Derivatives and Other Financial Instruments - Additional Information (Detail) In Millions, unless otherwise specified	1 Months Ended		6 Months Ended							1 Months Ended
	Jul. 31, 2012 Contract Plant	Jan. 31, 2012	Jun. 30, 2013 USD ($) Kmt Contract	Jun. 30, 2013 CAD Kmt Contract	Jun. 30, 2013 Level 1 [Member] USD ($)	Dec. 31, 2012 Level 1 [Member] USD ($)	Jun. 30, 2013 Level 2 [Member] USD ($)	Dec. 31, 2012 Level 2 [Member] USD ($)	Jun. 30, 2013 Level 3 [Member] USD ($)	Jul. 31, 2012 Power Contracts One [Member]	Jul. 31, 2012 Power Contracts Two [Member]	Jan. 31, 2012 Interest Rate Swap [Member] USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net unrealized gain on derivative contracts									$ 2
Other derivative contracts estimated term of quoted market prices, in years			10 years	10 years
Number of contracts	2
Power contracts maturity date										Mar. 31, 2014	Mar. 31, 2016
Number of smelters	2
Derivative asset amount			596
Power Purchase Period forecasted by derivative instrument			Through December 2036	Through December 2036
Period of natural gas supply contract			6 years	6 years
Number of commodities involved for valuation of supply contract			2	2
Number of contracts ceased to qualify as fair value hedge			0	0
Amount of aluminum kmt designated as fair value hedges			446	446
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
Interest rate cash flow hedges			0
Forward contract to purchase to mitigate the foreign currency risk related to a Canadian-denominated loan due in 2014			56	58
Long-term debt, fair value					$ 7,982	$ 8,936	$ 521	$ 569