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Derivatives and Other Financial Instruments - Schedule of Reconciliation of Activity for Derivative Contracts (Detail) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2012 | Sep. 30, 2012
Aluminum Contracts [Member] | Assets |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |
Beginning balance | $ 10 |  | $ 10 |
Sales | (2) |  | (7) |
Cost of goods sold | (53) |  | (53) |
Other expenses, net | (1) |  | (3) |
Other comprehensive loss | 1 |  | 8 |
Purchases, sales, issuances, and settlements* | 639 | [1] | 639 | [1]
Transfers into and (or) out of Level 3* |  | [1] |  | [1]
Foreign currency translation | 21 |  | 21 |
Ending balance | 615 |  | 615 |
Sales |  |  |  |
Cost of goods sold |  |  |  |
Other expenses, net | (1) |  | (3) |
Aluminum Contracts [Member] | Liabilities |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |
Beginning balance | 530 |  | 602 |
Sales | (7) |  | (25) |
Other comprehensive loss | 151 |  | 97 |
Transfers into and (or) out of Level 3* |  | [1] |  | [1]
Ending balance | 674 |  | 674 |
Sales |  |  |  |
Cost of goods sold |  |  |  |
Energy Contracts [Member] | Assets |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |
Beginning balance |  |  | 2 |
Other comprehensive loss |  |  | (2) |
Transfers into and (or) out of Level 3* |  | [1] |  | [1]
Sales |  |  |  |
Cost of goods sold |  |  |  |
Energy Contracts [Member] | Liabilities |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |
Beginning balance | 10 |  |  |
Other comprehensive loss | (2) |  | 8 |
Transfers into and (or) out of Level 3* |  | [1] |  | [1]
Ending balance | 8 |  | 8 |
Sales |  |  |  |
Cost of goods sold |  |  |  |
Embedded Credit Derivative [Member] | Liabilities |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |
Beginning balance | 39 |  | 28 |
Cost of goods sold | (1) |  | (3) |
Other expenses, net | (3) |  | 10 |
Transfers into and (or) out of Level 3* |  | [1] |  | [1]
Ending balance | 35 |  | 35 |
Sales |  |  |  |
Cost of goods sold |  |  |  |
Other expenses, net | $ (3) |  | $ 10 |
[1] | In July 2012, two embedded derivatives contained within existing power contracts became subject to derivative accounting under GAAP (see below). The amount reflected in this table represents the initial fair value of these embedded derivatives and was classified as an issuance of Level 3 financial instruments. There were no purchases, sales or settlements of Level 3 financial instruments. Additionally, there were no transfers of financial instruments into or out of Level 3.

X
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- Definition Amount of purchases, sales, issues, and settlements (net) which have taken place in relation to assets measured at fair value on a recurring basis using unobservable inputs (level 3). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 157 -Paragraph 32 -Subparagraph c(2) -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
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