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Pension and Other Postretirement Benefits - Schedule of Reconciliation of Activity for Investments (Detail) (Pension and Other Postretirement Benefit Plans' Assets [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Pension and Other Postretirement Benefit Plans' Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Balance at beginning of year	$ 3,421us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = aa_PensionAndOtherPostretirementBenefitPlansAssetsMember	$ 3,119us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = aa_PensionAndOtherPostretirementBenefitPlansAssetsMember
Realized gains	180us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1 / us-gaap_FairValueByFairValueHierarchyLevelAxis = aa_PensionAndOtherPostretirementBenefitPlansAssetsMember	140us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1 / us-gaap_FairValueByFairValueHierarchyLevelAxis = aa_PensionAndOtherPostretirementBenefitPlansAssetsMember
Unrealized gains	146us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss / us-gaap_FairValueByFairValueHierarchyLevelAxis = aa_PensionAndOtherPostretirementBenefitPlansAssetsMember	173us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss / us-gaap_FairValueByFairValueHierarchyLevelAxis = aa_PensionAndOtherPostretirementBenefitPlansAssetsMember
Purchases	868us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases / us-gaap_FairValueByFairValueHierarchyLevelAxis = aa_PensionAndOtherPostretirementBenefitPlansAssetsMember	636us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases / us-gaap_FairValueByFairValueHierarchyLevelAxis = aa_PensionAndOtherPostretirementBenefitPlansAssetsMember
Sales	(768)aa_FairValueAssetsMeasuredOnRecurringBasisGainLossIncludedInSales / us-gaap_FairValueByFairValueHierarchyLevelAxis = aa_PensionAndOtherPostretirementBenefitPlansAssetsMember	(626)aa_FairValueAssetsMeasuredOnRecurringBasisGainLossIncludedInSales / us-gaap_FairValueByFairValueHierarchyLevelAxis = aa_PensionAndOtherPostretirementBenefitPlansAssetsMember
Issuances	0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetIssues / us-gaap_FairValueByFairValueHierarchyLevelAxis = aa_PensionAndOtherPostretirementBenefitPlansAssetsMember	0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetIssues / us-gaap_FairValueByFairValueHierarchyLevelAxis = aa_PensionAndOtherPostretirementBenefitPlansAssetsMember
Settlements	0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements / us-gaap_FairValueByFairValueHierarchyLevelAxis = aa_PensionAndOtherPostretirementBenefitPlansAssetsMember	0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements / us-gaap_FairValueByFairValueHierarchyLevelAxis = aa_PensionAndOtherPostretirementBenefitPlansAssetsMember
Acquisitions	117aa_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAcquisitions / us-gaap_FairValueByFairValueHierarchyLevelAxis = aa_PensionAndOtherPostretirementBenefitPlansAssetsMember
Foreign currency translation impact	(39)aa_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetExchangeRate / us-gaap_FairValueByFairValueHierarchyLevelAxis = aa_PensionAndOtherPostretirementBenefitPlansAssetsMember	(21)aa_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetExchangeRate / us-gaap_FairValueByFairValueHierarchyLevelAxis = aa_PensionAndOtherPostretirementBenefitPlansAssetsMember
Transfers in and/or out of Level 3	0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersNet / us-gaap_FairValueByFairValueHierarchyLevelAxis = aa_PensionAndOtherPostretirementBenefitPlansAssetsMember	0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersNet / us-gaap_FairValueByFairValueHierarchyLevelAxis = aa_PensionAndOtherPostretirementBenefitPlansAssetsMember
Balance at end of year	$ 3,925us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = aa_PensionAndOtherPostretirementBenefitPlansAssetsMember	$ 3,421us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = aa_PensionAndOtherPostretirementBenefitPlansAssetsMember

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Fair value assets measured on recurring basis gain loss included in sales.

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+ References

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-Name Accounting Standards Codification

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Amount of settlement of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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