Contingencies And Commitments (Investments) (Narrative) (Details) In Millions, unless otherwise specified	12 Months Ended												3 Months Ended		12 Months Ended
	Dec. 31, 2011 MWH	Dec. 31, 2011 Alcoa Aluminio [Member] USD ($)	Dec. 31, 2011 Alcoa Aluminio [Member] BRL	Dec. 31, 2010 Alcoa Aluminio [Member] USD ($)	Dec. 31, 2010 Alcoa Aluminio [Member] BRL	Dec. 31, 2006 Alcoa Aluminio [Member]	Dec. 31, 2011 Barra Grande [Member]	Dec. 31, 2011 Machadinho [Member]	Dec. 31, 2011 Machadinho And Barra Grande [Member] USD ($) years	Dec. 31, 2011 Machadinho And Barra Grande [Member] BRL	Dec. 31, 2010 Machadinho And Barra Grande [Member] USD ($)	Dec. 31, 2010 Machadinho And Barra Grande [Member] BRL	Mar. 31, 2010 Estreito Project [Member] USD ($)	Mar. 31, 2010 Estreito Project [Member] ZAR	Dec. 31, 2011 Estreito Project [Member] MWH	Dec. 31, 2010 Estreito Project [Member] USD ($)	Dec. 31, 2010 Estreito Project [Member] ZAR	Dec. 31, 2006 Estreito Project [Member] Alcoa Aluminio [Member]	Dec. 31, 2011 Estreito Hydroelectric Power Project [Member]	Dec. 31, 2006 Estreito Hydroelectric Power Project [Member] MWH	Dec. 31, 2011 Serra Do Facao Project [Member] USD ($)	Dec. 31, 2011 Serra Do Facao Project [Member] BRL	Dec. 31, 2011 Serra Do Facao Project [Member] ZAR	Dec. 31, 2010 Serra Do Facao Project [Member] USD ($)	Dec. 31, 2010 Serra Do Facao Project [Member] BRL	Dec. 31, 2006 Serra Do Facao Project [Member] Alcoa Aluminio [Member]	Dec. 31, 2011 Dampier To Bunbry Natural Gas Pipeline (DBNGP) [Member] USD ($)	Dec. 31, 2011 Dampier To Bunbry Natural Gas Pipeline (DBNGP) [Member] AUD	Dec. 31, 2010 Dampier To Bunbry Natural Gas Pipeline (DBNGP) [Member] USD ($)	Dec. 31, 2010 Dampier To Bunbry Natural Gas Pipeline (DBNGP) [Member] AUD	Dec. 31, 2004 Dampier To Bunbry Natural Gas Pipeline (DBNGP) [Member] USD ($)	Dec. 31, 2004 Dampier To Bunbry Natural Gas Pipeline (DBNGP) [Member] AUD	Dec. 31, 2011 Three-Year Equity Call Plan [Member] Dampier To Bunbry Natural Gas Pipeline (DBNGP) [Member] USD ($)	Dec. 31, 2011 Three-Year Equity Call Plan [Member] Dampier To Bunbry Natural Gas Pipeline (DBNGP) [Member] AUD
Loss Contingencies [Line Items]
Investment percentage							42.18%	30.99%																							20.00%	20.00%
Commitment to taking share of output in associated projects at cost, in years									30	30
Total investment in project									$ 266	496	$ 274	461									$ 105	196		$ 116	195
Additional acquired share																		6.41%
Increase in estimated cost to complete project													720	1,300
Aluminio's share of project																670	1,250				190		350
Estimated project costs																2,600	4,900				540		1,000
Commitment expended on the project				630	1,170
Percentage of total participation in the consortium						25.49%																				34.97%
Initial cash investment																															17	24
Additional contributions																					180		340				16	16	9	9	141	176	5	6
Required plan contributions																																	40	40
Assured energy, megawatts	65																			38
Energy capacity installed, megawatts															280
Assured power from installed capacity															150
Repayments made under the agreement for future gas transmission services																											346	342
Alcoa's maximum exposure to loss in projects		$ 220	400								$ 300	570															$ 510	510
Percentage of current power self-sufficiency		67.00%	67.00%
Percentage of power self-sufficiency upon completion of project with full operating capacity																			70.00%
Total energy demand	690