Derivatives and Other Financial Instruments - Schedule of Outstanding Forward Contracts that were Entered into Hedge Forecasted Transactions (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013 Btu MWh	Dec. 31, 2012 Btu MWh
Energy Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Electricity (megawatt hours)	59,409,328	100,578,295
Natural gas (million British thermal units)	19,010,000	19,160,000
Foreign Exchange Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Foreign exchange contracts	$ 89	$ 71
Aluminum Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Aluminum contracts (kmt)	1,108	1,120