Derivatives and Other Financial Instruments - Schedule of Reconciliation of Activity for Derivative Contracts (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012		Sep. 30, 2012
Aluminum Contracts [Member] | Assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 10		$ 10
Sales	(2)		(7)
Cost of goods sold	(53)		(53)
Other expenses, net	(1)		(3)
Other comprehensive loss	1		8
Purchases, sales, issuances, and settlements*	639	[1]	639	[1]
Transfers into and (or) out of Level 3*		[1]		[1]
Foreign currency translation	21		21
Ending balance	615		615
Sales
Cost of goods sold
Other expenses, net	(1)		(3)
Aluminum Contracts [Member] | Liabilities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	530		602
Sales	(7)		(25)
Other comprehensive loss	151		97
Transfers into and (or) out of Level 3*		[1]		[1]
Ending balance	674		674
Sales
Cost of goods sold
Energy Contracts [Member] | Assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance			2
Other comprehensive loss			(2)
Transfers into and (or) out of Level 3*		[1]		[1]
Sales
Cost of goods sold
Energy Contracts [Member] | Liabilities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	10
Other comprehensive loss	(2)		8
Transfers into and (or) out of Level 3*		[1]		[1]
Ending balance	8		8
Sales
Cost of goods sold
Embedded Credit Derivative [Member] | Liabilities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	39		28
Cost of goods sold	(1)		(3)
Other expenses, net	(3)		10
Transfers into and (or) out of Level 3*		[1]		[1]
Ending balance	35		35
Sales
Cost of goods sold
Other expenses, net	$ (3)		$ 10

[1] In July 2012, two embedded derivatives contained within existing power contracts became subject to derivative accounting under GAAP (see below). The amount reflected in this table represents the initial fair value of these embedded derivatives and was classified as an issuance of Level 3 financial instruments. There were no purchases, sales or settlements of Level 3 financial instruments. Additionally, there were no transfers of financial instruments into or out of Level 3.