa_DebtAccruedAndUnpaidInterestMember		          
5.125% Senior Notes Due 2024 [Member]            
Debt Instrument [Line Items]            
Convertible notes, interest rate   5.125%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= aa_FivePointOneTwoFivePercentageSeniorNotesDueTwoThousandTwentyFourMember