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Earnings per Share - Additional Information (Detail) (USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Participating securities outstanding 0aa_ParticipatingNonvestedSharesWithNonForfeitableDividendRights    
Number of anti-dilutive securities     89,000,000us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
Convertible Notes [Member]      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Number of anti-dilutive securities 16,000,000us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= aa_ConvertibleNotesMember
89,000,000us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= aa_ConvertibleNotesMember
 
Shares that would have been included in diluted earnings per share calculation   89,000,000aa_SharesThatWouldHaveBeenIncludedInDilutedEarningsPerShareCalculation
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= aa_ConvertibleNotesMember
 
Mandatory Convertible Notes [Member]      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Number of anti-dilutive securities 22,000,000us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= aa_MandatoryConvertibleNotesMember
   
Stock and Performance Awards [Member]      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Participating securities outstanding, nonforfeitable 19,000,000aa_NonvestedSharesWithForfeitableDividends
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= aa_StockAndPerformanceAwardsMember
16,000,000aa_NonvestedSharesWithForfeitableDividends
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= aa_StockAndPerformanceAwardsMember
12,000,000aa_NonvestedSharesWithForfeitableDividends
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= aa_StockAndPerformanceAwardsMember
Equity Unit Purchase Agreements [Member]      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Number of anti-dilutive securities 3,000,000us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= us-gaap_EquityUnitPurchaseAgreementsMember
12,000,000us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= us-gaap_EquityUnitPurchaseAgreementsMember
27,000,000us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= us-gaap_EquityUnitPurchaseAgreementsMember
Stock Option [Member]      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Number of anti-dilutive securities   2,000,000us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= us-gaap_StockOptionMember
 
Shares that would have been included in diluted earnings per share calculation   12,000,000aa_SharesThatWouldHaveBeenIncludedInDilutedEarningsPerShareCalculation
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= us-gaap_StockOptionMember
 
Weighted average exercise price of options 16.24aa_WeightedAverageExercisePriceOfUnderwaterOptions
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= us-gaap_StockOptionMember
15.81aa_WeightedAverageExercisePriceOfUnderwaterOptions
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= us-gaap_StockOptionMember
15.41aa_WeightedAverageExercisePriceOfUnderwaterOptions
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= us-gaap_StockOptionMember
Stock Awards [Member]      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Number of anti-dilutive securities   9,000,000us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= aa_StockAwardsMember
 
Shares that would have been included in diluted earnings per share calculation   16,000,000aa_SharesThatWouldHaveBeenIncludedInDilutedEarningsPerShareCalculation
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= aa_StockAwardsMember