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Pension and Other Postretirement Benefits - Schedule of Pension and Postretirement Plans Investment Policy and Weighted Average Asset Allocations (Detail)	Dec. 31, 2014	Dec. 31, 2013
Equity Securities [Member] | Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, asset allocations	55.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember / us-gaap_RangeAxis = us-gaap_MaximumMember
Equity Securities [Member] | Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, asset allocations	20.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember / us-gaap_RangeAxis = us-gaap_MinimumMember
Fixed Income Securities [Member] | Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, asset allocations	55.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_RangeAxis = us-gaap_MaximumMember
Fixed Income Securities [Member] | Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, asset allocations	25.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_RangeAxis = us-gaap_MinimumMember
Other Investments [Member] | Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, asset allocations	55.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherInvestmentsMember / us-gaap_RangeAxis = us-gaap_MaximumMember
Other Investments [Member] | Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, asset allocations	15.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherInvestmentsMember / us-gaap_RangeAxis = us-gaap_MinimumMember
Plan Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, asset allocations	100.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = aa_PlanAssetsMember	100.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = aa_PlanAssetsMember
Plan Assets [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, asset allocations	33.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = aa_PlanAssetsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember	37.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = aa_PlanAssetsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember
Plan Assets [Member] | Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, asset allocations	45.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = aa_PlanAssetsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedIncomeSecuritiesMember	41.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = aa_PlanAssetsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedIncomeSecuritiesMember
Plan Assets [Member] | Other Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, asset allocations	22.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = aa_PlanAssetsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherInvestmentsMember	22.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = aa_PlanAssetsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherInvestmentsMember

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- Definition

Percentage of fair value of investments (categorized by debt securities, equity securities, real estate and other plan assets) to the fair value of plan assets held.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 5

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e2446-114920



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (d)(1)

-URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920

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