XML 24 R37.htm IDEA: XBRL DOCUMENT
v2.4.0.6

Inventories - Additional Information (Detail) (USD $) In Millions, unless otherwise specified | Mar. 31, 2013 | Dec. 31, 2012
--- | --- | ---
Inventories Net [Line Items] |  |
Percentage inventories valued on a last in, first out (LIFO) basis | 36.00% | 35.00%
Total inventories valued on an average-cost basis | $ 773 | $ 770

X
- Details
Name: | aa_InventoriesNetLineItems
Namespace Prefix: | aa_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition The percentage of LIFO (last in first out) inventory to total inventory as of the balance sheet date if other than 100 percent. + References No definition available. + Details
Name: | us-gaap_PercentageOfLIFOInventory
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition The amount of weighted average cost inventory present at the reporting date when inventory is valued using different valuation methods. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6361739&loc=d3e7789-107766 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Accounting Research Bulletin (ARB) -Number 43 -Section A -Paragraph 9 -Chapter 3 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. + Details
Name: | us-gaap_WeightedAverageCostInventoryAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant