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Earnings Per Share - Additional Information (Detail) (USD $) | 3 Months Ended
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Mar. 31, 2015 | Mar. 31, 2014
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] |  |
Common stock outstanding | 6,000,000 us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements |
Number of weighted average outstanding diluted shares | 1,238,000,000 us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding | 1,101,000,000 us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding
Number of common stock shares issued in a part of acquisition | 90,000,000 us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued |
Convertible Debt Securities [Member] |  |
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] |  |
Number of shares, option exercised |  | 89,000,000 us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ConvertibleDebtSecuritiesMember
Common stock outstanding |  | 89,000,000 us-gaap_CommonStockSharesOutstanding / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ConvertibleDebtSecuritiesMember
Number of weighted average outstanding shares included in both basic and diluted shares |  | 23,000,000 us-gaap_WeightedAverageNumberOfShareOutstandingBasicAndDiluted / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ConvertibleDebtSecuritiesMember
Common stock outstanding |  | 89,000,000 us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ConvertibleDebtSecuritiesMember
Number of weighted average outstanding diluted shares |  | 66,000,000 us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ConvertibleDebtSecuritiesMember
Mandatory Convertible Notes [Member] |  |
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] |  |
Number of anti-dilutive securities | 77,000,000 us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount / us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis = aa_MandatoryConvertibleNotesMember |
Convertible Notes [Member] |  |
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] |  |
Number of anti-dilutive securities |  | 66,000,000 us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount / us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis = aa_ConvertibleNotesMember
Shares that would have been included in diluted earnings per share calculation |  | 66,000,000 aa_SharesThatWouldHaveBeenIncludedInDilutedEarningsPerShareCalculation / us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis = aa_ConvertibleNotesMember
Stock Awards [Member] |  |
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] |  |
Number of anti-dilutive securities |  | 18,000,000 us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount / us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis = aa_StockAwardsMember
Shares that would have been included in diluted earnings per share calculation |  | 10,000,000 aa_SharesThatWouldHaveBeenIncludedInDilutedEarningsPerShareCalculation / us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis = aa_StockAwardsMember
Stock Option [Member] |  |
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] |  |
Number of anti-dilutive securities |  | 31,000,000 us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount / us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis = us-gaap_StockOptionMember
Shares that would have been included in diluted earnings per share calculation |  | 5,000,000 aa_SharesThatWouldHaveBeenIncludedInDilutedEarningsPerShareCalculation / us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis = us-gaap_StockOptionMember
Weighted average exercise price of options | 14.79 aa_WeightedAverageExercisePriceOfUnderwaterOptions / us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis = us-gaap_StockOptionMember | 14.53 aa_WeightedAverageExercisePriceOfUnderwaterOptions / us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis = us-gaap_StockOptionMember
Equity Unit Purchase Agreements [Member] |  |
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] |  |
Number of anti-dilutive securities | 13,000,000 us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount / us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis = us-gaap_EquityUnitPurchaseAgreementsMember | 10,000,000 us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount / us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis = us-gaap_EquityUnitPurchaseAgreementsMember

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- Definition Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Antidilution -URI http://asc.fasb.org/extlink&oid=6505113 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Diluted Earnings Per Share -URI http://asc.fasb.org/extlink&oid=6510752 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Contingent Stock Agreement -URI http://asc.fasb.org/extlink&oid=6508534 + Details
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