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Investments - Additional Information (Detail)
In Millions, unless otherwise specified
 12 Months Ended 12 Months Ended 12 Months Ended 1 Months Ended 12 Months Ended 12 Months Ended 12 Months Ended 12 Months Ended
Dec. 31, 2014
USD ($)
Subsidiary
Dec. 31, 2013
USD ($)
Dec. 31, 2012
USD ($)
kt
Dec. 31, 2014
Maaden Alcoa Joint Venture Smelter And Rolling Mill Companies [Member]
USD ($)
Dec. 31, 2014
Maaden Alcoa Joint Venture Mine And Refinery Company [Member]
USD ($)
Jan. 31, 2014
Maaden Alcoa Joint Venture Mine And Refinery Company [Member]
USD ($)
Dec. 31, 2013
Alcoa Joint Venture [Member]
Dec. 31, 2014
Alcoa Joint Venture [Member]
USD ($)
Dec. 31, 2014
Ma'aden Joint Venture [Member]
USD ($)
Dec. 31, 2014
Maaden Alcoa Joint Venture [Member]
USD ($)
Dec. 31, 2013
Maaden Alcoa Joint Venture [Member]
USD ($)
Apr. 11, 2014
Maaden Alcoa Joint Venture [Member]
Dec. 31, 2014
Becancour Smelter [Member]
Dec. 31, 2014
Boke Investment Company [Member]
Dec. 31, 2014
Compagnie Des Bauxites De Guinee [Member]
Jun. 30, 2013
Saudi Arabia Joint Venture [Member]
Dec. 31, 2014
Fixed Income Securities [Member]
USD ($)
Dec. 31, 2013
Equity Securities [Member]
USD ($)
Dec. 31, 2014
Maaden Alcoa Joint Venture Smelter And Rolling Mill Companies [Member]
USD ($)
Dec. 31, 2014
Maaden Alcoa Joint Venture Smelter And Rolling Mill Companies [Member]
Other Noncurrent Liabilities and Deferred Credits [Member]
USD ($)
Dec. 31, 2013
Maaden Alcoa Joint Venture Smelter And Rolling Mill Companies [Member]
Other Noncurrent Liabilities and Deferred Credits [Member]
USD ($)
Dec. 31, 2014
Maaden Alcoa Joint Venture Mine And Refinery Company [Member]
USD ($)
Jan. 31, 2014
Maaden Alcoa Joint Venture Mine And Refinery Company [Member]
USD ($)
Dec. 31, 2014
Maaden Alcoa Joint Venture Mine And Refinery Company [Member]
Other Noncurrent Liabilities and Deferred Credits [Member]
USD ($)
Dec. 31, 2013
Maaden Alcoa Joint Venture Mine And Refinery Company [Member]
Other Noncurrent Liabilities and Deferred Credits [Member]
USD ($)
Dec. 31, 2013
Scenario One [Member]
Saudi Arabia Joint Venture [Member]
USD ($)
Dec. 31, 2014
Scenario Two [Member]
Saudi Arabia Joint Venture [Member]
USD ($)
Dec. 31, 2014
Financial Guarantee [Member]
Maaden Alcoa Joint Venture Smelter And Rolling Mill Companies [Member]
USD ($)
Dec. 31, 2014
Financial Guarantee [Member]
Maaden Alcoa Joint Venture Mine And Refinery Company [Member]
USD ($)
Dec. 31, 2014
Maaden Alcoa Joint Venture [Member]
USD ($)
Agency
Dec. 31, 2014
Maaden Alcoa Joint Venture [Member]
SAR
Dec. 31, 2014
Brazil [Member]
Dec. 31, 2014
Alcoa World Alumina and Chemicals [Member]
Dec. 31, 2014
Alcoa World Alumina and Chemicals [Member]
Alcoa Joint Venture [Member]
Dec. 31, 2014
Alcoa World Alumina and Chemicals [Member]
Alumina Limited [Member]
Dec. 31, 2014
Smelter Sites [Member]
Becancour Smelter in Quebec, Canada [Member]
Dec. 31, 2014
Smelter Sites [Member]
Alcoa [Member]
Dec. 31, 2014
Smelter Sites [Member]
Pechiney Reynolds Quebec, Inc. [Member]
Dec. 31, 2013
Smelter Sites [Member]
Pechiney Reynolds Quebec, Inc. [Member]
Dec. 31, 2014
Bauxite Mining [Member]
kt
Dec. 31, 2014
Bauxite Mining [Member]
Halco Mining, Inc. [Member]
Dec. 31, 2013
Bauxite Mining [Member]
Halco Mining, Inc. [Member]
Dec. 31, 2014
Bauxite Mining [Member]
Minerao Rio Do Norte S.A. [Member]
Dec. 31, 2013
Bauxite Mining [Member]
Minerao Rio Do Norte S.A. [Member]
Dec. 31, 2014
Alumina Refinery [Member]
kt
Dec. 31, 2014
Primary Aluminum Smelter [Member]
kt
Dec. 31, 2014
Rolling Mill [Member]
kt
Schedule of Equity Method Investments [Line Items]                                                                                              
Percent of equity investments in other entity 40.00%us-gaap_EquityMethodInvestmentOwnershipPercentage                       49.90%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= aa_BecancourSmelterMember		 100.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= aa_BokeInvestmentCompanyMember		 51.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= aa_CompagnieDesBauxitesDeGuineeMember		                                   55.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_OtherOwnershipInterestsByNameAxis
= aa_AlcoaWorldAluminaAndChemicalsMember		 60.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_OtherOwnershipInterestsByNameAxis
= aa_AlcoaWorldAluminaAndChemicalsMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= aa_AlcoaJointVentureMember		 40.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_OtherOwnershipInterestsByNameAxis
= aa_AlcoaWorldAluminaAndChemicalsMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= aa_AluminaLimitedMember		 50.10%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= aa_BecancourSmelterInQuebecMember
/ us-gaap_StatementOperatingActivitiesSegmentAxis
= aa_SmelterOperationMember		 25.05%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= us-gaap_ParentCompanyMember
/ us-gaap_StatementOperatingActivitiesSegmentAxis
= aa_SmelterOperationMember		 50.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= aa_PechineyReynoldsQuebecIncMember
/ us-gaap_StatementOperatingActivitiesSegmentAxis
= aa_SmelterOperationMember		 50.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= aa_PechineyReynoldsQuebecIncMember
/ us-gaap_StatementOperatingActivitiesSegmentAxis
= aa_SmelterOperationMember		   45.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= aa_HalcoMiningIncMember
/ us-gaap_StatementOperatingActivitiesSegmentAxis
= aa_BauxiteMiningMember		 45.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= aa_HalcoMiningIncMember
/ us-gaap_StatementOperatingActivitiesSegmentAxis
= aa_BauxiteMiningMember		 18.20%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= aa_MineraoRioDoNorteSMember
/ us-gaap_StatementOperatingActivitiesSegmentAxis
= aa_BauxiteMiningMember		 18.20%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= aa_MineraoRioDoNorteSMember
/ us-gaap_StatementOperatingActivitiesSegmentAxis
= aa_BauxiteMiningMember		      
Number of hydroelectric power projects                                                               2aa_NumberOfHydroelectricPowerProjects
/ us-gaap_StatementGeographicalAxis
= country_BR		                              
Number of wholly-owned subsidiaries 2aa_NumberOfWhollyOwnedSubsidiaries                                                                                            
Dividends from equity investments $ 86us-gaap_EquityMethodInvestmentDividendsOrDistributions $ 89us-gaap_EquityMethodInvestmentDividendsOrDistributions $ 101us-gaap_EquityMethodInvestmentDividendsOrDistributions                                                                                        
Joint venture shareholders agreement period, years                 30 years                                                                            
Joint venture shareholders agreement, automatic extension additional period, years                 20 years                                                                            
Projected capacity amount of processed bauxite, in kmt                                                                               4,000aa_ProjectedCapacityAmountOfProcessedBauxite
/ us-gaap_StatementOperatingActivitiesSegmentAxis
= aa_BauxiteMiningMember		         1,800aa_ProjectedCapacityAmountOfProcessedBauxite
/ us-gaap_StatementOperatingActivitiesSegmentAxis
= aa_RefiningFacilitiesMember		 740aa_ProjectedCapacityAmountOfProcessedBauxite
/ us-gaap_StatementOperatingActivitiesSegmentAxis
= aa_PrimaryAluminumSmelterMember		 380aa_ProjectedCapacityAmountOfProcessedBauxite
/ us-gaap_StatementOperatingActivitiesSegmentAxis
= aa_RollingMillMember
Expanded capacity amount of processed bauxite, in Kmt     100aa_ExpandedCapacityAmountOfProcessedBauxite                                                                                        
Ownership interest in joint venture             25.10%aa_OwnershipInterestInJointVenture
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= aa_AlcoaJointVentureMember		   74.90%aa_OwnershipInterestInJointVenture
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= aa_MaadenJointVentureMember		                                                                            
Initial investment               80us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= aa_AlcoaJointVentureMember		                                                                              
Payment for pro rata share of pre-incorporation costs                 56aa_PaymentForProRataShareOfPreIncorporationCosts
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= aa_MaadenJointVentureMember		                                                                            
Ownership interest in joint venture at fair market value                   14.90%aa_OwnershipInterestInJointVentureAtFairMarketValue
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= aa_MaadenAlcoaJointVentureMember		                                                                          
Period Bracket To Exercise Option                   6 months                                                                          
Period To Open Option Exercise Bracket From Commercial Production Date                   5 years                                                                          
Outstanding receivable for labor and other employee-related expenses 30aa_OutstandingReceivableForLaborAndOtherEmployeeRelatedExpenses 31aa_OutstandingReceivableForLaborAndOtherEmployeeRelatedExpenses                                                                                          
Amount invested by Alcoa and Aluminum Financing Limited                   1,100us-gaap_LongTermPurchaseCommitmentAmount
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= aa_MaadenAlcoaJointVentureMember		                                       10,800us-gaap_LongTermPurchaseCommitmentAmount
/ us-gaap_InvestmentTypeAxis
= aa_MaadenAlcoaJointVentureMember		 40,500us-gaap_LongTermPurchaseCommitmentAmount
/ us-gaap_InvestmentTypeAxis
= aa_MaadenAlcoaJointVentureMember		                                
Capital investment commitment paid-to-date                   120us-gaap_PaymentsToAcquireEquityMethodInvestments
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= aa_MaadenAlcoaJointVentureMember		 171us-gaap_PaymentsToAcquireEquityMethodInvestments
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= aa_MaadenAlcoaJointVentureMember		                                     952us-gaap_PaymentsToAcquireEquityMethodInvestments
/ us-gaap_InvestmentTypeAxis
= aa_MaadenAlcoaJointVentureMember		                                  
Equity investments 1,780us-gaap_EquityMethodInvestments 1,777us-gaap_EquityMethodInvestments               983us-gaap_EquityMethodInvestments
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= aa_MaadenAlcoaJointVentureMember		 951us-gaap_EquityMethodInvestments
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= aa_MaadenAlcoaJointVentureMember		                                                                        
Capital investment       4,515us-gaap_CapitalizationLongtermDebtAndEquity
/ us-gaap_InvestmentSecondaryCategorizationAxis
= aa_MaadenAlcoaJointVentureSmelterAndRollingMillCompaniesMember		 1,992us-gaap_CapitalizationLongtermDebtAndEquity
/ us-gaap_InvestmentSecondaryCategorizationAxis
= aa_MaadenAlcoaJointVentureMineAndRefineryCompanyMember		 240us-gaap_CapitalizationLongtermDebtAndEquity
/ us-gaap_InvestmentSecondaryCategorizationAxis
= aa_MaadenAlcoaJointVentureMineAndRefineryCompanyMember		                         1,133us-gaap_CapitalizationLongtermDebtAndEquity
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= aa_MaadenAlcoaJointVentureSmelterAndRollingMillCompaniesMember		     500us-gaap_CapitalizationLongtermDebtAndEquity
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= aa_MaadenAlcoaJointVentureMineAndRefineryCompanyMember		 60us-gaap_CapitalizationLongtermDebtAndEquity
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= aa_MaadenAlcoaJointVentureMineAndRefineryCompanyMember		                                                
Debt service requirements, principal                                                       177us-gaap_DebtInstrumentPeriodicPaymentPrincipal
/ us-gaap_GuaranteeObligationsByNatureAxis
= us-gaap_FinancialGuaranteeMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= aa_MaadenAlcoaJointVentureSmelterAndRollingMillCompaniesMember		 120us-gaap_DebtInstrumentPeriodicPaymentPrincipal
/ us-gaap_GuaranteeObligationsByNatureAxis
= us-gaap_FinancialGuaranteeMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= aa_MaadenAlcoaJointVentureMineAndRefineryCompanyMember		                                    
Debt service requirements, interest maximum                                                       60us-gaap_DebtInstrumentPeriodicPaymentInterest
/ us-gaap_GuaranteeObligationsByNatureAxis
= us-gaap_FinancialGuaranteeMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= aa_MaadenAlcoaJointVentureSmelterAndRollingMillCompaniesMember		 30us-gaap_DebtInstrumentPeriodicPaymentInterest
/ us-gaap_GuaranteeObligationsByNatureAxis
= us-gaap_FinancialGuaranteeMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= aa_MaadenAlcoaJointVentureMineAndRefineryCompanyMember		                                    
Guarantee issued on behalf of smelting and rolling mill companies 394us-gaap_GuaranteeObligationsCurrentCarryingValue                                     8us-gaap_GuaranteeObligationsCurrentCarryingValue
/ us-gaap_BalanceSheetLocationAxis
= aa_OtherNoncurrentLiabilitiesAndDeferredCreditsMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= aa_MaadenAlcoaJointVentureSmelterAndRollingMillCompaniesMember		 10us-gaap_GuaranteeObligationsCurrentCarryingValue
/ us-gaap_BalanceSheetLocationAxis
= aa_OtherNoncurrentLiabilitiesAndDeferredCreditsMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= aa_MaadenAlcoaJointVentureSmelterAndRollingMillCompaniesMember		     4us-gaap_GuaranteeObligationsCurrentCarryingValue
/ us-gaap_BalanceSheetLocationAxis
= aa_OtherNoncurrentLiabilitiesAndDeferredCreditsMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= aa_MaadenAlcoaJointVentureMineAndRefineryCompanyMember		 4us-gaap_GuaranteeObligationsCurrentCarryingValue
/ us-gaap_BalanceSheetLocationAxis
= aa_OtherNoncurrentLiabilitiesAndDeferredCreditsMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= aa_MaadenAlcoaJointVentureMineAndRefineryCompanyMember		                                            
Percentage of amount required to be contributed in the event Alcoa would be required to make payments under the guarantees                                           40.00%aa_GuaranteeObligationPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= aa_MaadenAlcoaJointVentureMineAndRefineryCompanyMember		                                                  
Number of agencies needed to downgrade Alcoa's credit rating below investment grade in order to require Alcoa to provide a letter of credit or fund an escrow account                                                           2aa_NumberOfAgencies
/ us-gaap_InvestmentTypeAxis
= aa_MaadenAlcoaJointVentureMember		 2aa_NumberOfAgencies
/ us-gaap_InvestmentTypeAxis
= aa_MaadenAlcoaJointVentureMember		                                
Minimum percentage of equity commitment required                   67.00%aa_EquityMethodInvestmentContributionToJointVenturePercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= aa_MaadenAlcoaJointVentureMember		   67.00%aa_EquityMethodInvestmentContributionToJointVenturePercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= aa_MaadenAlcoaJointVentureMember		                                                                      
Potential forfeiture of letter of credit                                                   350aa_PotentialForfeitureOfLetterOfCredit
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= aa_SaudiArabiaJointVentureMember
/ us-gaap_StatementScenarioAxis
= aa_ScenarioOneMember		 60aa_PotentialForfeitureOfLetterOfCredit
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= aa_SaudiArabiaJointVentureMember
/ us-gaap_StatementScenarioAxis
= aa_ScenarioTwoMember		                                        
Joint venture agreement period                               20 years                                                              
Other investments $ 164us-gaap_OtherInvestments $ 130us-gaap_OtherInvestments                             $ 153us-gaap_OtherInvestments
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= us-gaap_FixedIncomeSecuritiesMember		 $ 153us-gaap_OtherInvestments
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= us-gaap_EquitySecuritiesMember