Debt - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	1 Months Ended		3 Months Ended	9 Months Ended		1 Months Ended	3 Months Ended	9 Months Ended	3 Months Ended		3 Months Ended		3 Months Ended				3 Months Ended					1 Months Ended
	Aug. 31, 2012	Jun. 01, 2012	Sep. 30, 2012	Sep. 30, 2012	Sep. 30, 2011	Jan. 31, 2012 Aluminio [Member] Day	Sep. 30, 2012 Aluminio [Member] Day	Sep. 30, 2012 Aluminio [Member] Day	Sep. 30, 2012 Two Term Loan Agreements [Member]	Jan. 31, 2012 Two Term Loan Agreements [Member]	Sep. 30, 2012 First Loan Agreement [Member]	Jun. 30, 2012 First Loan Agreement [Member]	Sep. 30, 2012 Third Loan Agreement [Member]	Sep. 30, 2012 Revolving Credit Agreement [Member] Year	Feb. 29, 2012 Revolving Credit Agreement [Member]	Jan. 31, 2012 Revolving Credit Agreement [Member]	Sep. 30, 2012 Revolving Credit Agreement [Member] Two Term Loan Agreements [Member]	Sep. 30, 2012 Revolving Credit Agreement [Member] First Loan Agreement [Member]	Sep. 30, 2012 Revolving Credit Agreement [Member] Fourth Loan Agreement [Member]	Sep. 30, 2012 Revolving Credit Agreement [Member] Third Loan Agreement [Member]	Sep. 30, 2012 Revolving Credit Agreement [Member] Second Loan Agreement [Member]	Aug. 31, 2012 Bond Series 2012 [Member]	Sep. 30, 2012 Current Asset [Member]
Debt Instrument [Line Items]
Repayment of outstanding debt		$ 322		$ 793	$ 1,122		$ 5	$ 280
Outstanding debt, interest rate		6.00%
Line Of Credit Facility Amount Outstanding						280				350	100	150	150		100	100			100
Pension Contributions			163	515	217
Applicable Margin On LIBOR									1.50%		1.25%		1.375%
Line of credit facility, expiration date									Dec. 31, 2012								Jul. 30, 2012	Sep. 30, 2013	Sep. 30, 2013	Mar. 31, 2013	Dec. 31, 2012
Revolving credit facility, term														5
Weighted-average interest rate							2.32%
Weighted-average maturity days
Proceeds from issuance of bond	250			886	1,254
Debt instruments maturity date																						2042
Net proceeds																						248
Restricted cash			$ 212	$ 212																			$ 198
Loan agreement to redeem the Bonds, descriptions				Alcoa has the option through the loan agreement to redeem the Bonds, as a whole or in part, on or after August 1, 2022, on at least 30 days, but not more than 60 days, prior notice to the holders of the Bonds at a redemption price equal to 100% of the principal amount thereof, without premium, plus accrued interest, if any, to the redemption date.
Minimum number of days prior notice to redeem bonds				30 days
Maximum number of days prior notice to redeem bonds				60 days
Percentage of principal amount			100.00%	100.00%