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Preferred and Common Stock - Schedule of Activity for Stock Options and Stock Awards (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014
Schedule Of Stock Option Activity [Line Items]
Number of options, Exercised	(18.1)us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised
Stock options [Member]
Schedule Of Stock Option Activity [Line Items]
Number of options, Outstanding beginning of year	45us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber / us-gaap_OptionIndexedToIssuersEquityTypeAxis = us-gaap_EmployeeStockOptionMember
Number of options, Granted	6us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod / us-gaap_OptionIndexedToIssuersEquityTypeAxis = us-gaap_EmployeeStockOptionMember
Number of options, Exercised	(17.0)us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised / us-gaap_OptionIndexedToIssuersEquityTypeAxis = us-gaap_EmployeeStockOptionMember
Converted	0aa_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsConverted / us-gaap_OptionIndexedToIssuersEquityTypeAxis = us-gaap_EmployeeStockOptionMember
Number of options, Expired or forfeited	(2)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriod / us-gaap_OptionIndexedToIssuersEquityTypeAxis = us-gaap_EmployeeStockOptionMember
Number of options, Performance share adjustment	0aa_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsPerformanceSharesActualPayout / us-gaap_OptionIndexedToIssuersEquityTypeAxis = us-gaap_EmployeeStockOptionMember
Number of options, Outstanding end of year	32us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber / us-gaap_OptionIndexedToIssuersEquityTypeAxis = us-gaap_EmployeeStockOptionMember
Weighted average exercise price, Outstanding beginning of year	10.78us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice / us-gaap_OptionIndexedToIssuersEquityTypeAxis = us-gaap_EmployeeStockOptionMember
Weighted average exercise price, granted	11.06us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice / us-gaap_OptionIndexedToIssuersEquityTypeAxis = us-gaap_EmployeeStockOptionMember
Weighted average exercise price, exercised	8.70us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice / us-gaap_OptionIndexedToIssuersEquityTypeAxis = us-gaap_EmployeeStockOptionMember
Weighted average exercise price, Converted	0aa_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsConvertedOrTransferredInPeriodWeightedAverageExercisePrice / us-gaap_OptionIndexedToIssuersEquityTypeAxis = us-gaap_EmployeeStockOptionMember
Weighted average exercise price, expired or forfeited	20.93us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriodWeightedAverageExercisePrice / us-gaap_OptionIndexedToIssuersEquityTypeAxis = us-gaap_EmployeeStockOptionMember
Weighted average exercise price, Performance share adjustment	0aa_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsPerformanceAdjustmentWeightedAverageGrantDateFairValue / us-gaap_OptionIndexedToIssuersEquityTypeAxis = us-gaap_EmployeeStockOptionMember
Weighted average exercise price Outstanding, end of year	11.26us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice / us-gaap_OptionIndexedToIssuersEquityTypeAxis = us-gaap_EmployeeStockOptionMember
Stock Awards [Member]
Schedule Of Stock Option Activity [Line Items]
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Number of awards, Granted	7us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_AwardTypeAxis = aa_StockAwardsMember
Number of awards, Exercised	0aa_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExercisedInPeriod / us-gaap_AwardTypeAxis = aa_StockAwardsMember
Number of awards, Converted	(4)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod / us-gaap_AwardTypeAxis = aa_StockAwardsMember
Number of awards, Expired or forfeited	(1)aa_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedAndExpirationsInPeriod / us-gaap_AwardTypeAxis = aa_StockAwardsMember
Number of awards, Performance share adjustment	1us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsPeriodIncreaseDecrease / us-gaap_AwardTypeAxis = aa_StockAwardsMember
Number of awards, Outstanding end of year	19us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = aa_StockAwardsMember
Weighted average FMV per award, Outstanding beginning of year	10.88us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = aa_StockAwardsMember
Weighted average FMV per award, Granted	11.14us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = aa_StockAwardsMember
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Weighted average FMV per award, Converted	16.18us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = aa_StockAwardsMember
Weighted average FMV per award, Expired or forfeited	9.98aa_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresAndExpiredWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = aa_StockAwardsMember
Weighted average FMV per award, Performance share adjustment	10.00aa_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsPerformanceShareAdjustment / us-gaap_AwardTypeAxis = aa_StockAwardsMember
Weighted average FMV per award, Outstanding, end of year	9.98us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = aa_StockAwardsMember

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The net total number of shares (or other type of equity) under an equity-based award plan, other than a stock option plan, that were granted, vested and forfeited during the reporting period.

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Weighted average price of options that were either forfeited or expired.

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Number of options outstanding, including both vested and non-vested options.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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