Derivatives and Other Financial Instruments - Schedule of Outstanding Forward Contracts that were Entered into Hedge Forecasted Transactions (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014 t	Dec. 31, 2013 t
Aluminum Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Aluminum contracts (000 metric tons)	707,000	841,000
Energy Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Electricity (megawatt hours)	59,409,328	59,409,328
Natural gas (million British thermal units)	20,570,000	19,980,000
Foreign Exchange Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Foreign exchange contracts	$ 632	$ 335