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Contingencies And Commitments (Investments) (Narrative) (Details) In Millions, unless otherwise specified | 12 Months Ended | 3 Months Ended | 12 Months Ended
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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 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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