Commitments And Contingencies (Investments) (Details) In Millions, unless otherwise specified	9 Months Ended					12 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended			9 Months Ended					12 Months Ended		3 Months Ended		9 Months Ended		12 Months Ended	6 Months Ended		12 Months Ended			9 Months Ended		12 Months Ended				12 Months Ended
	Sep. 30, 2011 USD ($)	Dec. 31, 2010 USD ($)	Dec. 31, 2004	Sep. 30, 2011 Non Current Liabilities and Deferred Credits [Member] USD ($)	Dec. 31, 2010 Non Current Liabilities and Deferred Credits [Member] USD ($)	Dec. 31, 2010 Smelting and Rolling Mill Companies [Member] USD ($)	Sep. 30, 2011 Ma'aden Joint Venture [Member]	Sep. 30, 2011 Alcoa Joint Venture [Member] USD ($)	Sep. 30, 2011 Alcoa Joint Venture [Member] USD ($)	Sep. 30, 2011 Alcoa Joint Venture [Member] SAR	Dec. 31, 2010 Alcoa Joint Venture [Member] USD ($)	Sep. 30, 2011 Brazilian Real [Member]	Sep. 30, 2011 Barra Grande [Member]	Sep. 30, 2011 Machadinho [Member]	Sep. 30, 2011 Machadinho and Barra Grande [Member] BRL MWH	Sep. 30, 2011 Machadinho and Barra Grande [Member] USD ($)	Dec. 31, 2010 Machadinho and Barra Grande [Member] BRL	Dec. 31, 2010 Machadinho and Barra Grande [Member] USD ($)	Mar. 31, 2010 Estreito Project [Member] USD ($)	Mar. 31, 2010 Estreito Project [Member] BRL	Sep. 30, 2011 Estreito Project [Member] BRL MWH	Sep. 30, 2011 Estreito Project [Member] USD ($)	Dec. 31, 2006 Estreito Project [Member] Alcoa Aluminio [Member]	Jun. 30, 2011 Serra do Facao Project [Member] USD ($)	Jun. 30, 2011 Serra do Facao Project [Member] BRL MWH	Dec. 31, 2010 Serra do Facao Project [Member] USD ($)	Dec. 31, 2010 Serra do Facao Project [Member] BRL	Dec. 31, 2006 Serra do Facao Project [Member] Alcoa Aluminio [Member]	Sep. 30, 2011 Dampier to Bunbry Natural Gas Pipeline (DBNGP) [Member] USD ($)	Sep. 30, 2011 Dampier to Bunbry Natural Gas Pipeline (DBNGP) [Member] AUD	Dec. 31, 2004 Dampier to Bunbry Natural Gas Pipeline (DBNGP) [Member] USD ($)	Dec. 31, 2004 Dampier to Bunbry Natural Gas Pipeline (DBNGP) [Member] AUD	Dec. 31, 2011 Dampier to Bunbry Natural Gas Pipeline (DBNGP) [Member] AUD	Dec. 31, 2011 Dampier to Bunbry Natural Gas Pipeline (DBNGP) [Member] USD ($)	Dec. 31, 2006 Alcoa Aluminio [Member]	Sep. 30, 2011 Alcoa Aluminio [Member] USD ($) MWH	Sep. 30, 2011 Alcoa Aluminio [Member] BRL	Dec. 31, 2010 Alcoa [Member] USD ($)
Ownership interest in joint venture at fair market value							74.90%		25.10%
Capital investment	$ 1,521	$ 1,340				$ 4,000		$ 10,800	$ 10,800	40,500
Amount invested by Alcoa and Aluminum Financing Limited								1,100	1,100
Amount invested by Alcoa and Aluminum Financing Limited, over year	4
Capital investment commitment paid-to-date								325	325															180	340
Capital investment commitment paid in period								13	165
Duration of sharing capital commitment, in years															30	30
Power self sufficiency												70.00%			40.00%	40.00%
Investment in SPPL as an equity method investment								461	461		285
Guarantee issued on behalf of smelting and rolling mill companies				8	8																																	1,004
Ownership interest issued as a guarantee for smelting and rolling companies							74.90%			25.10%
Debt service requirements, principal						108
Debt service requirements, interest						50
Investment percentage													42.18%	30.99%
Total energy demand															690	690
Total investment in project															487	266	461	274
Aluminio's share of Serra do Facao																								190	350
Additional acquired share																							6.41%
Increase in estimated cost to complete Estreito project																			720	1,300
Aluminio's share of Estreito project																					1,250	680
Estimated project costs																					4,900	2,700		550	1,000
Total investment in Serra do Facao																								106	193	116	195
Percent interest in a consortium																												34.97%							25.49%
Assured energy																								65	65											38	38
Energy capacity installed																					280	280
Assured power from installed capacity																					150	150
Initial cash investment																															17	24
Additional contributions																													16	16	141	176
Additional amount committed to invest																																	9	10
Amount contributed																																				620	1,100
Alcoa's maximum exposure to loss in projects															570	$ 310													$ 470	480						$ 220	400
Interest in consortium			20.00%