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Derivatives and Other Financial Instruments - Additional Information (Detail) In Millions, unless otherwise specified | 12 Months Ended |  |  |  | 1 Months Ended | 12 Months Ended |  |  |  |  |  |  |
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Dec. 31, 2012 USD ($) Kmt Y | Dec. 31, 2011 USD ($) | Dec. 31, 2010 USD ($) | Jul. 30, 2012 Contract | Jan. 31, 2012 USD ($) | Dec. 31, 2011 CAD | Jul. 30, 2012 Power contracts one [Member] | Jul. 30, 2012 Power contracts two [Member] | Dec. 31, 2012 6% Notes, due 2012 [Member] | Jan. 31, 2012 6% Notes, due 2012 [Member] | Dec. 31, 2012 Six Year Natural Gas Supply Contract [Member] USD ($) | Dec. 31, 2012 Level 3 [Member] USD ($) | Dec. 31, 2011 Level 3 [Member] USD ($) | Dec. 31, 2012 Level 1 [Member] USD ($) | Dec. 31, 2011 Level 1 [Member] USD ($) | Dec. 31, 2012 Level 2 [Member] USD ($) | Dec. 31, 2011 Level 2 [Member] USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net unrealized loss on derivative contracts |  |  |  |  |  |  |  |  |  |  | $ 5 | $ 88 | $ 618 |  |  |  |
Other derivative contracts estimated term of quoted market prices, in years | 10 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of contracts |  |  |  | 2 |  |  |  |  |  |  |  |  |  |  |  |  |
Power contracts maturity date |  |  |  |  |  |  | Dec. 31, 2014 | Dec. 31, 2016 |  |  |  |  |  |  |  |  |
Derivative asset amount | 596 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount of aluminum kmt designated as fair value hedges | 386 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Face amount of debt the hedges effectively convert from fixed to floating through 2018 | 200 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Notional amount interest rate swaps | 550 | 550 | 825 |  | 315 |  |  |  |  |  |  |  |  |  |  |  |
Gain on termination of interest rate swaps |  | 33 | 28 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Notes interest rate | 4.75% |  |  |  | 6.00% |  |  |  | 6.00% | 6.00% |  |  |  |  |  |  |
Notes maturity date |  |  |  |  |  |  |  |  | 2012 |  |  |  |  |  |  |  |
Contracts expiration date | September 2011 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Forward contract to purchase to mitigate the foreign currency risk related to a Canadian-denominated loan due in 2014 | 58 |  |  |  |  | 58 |  |  |  |  |  |  |  |  |  |  |
Fair value hierarchy for public debt |  |  |  |  |  |  |  |  |  |  |  |  |  | (422) |  |  |
Fair value hierarchy for non-public debt |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (43) |
Long-term debt, fair value |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 8,456 | $ 8,576 | $ 572 | $ 698

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