Derivatives and Other Financial Instruments - Schedule of Outstanding Forward Contracts that were Entered into Hedge Forecasted Transactions (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Energy contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Electricity (megawatt hours)	59,409,328	100,578,295
Natural gas (million British thermal units)	19,250,000	19,160,000
Foreign exchange contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Foreign exchange contracts	$ 86	$ 71
Aluminum contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Aluminum contracts (kmt)	1,085	1,120