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Income Taxes - Schedule of Changes in Valuation Allowance (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Balance at beginning of year	$ 1,804us-gaap_DeferredTaxAssetsValuationAllowance	$ 1,400us-gaap_DeferredTaxAssetsValuationAllowance	$ 1,398us-gaap_DeferredTaxAssetsValuationAllowance
Increase to allowance	117us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount	471us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount	45us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount
Release of allowance	(77)aa_DeferredTaxAssetsValuationReleaseOfAllowance	(41)aa_DeferredTaxAssetsValuationReleaseOfAllowance	(31)aa_DeferredTaxAssetsValuationReleaseOfAllowance
Acquisitions and divestitures (F)	(37)aa_DeferredTaxAssetsValuationAcquisitionsDivestitures
U.S. state tax apportionment and tax rate changes	(80)aa_ChangeInValuationAllowanceRelatedToDeferredStateIncomeTaxes	(32)aa_ChangeInValuationAllowanceRelatedToDeferredStateIncomeTaxes	(17)aa_ChangeInValuationAllowanceRelatedToDeferredStateIncomeTaxes
Foreign currency translation	(59)aa_DeferredTaxAssetsValuationForeignCurrencyTranslation	6aa_DeferredTaxAssetsValuationForeignCurrencyTranslation	5aa_DeferredTaxAssetsValuationForeignCurrencyTranslation
Balance at end of year	$ 1,668us-gaap_DeferredTaxAssetsValuationAllowance	$ 1,804us-gaap_DeferredTaxAssetsValuationAllowance	$ 1,400us-gaap_DeferredTaxAssetsValuationAllowance

X

- Definition

Change in valuation allowance related to deferred state income taxes.

+ References

No definition available.

+ Details

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Balance Type:	credit
Period Type:	duration

X

- Definition

Deferred tax assets valuation acquisitions divestitures.

+ References

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+ Details

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Namespace Prefix:	aa_
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Balance Type:	debit
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X

- Definition

Deferred tax assets valuation foreign currency translation.

+ References

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+ Details

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Namespace Prefix:	aa_
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Balance Type:	debit
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- Definition

Deferred tax assets valuation release of allowance.

+ References

No definition available.

+ Details

Name:	aa_DeferredTaxAssetsValuationReleaseOfAllowance
Namespace Prefix:	aa_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

Amount of deferred tax assets 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

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- Details

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X

- Definition

Amount of increase (decrease) in the valuation allowance for a specified deferred tax asset.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 45

-Paragraph 20

-URI http://asc.fasb.org/extlink&oid=37586315&loc=d3e32123-109318

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