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Properties, Plants, and Equipment, Net - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014 kt	Dec. 31, 2013 kt
Smelting Assets [Member]
Property, Plant and Equipment [Line Items]
Net carrying value of assets	$ 419aa_NetCarryingValueOfAssets / us-gaap_ChangeInAccountingEstimateByTypeAxis = aa_SmeltingAssetsMember	$ 404aa_NetCarryingValueOfAssets / us-gaap_ChangeInAccountingEstimateByTypeAxis = aa_SmeltingAssetsMember
Idle capacity of assets, units	665aa_IdleCapacityOfAsset / us-gaap_ChangeInAccountingEstimateByTypeAxis = aa_SmeltingAssetsMember	655aa_IdleCapacityOfAsset / us-gaap_ChangeInAccountingEstimateByTypeAxis = aa_SmeltingAssetsMember
Idled Refining Assets [Member]
Property, Plant and Equipment [Line Items]
Net carrying value of assets	$ 62aa_NetCarryingValueOfAssets / us-gaap_ChangeInAccountingEstimateByTypeAxis = aa_IdledRefiningMember	$ 60aa_NetCarryingValueOfAssets / us-gaap_ChangeInAccountingEstimateByTypeAxis = aa_IdledRefiningMember
Idle capacity of assets, units	1,216aa_IdleCapacityOfAsset / us-gaap_ChangeInAccountingEstimateByTypeAxis = aa_IdledRefiningMember	1,216aa_IdleCapacityOfAsset / us-gaap_ChangeInAccountingEstimateByTypeAxis = aa_IdledRefiningMember

X

- Definition

Idle capacity of asset.

+ References

No definition available.

+ Details

Name:	aa_IdleCapacityOfAsset
Namespace Prefix:	aa_
Data Type:	num:massItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Net carrying value of assets.

+ References

No definition available.

+ Details

Name:	aa_NetCarryingValueOfAssets
Namespace Prefix:	aa_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Details

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

X

- Details

Name:	us-gaap_ChangeInAccountingEstimateByTypeAxis=aa_IdledRefiningMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_ChangeInAccountingEstimateByTypeAxis=aa_SmeltingAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: