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Debt (Credit Facilities) - Additional Information (Detail)
 12 Months Ended 12 Months Ended 12 Months Ended
Dec. 31, 2014
USD ($)
Dec. 31, 2014
BRL
Dec. 31, 2013
USD ($)
Dec. 31, 2013
BRL
Dec. 31, 2012
Jul. 25, 2014
USD ($)
Dec. 31, 2014
Alcoa [Member]
USD ($)
Dec. 31, 2013
Alcoa [Member]
USD ($)
Dec. 31, 2012
Alcoa [Member]
Dec. 31, 2014
Credit Facility [Member]
USD ($)
Dec. 31, 2013
Credit Facility [Member]
USD ($)
Dec. 31, 2014
Minimum [Member]
Dec. 31, 2014
Maximum [Member]
Dec. 31, 2014
Revolving Credit Agreement [Member]
USD ($)
Dec. 31, 2013
Revolving Credit Agreement [Member]
USD ($)
Dec. 31, 2014
Commercial Paper [Member]
USD ($)
Dec. 31, 2014
Expected Maturity Date Year 2015 [Member]
USD ($)
Dec. 31, 2014
Expected Maturity Date Year 2016 [Member]
USD ($)
Debt Instrument [Line Items]                                    
Revolving credit facility, term                           5 years        
Line of credit facility, outstanding borrowings           $ 4,000,000,000us-gaap_LineOfCredit       $ 0us-gaap_LineOfCredit
/ us-gaap_CreditFacilityAxis
= aa_CreditFacilityMember		 $ 0us-gaap_LineOfCredit
/ us-gaap_CreditFacilityAxis
= aa_CreditFacilityMember		              
Limit of requested increases in lender commitments 1,040,000,000us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity         500,000,000us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity                     740,000,000us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
/ us-gaap_LongtermDebtTypeAxis
= aa_ExpectedMaturityDateYear2015Member		 300,000,000us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
/ us-gaap_LongtermDebtTypeAxis
= aa_ExpectedMaturityDateYear2016Member
Letters of credit sublimit under credit facility 1,000,000,000aa_LettersOfCreditSublimitUnderCreditFacility                                  
Debt instrument, maturity date July 25, 2019 July 25, 2019                       July 25, 2017        
Fees paid to maintain credit facility                               0.25%us-gaap_LineOfCreditFacilityCommitmentFeePercentage
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_CommercialPaperMember		    
Extension of maturity period 1 year 1 year                                
Applicable margin on LIBOR loans                       0.50%aa_ApplicableMarginOnLiborLoans
/ us-gaap_RangeAxis
= us-gaap_MinimumMember		 1.50%aa_ApplicableMarginOnLiborLoans
/ us-gaap_RangeAxis
= us-gaap_MaximumMember		          
Outstanding borrowings                               3,750,000,000us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_CommercialPaperMember		    
Agreement termination date Jul. 25, 2014 Jul. 25, 2014                             Dec. 31, 2015 Dec. 31, 2016
Amounts borrowed under the credit facility                   0us-gaap_ProceedsFromLongTermLinesOfCredit
/ us-gaap_CreditFacilityAxis
= aa_CreditFacilityMember		       0us-gaap_ProceedsFromLongTermLinesOfCredit
/ us-gaap_LongtermDebtTypeAxis
= aa_RevolvingCreditAgreementMember		 0us-gaap_ProceedsFromLongTermLinesOfCredit
/ us-gaap_LongtermDebtTypeAxis
= aa_RevolvingCreditAgreementMember		      
Line of credit facility, borrowing 1,640,000,000us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity   1,850,000,000us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity                              
Line of credit facility, repayment $ 20,000,000us-gaap_RepaymentsOfLinesOfCredit 47,000,000us-gaap_RepaymentsOfLinesOfCredit $ 22,000,000us-gaap_RepaymentsOfLinesOfCredit 47,000,000us-gaap_RepaymentsOfLinesOfCredit     $ 1,640,000,000us-gaap_RepaymentsOfLinesOfCredit
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember		 $ 1,850,000,000us-gaap_RepaymentsOfLinesOfCredit
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember		                    
Weighted-average interest rate 6.49%aa_WeightedAverageInterestRateOutstandingBorrowings 6.49%aa_WeightedAverageInterestRateOutstandingBorrowings 6.49%aa_WeightedAverageInterestRateOutstandingBorrowings 6.49%aa_WeightedAverageInterestRateOutstandingBorrowings     1.54%aa_WeightedAverageInterestRateOutstandingBorrowings
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember		 1.57%aa_WeightedAverageInterestRateOutstandingBorrowings
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember		 1.89%aa_WeightedAverageInterestRateOutstandingBorrowings
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember		                  
Weighted-average maturity days 67 days 67 days 213 days 213 days 260 days