Contingencies And Commitments - Additional Information (Detail) In Millions, unless otherwise specified	1 Months Ended		3 Months Ended		9 Months Ended			12 Months Ended				9 Months Ended		1 Months Ended			1 Months Ended			1 Months Ended	3 Months Ended		9 Months Ended	12 Months Ended		3 Months Ended		12 Months Ended					3 Months Ended	9 Months Ended			12 Months Ended			9 Months Ended			12 Months Ended	9 Months Ended						9 Months Ended					12 Months Ended			3 Months Ended		9 Months Ended		18 Months Ended			3 Months Ended	9 Months Ended	3 Months Ended				9 Months Ended	12 Months Ended								3 Months Ended		9 Months Ended						1 Months Ended	9 Months Ended	1 Months Ended	12 Months Ended	0 Months Ended
	Jan. 31, 2007 Day	Nov. 30, 2006 Day	Sep. 30, 2012 USD ($)	Sep. 30, 2011 USD ($)	Sep. 30, 2012 USD ($) mwh MW	Sep. 30, 2012 EUR (€)	Sep. 30, 2011 USD ($)	Dec. 31, 2011 USD ($)	Nov. 08, 2011 Day	Sep. 30, 2012 Non Current Liabilities And Deferred Credits [Member] USD ($)	Dec. 31, 2011 Parent Company [Member] USD ($)	Sep. 30, 2012 Alcoa Aluminio [Member] USD ($)	Sep. 30, 2012 Alcoa Aluminio [Member] BRL	Jul. 31, 2006 Italian Government [Member] Year	Nov. 19, 2009 Italian Government [Member] USD ($)	Nov. 19, 2009 Italian Government [Member] EUR (€)	Jan. 31, 2007 Spanish Government [Member] Year	Jun. 30, 2003 Massena Ny [Member] Year	Sep. 30, 2012 Massena Ny [Member] USD ($)	Nov. 30, 2008 Grasse River [Member] USD ($)	Sep. 30, 2012 Grasse River [Member] USD ($)	Jun. 30, 2012 Grasse River [Member] USD ($)	Sep. 30, 2012 Grasse River [Member] USD ($)	Dec. 31, 2010 Grasse River [Member] USD ($)	Mar. 24, 2010 Grasse River [Member] USD ($)	Sep. 30, 2012 Smelter Sites [Member] USD ($)	Sep. 30, 2012 East St Louis Site [Member] USD ($)	Dec. 31, 2005 East St Louis Site [Member] USD ($)	Dec. 31, 2009 Portovesme Site [Member] USD ($)	Dec. 31, 2010 Smelting And Rolling Mill Companies [Member] USD ($)	Sep. 30, 2012 Smelting And Rolling Mill Companies [Member] Non Current Liabilities And Deferred Credits [Member] USD ($)	Dec. 31, 2011 Smelting And Rolling Mill Companies [Member] Non Current Liabilities And Deferred Credits [Member] USD ($)	Sep. 30, 2012 Alcoa Joint Venture [Member] USD ($)	Sep. 30, 2012 Alcoa Joint Venture [Member] USD ($) Year	Sep. 30, 2012 Alcoa Joint Venture [Member] SAR	Dec. 31, 2011 Alcoa Joint Venture [Member] USD ($)	Dec. 31, 2011 Refining And Mining Company [Member] USD ($)	Sep. 30, 2012 Maaden Joint Venture [Member]	Dec. 31, 2011 Maaden Joint Venture [Member]	Sep. 30, 2012 Alumina Limited [Member]	Jan. 31, 2007 Tariffs Granted By Spain [Member] USD ($)	Jan. 31, 2007 Tariffs Granted By Spain [Member] EUR (€)	Dec. 31, 2009 Fusina Site [Member] USD ($)	Sep. 30, 2012 Machadinho [Member]	Sep. 30, 2012 Barra Grande [Member]	Sep. 30, 2012 Machadinho And Barra Grande [Member] USD ($)	Sep. 30, 2012 Machadinho And Barra Grande [Member] BRL	Dec. 31, 2011 Machadinho And Barra Grande [Member] USD ($)	Dec. 31, 2011 Machadinho And Barra Grande [Member] BRL	Sep. 30, 2012 Serra Do Facao Project [Member] USD ($)	Sep. 30, 2012 Serra Do Facao Project [Member] ZAR	Sep. 30, 2012 Serra Do Facao Project [Member] BRL	Dec. 31, 2011 Serra Do Facao Project [Member] USD ($)	Dec. 31, 2011 Serra Do Facao Project [Member] BRL	Dec. 31, 2007 Serra Do Facao Project [Member] Alcoa Aluminio [Member]	Dec. 31, 2004 Dampier Gas Pipeline [Member] USD ($)	Dec. 31, 2004 Dampier Gas Pipeline [Member] AUD	Sep. 30, 2012 Dampier Gas Pipeline [Member] Three Year Equity Call Plan [Member] USD ($)	Sep. 30, 2012 Dampier Gas Pipeline [Member] Three Year Equity Call Plan [Member] AUD	Sep. 30, 2012 Dampier Gas Pipeline [Member] Three Year Equity Call Plan [Member] USD ($)	Sep. 30, 2012 Dampier Gas Pipeline [Member] Three Year Equity Call Plan [Member] AUD	Sep. 30, 2012 Dampier Gas Pipeline [Member] Three Year Equity Call Plan [Member] USD ($)	Sep. 30, 2012 Dampier Gas Pipeline [Member] Three Year Equity Call Plan [Member] AUD	Sep. 30, 2012 Sherwin, TX [Member] USD ($)	Sep. 30, 2012 Other Sites [Member] USD ($)	Sep. 30, 2012 Other Sites [Member] USD ($)	Sep. 30, 2012 Baie Comeau [Member] USD ($)	Sep. 30, 2012 Mosjoen, Norway [Member] USD ($)	Jun. 30, 2012 Estreito Project [Member] USD ($)	Jun. 30, 2012 Estreito Project [Member] BRL	Sep. 30, 2012 Estreito Project [Member] mwh	Dec. 31, 2011 Estreito Project [Member] USD ($)	Dec. 31, 2011 Estreito Project [Member] ZAR	Dec. 31, 2010 Estreito Project [Member] USD ($)	Dec. 31, 2010 Estreito Project [Member] ZAR	Dec. 31, 2006 Estreito Project [Member] Alcoa Aluminio [Member]	Dec. 31, 2006 Estreito Hydroelectric Power Project [Member] mwh	Sep. 30, 2012 Minimum [Member] USD ($)	Sep. 30, 2012 Maximum [Member] USD ($)	Sep. 30, 2012 Alba [Member] USD ($)	Jun. 30, 2012 Alba [Member] USD ($)	Sep. 30, 2012 Alba [Member] USD ($)	Feb. 27, 2008 Alba [Member] Year	Sep. 30, 2012 Alba [Member] Minimum [Member] USD ($)	Sep. 30, 2012 Alba [Member] Maximum [Member] USD ($)	Sep. 30, 2012 Canada And Norway [Member] Massena Ny [Member] USD ($)	May 31, 2011 Breaches Of Warranty [Member] USD ($)	May 31, 2011 Negligence Award [Member] USD ($)	Sep. 30, 2012 Recurring Costs Of Managing Hazardous Substances And Environmental Programs [Member]	Nov. 30, 2006 Curtis Vs Alcoa [Member] Person	Dec. 31, 2011 Scrg Brownfield Recovery Corp Energy Answers Corporation Of Puerto Rico [Member] USD ($)	Jan. 20, 2011 Scrg Brownfield Recovery Corp Energy Answers Corporation Of Puerto Rico [Member] Breaches Of Warranty [Member] USD ($)	Jan. 20, 2011 Scrg Brownfield Recovery Corp Energy Answers Corporation Of Puerto Rico [Member] Punitive Damages [Member] USD ($)	Jan. 20, 2011 Scrg Brownfield Recovery Corp Energy Answers Corporation Of Puerto Rico [Member] Property Damages [Member] USD ($)
Loss Contingencies [Line Items]
Period of alleged conspiracy, years																																																																																			15
Damages claimed, value																																																																																$ 1,000
Number of days required to dismiss lawsuit									30
Cash payment for suit settlement																																																																																45		40
Restructuring and other charges			2	9	27		49	130																																																																								45	45
Restructuring and other charges, after-tax and noncontrolling interests			2	5	19		26																																																																									18	18	15
Alcoa's maximum exposure to loss in projects			375		375	303						200	400																												90	70				280	560											520	500	520	500	520	500																	75		75
Contingency settlement agreement amount, total																																																																																		85
Contingency settlement agreement amount paid																																																																																42.5		42.5
Contingency settlement agreement amount payable																																																																																42.5		42.5
Contingency settlement agreement future payable date																																																																																		October 9, 2013
Percentage of equity interest																																																																																		40.00%
Estimated contingency settlement changes, net of tax																																																																																				25	30
Retired former employees involved in class action																																																																																										13,000
Trial length, number of days		8
Court granted motion for judgment																																																																																								10			20	13	6	10
Damages awarded to plaintiff, pre-judgment interest																																																																																							2
Management estimate for maximum exposure from class action			110		110	85
Provision for contract losses						39																																																																								0	50
Number of years operating under a power supply structure														10			9
Charge related to European commission announcement															250	173
Write-off of receivable															20	14
Establishment reserve															230	159
Recovery amount					375	303
Reduction in recovery amount			65		65	53
Payments to Italian government					310	250
Payments to Italian government, installment amount					62	50
Number of installments, litigation payment					5	5
Litigation reserve			204		204	159
Range of reasonable possible loss, minimum			204		204	159
Days provided to submit observations and comments to the EC	30
Number of cleanup locations					More than 100	More than 100
Remediation reserve balance			535		535			347																																																								30
Remediation reserve balance, classified as a current liability			63		63			58
Remediation reserve increase			174		204															40	128	37	1	2		30	14	15	3														12																						1	10	25	20
Remediation reserve adjustment																			128						17	45	14																																																											165
Payments related to remediation expenses applied against the reserve			5		16
Increase in reserves due to effects of foreign currency translation			1
Percentage of cost of goods sold																																																																																									2.00%
Period of time between potential phenomenon occurrences, years																		10
Ownership interest issued as guarantee for smelting and rolling mill companies																																	25.10%	25.10%		25.10%		74.90%	74.90%
Capital investment			1,831		1,831			1,626																						4,035			10,800	10,800	40,500		1,992
Amount invested by Alcoa and Aluminum Financing Limited																																	1,100	1,100
Term of investment by Alcoa and Aluminum Financing Limited, years																																		4
Capital investment commitment paid-to-date																																	611	611
Capital investment commitment paid in period																																	37	202
Equity method investment, carrying value																																	774	774		565
Guarantee issued on behalf of smelting and rolling mill companies										5	1,013																				9	8					500
Debt service requirements, principal																														108							60
Debt service requirements, interest maximum																														50							25
Ownership interest in joint venture at fair market value																																								40.00%
Investment percentage																																												30.99%	42.18%											20.00%	20.00%
Total investment in project																																														252	513	266	496	99		201	105	196
Additional acquired share																																																																												6.41%
Percentage of total participation in the consortium																																																							34.97%																					25.49%
Assured energy, megawatts					65	65																																																																							38
Energy capacity installed, megawatts																																																																							280
Assured power from installed capacity																																																																							150
Increase in estimated cost to complete project																																																																					130	270		720	1,300
Estimated project costs																																																		500	1,000																							2,550	5,170
Aluminio's share of project																																																		170	350																					650	1,320
Commitment expended on the project												610	1,230
Additional contributions																																																		170	350					141	176	3	2	10	8	15	14
Guarantee expiration year					2027	2027
Percentage of power self-sufficiency upon completion of project with full operating capacity					70.00%	70.00%
Total energy demand					690	690
Initial cash investment																																																								17	24
Required plan contributions																																																												40	40
Repayments made under the agreement for future gas transmission services																																																												$ 361	347