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Derivatives and Other Financial Instruments - Additional Information (Detail) In Millions, unless otherwise specified | 1 Months Ended | 3 Months Ended |  |  |  |  | 1 Months Ended |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Jul. 31, 2012 Plant Contract | Jan. 31, 2012 USD ($) | Mar. 31, 2013 USD ($) Kmt Y | Mar. 31, 2013 CAD Kmt Y | Mar. 31, 2013 Level 3 [Member] USD ($) | Mar. 31, 2013 Level 1 [Member] USD ($) | Dec. 31, 2012 Level 1 [Member] USD ($) | Mar. 31, 2013 Level 2 [Member] USD ($) | Dec. 31, 2012 Level 2 [Member] USD ($) | Jul. 31, 2012 Power contracts one [Member] | Jul. 31, 2012 Power contracts two [Member] | Mar. 31, 2013 Six Year Natural Gas Supply Contract [Member] USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net unrealized gain on derivative contracts |  |  |  |  | $ 20 |  |  |  |  |  |  | $ 5
Other derivative contracts estimated term of quoted market prices, in years |  |  | 10 | 10 |  |  |  |  |  |  |  |
Number of contracts | 2 |  |  |  |  |  |  |  |  |  |  |
Power contracts maturity date |  |  |  |  |  |  |  |  |  | Mar. 31, 2014 | Mar. 31, 2016 |
Number of smelters | 2 |  |  |  |  |  |  |  |  |  |  |
Derivative asset amount |  |  | 596 |  |  |  |  |  |  |  |  |
Amount of aluminum kmt designated as fair value hedges |  |  | 430 | 430 |  |  |  |  |  |  |  |
Face amount of debt the hedges effectively convert from fixed to floating through 2018 |  |  | 200 |  |  |  |  |  |  |  |  |
Notional amount of interest rate swaps |  | 315 |  |  |  |  |  |  |  |  |  |
Long-term debt, interest rate |  | 6.00% |  |  |  |  |  |  |  |  |  |
Long-term debt, maturity date |  | 2012 |  |  |  |  |  |  |  |  |  |
Forward contract to purchase to mitigate the foreign currency risk related to a Canadian-denominated loan due in 2014 |  |  | 57 | 58 |  |  |  |  |  |  |  |
Long-term debt, fair value |  |  |  |  |  | $ 9,007 | $ 8,936 | $ 561 | $ 569 |  |  |

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- Definition Number Of Plants + References No definition available. + Details
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- Definition The amount of foreign currency that the entity has contractually agreed, or for which it has the option, to buy or sell with respect to the contract being described. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
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- Definition The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. + References No definition available. + Details
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