Derivatives and Other Financial Instruments - Gross Amounts of Recognized Derivative Assets and Liabilities and Gross Amounts Offset in Accompanying Consolidated Balance Sheet (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross amounts recognized	$ 111	$ 117
Gross amounts offset	(78)	(72)
Net amount presented in consolidated balance sheet	563	596
Gross amounts recognized	146	86
Gross amounts offset	(78)	(72)
Net amount presented in consolidated balance sheet	68	14
Aluminum contracts [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross amounts recognized	69	72
Gross amounts offset	(64)	(55)
Net amount presented in consolidated balance sheet	5	17
Gross amounts recognized	132	69
Gross amounts offset	(64)	(55)
Net amount presented in consolidated balance sheet	68	14
Interest rate contracts [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross amounts recognized	42	45
Gross amounts offset	(14)	(17)
Net amount presented in consolidated balance sheet	28	28
Gross amounts recognized	14	17
Gross amounts offset	(14)	(17)
Net amount presented in consolidated balance sheet