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Contingencies And Commitments (Investments) (Narrative) (Details)
In Millions, unless otherwise specified
 3 Months Ended 3 Months Ended 3 Months Ended 3 Months Ended 12 Months Ended 3 Months Ended 12 Months Ended 3 Months Ended 3 Months Ended 3 Months Ended 12 Months Ended 3 Months Ended 15 Months Ended
Mar. 31, 2012
USD ($)
MWh
MW
Dec. 31, 2011
USD ($)
Mar. 31, 2012
Aluminio [Member]
USD ($)
Mar. 31, 2012
Aluminio [Member]
BRL
Dec. 31, 2011
Alcoa Inc. [Member]
USD ($)
Mar. 31, 2012
Barra Grande [Member]
Mar. 31, 2012
Machadinho [Member]
Mar. 31, 2012
Machadinho And Barra Grande [Member]
USD ($)
Y
Mar. 31, 2012
Machadinho And Barra Grande [Member]
BRL
Y
Dec. 31, 2011
Machadinho And Barra Grande [Member]
USD ($)
Dec. 31, 2011
Machadinho And Barra Grande [Member]
BRL
Mar. 31, 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]
Aluminio [Member]
Dec. 31, 2006
Estreito Hydroelectric Power Project [Member]
MWh
Mar. 31, 2012
Serra Do Facao Project [Member]
USD ($)
Mar. 31, 2012
Serra Do Facao Project [Member]
ZAR
Mar. 31, 2012
Serra Do Facao Project [Member]
BRL
Dec. 31, 2011
Serra Do Facao Project [Member]
USD ($)
Dec. 31, 2011
Serra Do Facao Project [Member]
BRL
Dec. 31, 2007
Serra Do Facao Project [Member]
Aluminio [Member]
Dec. 31, 2004
Dampier To Bunbury Natural Gas Pipeline (DBNGP) [Member]
USD ($)
Dec. 31, 2004
Dampier To Bunbury Natural Gas Pipeline (DBNGP) [Member]
AUD
Mar. 31, 2012
Ma'aden Joint Venture [Member]
Dec. 31, 2011
Ma'aden Joint Venture [Member]
Mar. 31, 2012
Alcoa Joint Venture [Member]
USD ($)
Y
Mar. 31, 2012
Alcoa Joint Venture [Member]
SAR
Dec. 31, 2011
Alcoa Joint Venture [Member]
USD ($)
Mar. 31, 2012
Alumina Limited [Member]
Dec. 31, 2010
Smelting And Rolling Mill Companies [Member]
USD ($)
Dec. 31, 2011
Refining And Mining Company [Member]
USD ($)
Mar. 31, 2012
Three-Year Equity Call Plan [Member]
Dampier To Bunbury Natural Gas Pipeline (DBNGP) [Member]
USD ($)
Mar. 31, 2012
Three-Year Equity Call Plan [Member]
Dampier To Bunbury Natural Gas Pipeline (DBNGP) [Member]
AUD
Mar. 31, 2012
Three-Year Equity Call Plan [Member]
Dampier To Bunbury Natural Gas Pipeline (DBNGP) [Member]
USD ($)
Mar. 31, 2012
Three-Year Equity Call Plan [Member]
Dampier To Bunbury Natural Gas Pipeline (DBNGP) [Member]
AUD
Mar. 31, 2012
Other Noncurrent Liabilities And Deferred Credits [Member]
Smelting And Rolling Mill Companies [Member]
USD ($)
Dec. 31, 2011
Other Noncurrent Liabilities And Deferred Credits [Member]
Smelting And Rolling Mill Companies [Member]
USD ($)
Dec. 31, 2011
Other Noncurrent Liabilities And Deferred Credits [Member]
Refining And Mining Company [Member]
USD ($)
Loss Contingencies [Line Items]                                                                                  
Ownership interest in joint venture at fair market value                                                     74.90%   25.10%     40.00%                  
Capital investment $ 1,750 $ 1,626                                                     $ 10,800 40,500     $ 4,035 $ 1,992              
Amount invested by Alcoa and Aluminum Financing Limited                                                         1,100                        
Term of investment by Alcoa and Aluminum Financing Limited, years                                                         4                        
Capital investment commitment paid-to-date                                                         501                        
Capital investment commitment paid in period                                                         93                        
Equity method investment, carrying value                                                         664   565                    
Guarantee issued on behalf of smelting and rolling mill companies         1,013                                                         500         9 8 5
Ownership interest issued as guarantee for smelting and rolling mill companies                                                       74.90%     25.10%                    
Debt service requirements, principal                                                                 108 60              
Debt service requirements, interest maximum                                                                 50 25              
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               275 504 266 496               106   195 105 196                                    
Additional acquired share                                 6.41%                                                
Increase in estimated cost to complete project                         720 1,300                                                      
Aluminio's share of project                         680 1,250         190 350                                          
Estimated project costs                             2,700 4,900     550 1,000                                          
Commitment expended on the project     650 1,190                                                                          
Percentage of total participation in the consortium                                 25.49%             34.97%                                  
Initial cash investment                                                 17 24                              
Additional contributions                                     190 340         141 176                 4 3 9 9      
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                                                                     358 344          
Alcoa's maximum exposure to loss in projects     $ 220 400       $ 310 570                                                   $ 530 510 $ 530 510      
Percentage of current power self-sufficiency 67.00%                                                                                
Percentage of power self-sufficiency upon completion of project with full operating capacity 70.00%                                                                                
Total energy demand 690                                                                                
Guarantee expiration year 2027