Derivatives and Other Financial Instruments - Schedule of Derivative Contract Assets and Liabilities that are Measured and Recognized at Fair Value on Recurring Basis (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value asset derivatives	$ 688		$ 94
Fair value liability derivatives	769	[1]	737	[1]
Level 1 [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value asset derivatives	48		57
Fair value liability derivatives	20		64
Level 2 [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value asset derivatives	48		47
Fair value liability derivatives	32		44
Level 3 [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value asset derivatives	615		12
Fair value liability derivatives	717		630
Margin Held [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Margin held	(23)		(22)
Less Margin Posted [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Margin posted			$ (1)

[1] All margin posted is in the form of cash and is valued under a Level 1 technique. The levels that correspond to the margin posted in the table above reference the level of the corresponding liability for which it is posted. Alcoa elected to net the margin posted against the fair value amounts recognized for derivative instruments executed with the same counterparties under master netting arrangements.