Derivatives and Other Financial Instruments - Additional Information (Detail) In Millions, unless otherwise specified	12 Months Ended						1 Months Ended		12 Months Ended
	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