Derivatives and Other Financial Instruments - Schedule of Outstanding Forward Contracts that were Entered into Hedge Forecasted Transactions (Detail) (USD $) In Millions, unless otherwise specified				9 Months Ended
	Sep. 30, 2012 MMBtu	Sep. 30, 2012 Energy Contracts [Member] mwh	Dec. 31, 2011 Energy Contracts [Member] mwh	Sep. 30, 2012 Foreign Exchange Contract [Member]	Sep. 30, 2012 Aluminum Contracts [Member] MetricTon	Dec. 31, 2011 Aluminum Contracts [Member] MetricTon
Derivative Instruments, Gain (Loss) [Line Items]
Aluminum contracts (kmt)					1,343	1,294
Energy contracts (electricity-megawatt hours)		100,578,295	100,578,295
Natural gas (million British thermal units)	18,320,000
Foreign exchange contracts				$ 100