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Contingencies and Commitments - Summary of Activity Related to Alba Matter (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Commitments Contingencies And Litigation [Line Items]
Government investigations	$ 1,168us-gaap_RestructuringCharges	$ 782us-gaap_RestructuringCharges	$ 172us-gaap_RestructuringCharges
Civil suit	19,137us-gaap_CostOfGoodsSold	19,286us-gaap_CostOfGoodsSold	20,401us-gaap_CostOfGoodsSold
(Loss) income before income taxes	497us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments	(1,816)us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments	324us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments
Benefit for income taxes	320us-gaap_IncomeTaxExpenseBenefit	428us-gaap_IncomeTaxExpenseBenefit	162us-gaap_IncomeTaxExpenseBenefit
Net (loss) income	177us-gaap_ProfitLoss	(2,244)us-gaap_ProfitLoss	162us-gaap_ProfitLoss
Alba [Member]
Commitments Contingencies And Litigation [Line Items]
(Loss) income before income taxes		384us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments / us-gaap_CounterpartyNameAxis = aa_AluminiumBahrainBSCVsAlcoaIncMember	85us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments / us-gaap_CounterpartyNameAxis = aa_AluminiumBahrainBSCVsAlcoaIncMember
Benefit for income taxes		66us-gaap_IncomeTaxExpenseBenefit / us-gaap_CounterpartyNameAxis = aa_AluminiumBahrainBSCVsAlcoaIncMember	18us-gaap_IncomeTaxExpenseBenefit / us-gaap_CounterpartyNameAxis = aa_AluminiumBahrainBSCVsAlcoaIncMember
Net (loss) income		318us-gaap_ProfitLoss / us-gaap_CounterpartyNameAxis = aa_AluminiumBahrainBSCVsAlcoaIncMember	67us-gaap_ProfitLoss / us-gaap_CounterpartyNameAxis = aa_AluminiumBahrainBSCVsAlcoaIncMember
Alba [Member] | Government Investigation [Member]
Commitments Contingencies And Litigation [Line Items]
Government investigations		384us-gaap_RestructuringCharges / us-gaap_CounterpartyNameAxis = aa_AluminiumBahrainBSCVsAlcoaIncMember / us-gaap_LitigationCaseAxis = aa_GovernmentInvestigationMember
Alba [Member] | Civil Suit [Member]
Commitments Contingencies And Litigation [Line Items]
Civil suit			85us-gaap_CostOfGoodsSold / us-gaap_CounterpartyNameAxis = aa_AluminiumBahrainBSCVsAlcoaIncMember / us-gaap_LitigationCaseAxis = aa_CivilSuitMember
Alcoa [Member] | Alba [Member]
Commitments Contingencies And Litigation [Line Items]
Reallocation of civil suit		21aa_ReallocationChangeInCivilCost / us-gaap_CounterpartyNameAxis = aa_AluminiumBahrainBSCVsAlcoaIncMember / dei_LegalEntityAxis = us-gaap_ParentCompanyMember
Reallocation of legal costs		20aa_ReallocationChangeInLegalCost / us-gaap_CounterpartyNameAxis = aa_AluminiumBahrainBSCVsAlcoaIncMember / dei_LegalEntityAxis = us-gaap_ParentCompanyMember
(Loss) income before income taxes		367us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments / us-gaap_CounterpartyNameAxis = aa_AluminiumBahrainBSCVsAlcoaIncMember / dei_LegalEntityAxis = us-gaap_ParentCompanyMember	51us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments / us-gaap_CounterpartyNameAxis = aa_AluminiumBahrainBSCVsAlcoaIncMember / dei_LegalEntityAxis = us-gaap_ParentCompanyMember
Benefit for income taxes		66us-gaap_IncomeTaxExpenseBenefit / us-gaap_CounterpartyNameAxis = aa_AluminiumBahrainBSCVsAlcoaIncMember / dei_LegalEntityAxis = us-gaap_ParentCompanyMember	18us-gaap_IncomeTaxExpenseBenefit / us-gaap_CounterpartyNameAxis = aa_AluminiumBahrainBSCVsAlcoaIncMember / dei_LegalEntityAxis = us-gaap_ParentCompanyMember
Net (loss) income		301us-gaap_ProfitLoss / us-gaap_CounterpartyNameAxis = aa_AluminiumBahrainBSCVsAlcoaIncMember / dei_LegalEntityAxis = us-gaap_ParentCompanyMember	33us-gaap_ProfitLoss / us-gaap_CounterpartyNameAxis = aa_AluminiumBahrainBSCVsAlcoaIncMember / dei_LegalEntityAxis = us-gaap_ParentCompanyMember
Alcoa [Member] | Alba [Member] | Government Investigation [Member]
Commitments Contingencies And Litigation [Line Items]
Government investigations		326us-gaap_RestructuringCharges / us-gaap_CounterpartyNameAxis = aa_AluminiumBahrainBSCVsAlcoaIncMember / dei_LegalEntityAxis = us-gaap_ParentCompanyMember / us-gaap_LitigationCaseAxis = aa_GovernmentInvestigationMember
Alcoa [Member] | Alba [Member] | Civil Suit [Member]
Commitments Contingencies And Litigation [Line Items]
Civil suit			51us-gaap_CostOfGoodsSold / us-gaap_CounterpartyNameAxis = aa_AluminiumBahrainBSCVsAlcoaIncMember / dei_LegalEntityAxis = us-gaap_ParentCompanyMember / us-gaap_LitigationCaseAxis = aa_CivilSuitMember
Alumina [Member] | Alba [Member]
Commitments Contingencies And Litigation [Line Items]
Reallocation of civil suit		(21)aa_ReallocationChangeInCivilCost / us-gaap_CounterpartyNameAxis = aa_AluminiumBahrainBSCVsAlcoaIncMember / dei_LegalEntityAxis = aa_AluminaMember
Reallocation of legal costs		(20)aa_ReallocationChangeInLegalCost / us-gaap_CounterpartyNameAxis = aa_AluminiumBahrainBSCVsAlcoaIncMember / dei_LegalEntityAxis = aa_AluminaMember
(Loss) income before income taxes		17us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments / us-gaap_CounterpartyNameAxis = aa_AluminiumBahrainBSCVsAlcoaIncMember / dei_LegalEntityAxis = aa_AluminaMember	34us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments / us-gaap_CounterpartyNameAxis = aa_AluminiumBahrainBSCVsAlcoaIncMember / dei_LegalEntityAxis = aa_AluminaMember
Net (loss) income		17us-gaap_ProfitLoss / us-gaap_CounterpartyNameAxis = aa_AluminiumBahrainBSCVsAlcoaIncMember / dei_LegalEntityAxis = aa_AluminaMember	34us-gaap_ProfitLoss / us-gaap_CounterpartyNameAxis = aa_AluminiumBahrainBSCVsAlcoaIncMember / dei_LegalEntityAxis = aa_AluminaMember
Alumina [Member] | Alba [Member] | Government Investigation [Member]
Commitments Contingencies And Litigation [Line Items]
Government investigations		58us-gaap_RestructuringCharges / us-gaap_CounterpartyNameAxis = aa_AluminiumBahrainBSCVsAlcoaIncMember / dei_LegalEntityAxis = aa_AluminaMember / us-gaap_LitigationCaseAxis = aa_GovernmentInvestigationMember
Alumina [Member] | Alba [Member] | Civil Suit [Member]
Commitments Contingencies And Litigation [Line Items]
Civil suit			$ 34us-gaap_CostOfGoodsSold / us-gaap_CounterpartyNameAxis = aa_AluminiumBahrainBSCVsAlcoaIncMember / dei_LegalEntityAxis = aa_AluminaMember / us-gaap_LitigationCaseAxis = aa_CivilSuitMember

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