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