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Income Taxes - Schedule of Expiration Periods of Deferred Tax Assets (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Tax Credit Carryforward [Line Items]
Tax loss carryforwards	$ 2,075us-gaap_OperatingLossCarryforwards
Tax credit carryforwards	625us-gaap_TaxCreditCarryforwardAmount
Other	3,557us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsOther
Valuation allowance	(1,668)us-gaap_TaxCreditCarryforwardValuationAllowance
Deferred tax assets, net	4,589us-gaap_DeferredTaxAssetsNet	4,584us-gaap_DeferredTaxAssetsNet
Expires Within 10 Years [Member]
Tax Credit Carryforward [Line Items]
Tax loss carryforwards	330us-gaap_OperatingLossCarryforwards / us-gaap_ValuationAllowancesAndReservesTypeAxis = aa_ExpiresWithinTenYearsMember
Tax credit carryforwards	428us-gaap_TaxCreditCarryforwardAmount / us-gaap_ValuationAllowancesAndReservesTypeAxis = aa_ExpiresWithinTenYearsMember
Valuation allowance	(341)us-gaap_TaxCreditCarryforwardValuationAllowance / us-gaap_ValuationAllowancesAndReservesTypeAxis = aa_ExpiresWithinTenYearsMember
Deferred tax assets, net	417us-gaap_DeferredTaxAssetsNet / us-gaap_ValuationAllowancesAndReservesTypeAxis = aa_ExpiresWithinTenYearsMember
Expires Within 11-20 Years [Member]
Tax Credit Carryforward [Line Items]
Tax loss carryforwards	619us-gaap_OperatingLossCarryforwards / us-gaap_ValuationAllowancesAndReservesTypeAxis = aa_ExpiresWithinElevenToTwentyYearsMember
Tax credit carryforwards	86us-gaap_TaxCreditCarryforwardAmount / us-gaap_ValuationAllowancesAndReservesTypeAxis = aa_ExpiresWithinElevenToTwentyYearsMember
Valuation allowance	(645)us-gaap_TaxCreditCarryforwardValuationAllowance / us-gaap_ValuationAllowancesAndReservesTypeAxis = aa_ExpiresWithinElevenToTwentyYearsMember
Deferred tax assets, net	60us-gaap_DeferredTaxAssetsNet / us-gaap_ValuationAllowancesAndReservesTypeAxis = aa_ExpiresWithinElevenToTwentyYearsMember
No Expiration [Member]
Tax Credit Carryforward [Line Items]
Tax loss carryforwards	1,126us-gaap_OperatingLossCarryforwards / us-gaap_ValuationAllowancesAndReservesTypeAxis = aa_NoExpirationMember
Tax credit carryforwards	111us-gaap_TaxCreditCarryforwardAmount / us-gaap_ValuationAllowancesAndReservesTypeAxis = aa_NoExpirationMember
Other	488us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsOther / us-gaap_ValuationAllowancesAndReservesTypeAxis = aa_NoExpirationMember
Valuation allowance	(392)us-gaap_TaxCreditCarryforwardValuationAllowance / us-gaap_ValuationAllowancesAndReservesTypeAxis = aa_NoExpirationMember
Deferred tax assets, net	1,333us-gaap_DeferredTaxAssetsNet / us-gaap_ValuationAllowancesAndReservesTypeAxis = aa_NoExpirationMember
Other [Member]
Tax Credit Carryforward [Line Items]
Other	3,069us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsOther / us-gaap_ValuationAllowancesAndReservesTypeAxis = aa_OtherDeferredTaxAssetsMember
Valuation allowance	(290)us-gaap_TaxCreditCarryforwardValuationAllowance / us-gaap_ValuationAllowancesAndReservesTypeAxis = aa_OtherDeferredTaxAssetsMember
Deferred tax assets, net	$ 2,779us-gaap_DeferredTaxAssetsNet / us-gaap_ValuationAllowancesAndReservesTypeAxis = aa_OtherDeferredTaxAssetsMember

X

- Definition

Amount after allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences and carryforwards.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (b),(c)

-URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32537-109319

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

Amount before allocation of valuation allowances of deferred tax asset attributable to other deductible tax credit carryforwards not separately disclosed.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 50

-Paragraph 6

-URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32621-109319



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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 50

-Paragraph 8

-URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32632-109319



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 50

-Paragraph 3

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32559-109319

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Namespace Prefix:	us-gaap_
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Period Type:	instant

X

- Definition

Amount of operating loss carryforward, before tax effects, available to reduce future taxable income under enacted tax laws.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 50

-Paragraph 3

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32559-109319

+ Details

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Namespace Prefix:	us-gaap_
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Balance Type:	debit
Period Type:	instant

X

- Definition

The amount of the tax credit carryforward, before tax effects, available to reduce future taxable income under enacted tax laws.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 50

-Paragraph 3

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32559-109319



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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Carryforwards

-URI http://asc.fasb.org/extlink&oid=6506874

+ Details

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X

- Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Amount of valuation allowance pertaining to the deferred tax asset representing potential future taxable deductions from tax credit carryforwards for which it is more likely than not that a tax benefit will not be realized.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32537-109319



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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Valuation Allowance

-URI http://asc.fasb.org/extlink&oid=6528051

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X

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Namespace Prefix:
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X

- Details

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