Derivatives And Other Financial Instruments - Additional Information (Detail) In Millions, unless otherwise specified	9 Months Ended				1 Months Ended		9 Months Ended
	Sep. 30, 2012 USD ($) MetricTon	Jul. 30, 2012 USD ($) Contract	Dec. 31, 2009 USD ($)	Dec. 31, 2009 CAD	Jul. 30, 2012 Power Contract One [Member]	Jul. 30, 2012 Power Contract Two [Member]	Sep. 30, 2012 Six Year Natural Gas Supply Contract [Member] USD ($)	Sep. 30, 2012 Level 3 [Member] USD ($)	Sep. 30, 2012 Level 1 [Member] USD ($)	Dec. 31, 2011 Level 1 [Member] USD ($)	Sep. 30, 2012 Level 2 [Member] USD ($)	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	$ 102
Number of contract		2
Power contract maturity date					Dec. 31, 2014	Dec. 31, 2016
Derivative asset amount		639
Supply contract duration							6 years
Amount of aluminum kmt designated as fair value hedges	401
Face amount of debt the hedges effectively convert from fixed to floating through 2018	200
Contracts expiration date	September 2011
Forward contract to purchase to mitigate the foreign currency risk related to a Canadian-denominated loan due in 2014			59	58
Fair value hierarchy for public debt									(422)
Fair value hierarchy for non-public debt											(118)
Long-term debt, fair value									$ 8,638	$ 8,576	$ 615	$ 698