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Contingencies and Commitments - Additional Information - 2 (Detail) BRL in Millions, SAR in Billions |  |  |  | 1 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended | 36 Months Ended | 93 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Apr. 08, 2015 USD ($) Installment Refinery | Feb. 27, 2014 EUR (€) | Feb. 27, 2014 USD ($) | Apr. 08, 2013 USD ($) | Apr. 08, 2013 BRL | Jan. 31, 2014 | Oct. 31, 2013 USD ($) | Mar. 31, 2013 USD ($) | May. 31, 2012 USD ($) | May. 31, 2012 BRL | Sep. 30, 2015 USD ($) | Sep. 30, 2015 AUD | Jun. 30, 2015 USD ($) | Mar. 31, 2015 USD ($) | Sep. 30, 2015 USD ($) MWh Entity | Sep. 30, 2015 BRL MWh Entity | Sep. 30, 2015 AUD MWh Entity | Sep. 30, 2015 SAR MWh Entity | Dec. 31, 2014 USD ($) | Dec. 31, 2014 AUD | Dec. 31, 2012 USD ($) | Dec. 31, 2009 USD ($) | Dec. 31, 2004 USD ($) | Dec. 31, 2004 AUD | Dec. 31, 2002 t | Sep. 30, 2011 USD ($) | Sep. 30, 2011 AUD | Sep. 30, 2015 EUR (€) | Sep. 30, 2015 USD ($) | Sep. 30, 2015 BRL | Sep. 30, 2015 AUD | Apr. 08, 2013 BRL | Mar. 31, 2013 BRL | Jun. 30, 2012 EUR (€) Installment | Jun. 30, 2012 USD ($) Installment
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase in remediation reserve |  |  |  |  |  |  |  |  |  |  | $ 1,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Revised project cost submitted to reflect the removal of a larger volume of contaminated soil |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 7,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total combined assessments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | € 239,000,000 | $ 268,000,000 |  |  |  |  |  |
Range of reasonable possible loss, maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | € 303,000,000 | $ 340,000,000
Number of installments | Installment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5 | 5
State and Local Jurisdiction [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total combined assessments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 32,000,000 | BRL 133 |  |  |  |  |
Maaden Alcoa Joint Venture Mine And Refinery Company [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Capital investment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 560,000,000 |  |  |  |  |  |
Percentage of amount required to be contributed in the event Alcoa would be required to make payments under the guarantees |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 40.00% | 40.00% | 40.00% | 40.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maaden Alcoa Joint Venture Mine And Refinery Company [Member] | Other Noncurrent Liabilities and Deferred Credits [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Guarantee issued on behalf of smelting and rolling mill companies |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 4,000,000 |  |  |  |  |  |  |  |  |  | $ 3,000,000 |  |  |  |  |  |
Maaden Alcoa Joint Venture Mine And Refinery Company [Member] | Financial Guarantee [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt service requirements, principal |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 120,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt service requirements, interest maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 30,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maaden Alcoa Joint Venture [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest issued as guarantee for Saudi Arabian Joint Venture |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 25.10% | 25.10% | 25.10% | 25.10% |  |  |  |
Number of companies in Saudi Arabian joint venture | Entity |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1 | 1 | 1 | 1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount invested by Alcoa and Aluminum Financing Limited |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 10,800,000,000 |  |  | SAR 40.5 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Capital investment |  |  |  |  |  |  |  |  |  |  | 6,000,000 |  |  |  | 15,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity method investment, carrying value |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 983,000,000 |  |  |  |  |  |  |  |  |  | $ 935,000,000 |  |  |  |  |  |
Maaden Alcoa Joint Venture Smelter And Rolling Mill Companies [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Capital investment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,098,000,000 |  |  |  |  |  |
Maaden Alcoa Joint Venture Smelter And Rolling Mill Companies [Member] | Other Noncurrent Liabilities and Deferred Credits [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Guarantee issued on behalf of smelting and rolling mill companies |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 8,000,000 |  |  |  |  |  |  |  |  |  | 7,000,000 |  |  |  |  |  |
Maaden Alcoa Joint Venture Smelter And Rolling Mill Companies [Member] | Financial Guarantee [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt service requirements, principal |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 176,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt service requirements, interest maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 50,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Alcoa [Member] | Maaden Alcoa Joint Venture Mine And Refinery Company [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Capital investment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2,232,000,000 |  |  |  |  |  |
Alcoa [Member] | Maaden Alcoa Joint Venture [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount invested by Alcoa and Aluminum Financing Limited |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,100,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Capital investment |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 967,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Alcoa [Member] | Maaden Alcoa Joint Venture Smelter And Rolling Mill Companies [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Capital investment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 4,374,000,000 |  |  |  |  |  |
Alcoa World Alumina Brasil [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disallowed tax credits |  |  |  |  |  |  |  | $ 110,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | BRL 220 |  |
Percentage of penalty |  |  |  | 50.00% | 50.00% |  |  | 50.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Value added tax receivable |  |  |  |  |  |  |  |  | $ 41,000,000 | BRL 82 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Estimated range of reasonably possible loss, minimum | BRL |  |  |  |  | BRL 103 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Alcoa World Alumina Brasil [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Estimated range of reasonably possible loss, minimum |  |  |  | $ 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Alcoa World Alumina Brasil [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Estimated range of reasonably possible loss, minimum |  |  |  | 25,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Alcoa World Alumina Brasil [Member] | Fixed Assets [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disallowed tax credits | BRL |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | BRL 117 |  |  |
Alcoa World Alumina Brasil [Member] | Fixed Assets [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disallowed tax credits |  |  |  | 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Alcoa World Alumina Brasil [Member] | Fixed Assets [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disallowed tax credits |  |  |  | $ 28,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Alcoa Aluminio [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Metal sold per month | t |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2,000 |  |  |  |  |  |  |  |  |  |
Maaden Company [Member] | Maaden Alcoa Joint Venture [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percent of guarantee issued by co-venturing entity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 74.90% | 74.90% | 74.90% | 74.90% |  |  |  |
Estreito Project [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investment percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 25.49% | 25.49% | 25.49% | 25.49% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Assured power from installed capacity | MWh |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 150 | 150 | 150 | 150 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Estimated project costs |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,260,000,000 | BRL 5,170 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aluminio's share of project |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 320,000,000 | 1,320 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Commitment expended on the project |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 310,000,000 | BRL 1,270 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ligestra [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Costs and payments |  | € 12,000,000 | $ 17,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Costs and payments related to damages |  | 6,000,000 | 9,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payment period |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 10 years | 10 years | 10 years | 10 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ligestra [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of payments and remediation costs |  |  |  |  |  | 50.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ligestra [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of payments and remediation costs |  |  |  |  |  | 80.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Italian Ministry of Environment and Protection of Land and Sea[Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Soil remediation project, estimated cost |  | 24,000,000 | 33,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments for emergency action and natural resource damages |  | € 18,000,000 | $ 25,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Majority-Owned Subsidiary | Alcoa Of Australia Limited [Member] | Service Agreements [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gas supply agreement period |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 12 years | 12 years | 12 years | 12 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of alumina refineries to be powered under supplied agreement | Refinery | 3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of installments | Installment | 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gas supply agreement prepayment amount | $ 500,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Majority-Owned Subsidiary | Alcoa Of Australia Limited [Member] | Service Agreements [Member] | First Installment [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gas supply agreement prepayment amount | 300,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Majority-Owned Subsidiary | Alcoa Of Australia Limited [Member] | Service Agreements [Member] | Second Installment [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gas supply agreement prepayment amount | $ 200,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fusina [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase in remediation reserve |  |  |  |  |  |  | $ 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 12,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Portovesme [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase in remediation reserve |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3,000,000 |  |  | $ 3,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Baie Comeau [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase in remediation reserve |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 25,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Mosjoen [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase in remediation reserve |  |  |  |  |  |  |  |  |  |  | 1,000,000 |  | $ 11,000,000 |  | $ 12,000,000 |  |  |  |  |  | $ 20,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dampier to Bunbury Natural Gas Pipeline [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Capital investment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 17,000,000 | AUD 24,000,000 |  | $ 141,000,000 | AUD 176,000,000 |  |  |  |  |  |  |  |
Investment percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 20.00% | 20.00% |  |  |  |  |  |  |  |  |  |  |
Dampier to Bunbury Natural Gas Pipeline [Member] | Three-Year Equity Call Plan [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount invested by Alcoa and Aluminum Financing Limited |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 39,000,000 | AUD 40,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Capital investment |  |  |  |  |  |  |  |  |  |  | $ 3,000,000 | AUD 4,000,000 |  |  | 11,000,000 |  | AUD 15,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Prepayments made under the agreement for future gas transmission services |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 256,000,000 |  | AUD 366,000,000 |  |  |  |
Range of reasonable possible loss, maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 370,000,000 |  | AUD 520,000,000 |  |  |  |
Dampier to Bunbury Natural Gas Pipeline [Member] | New Equity Call Plan [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount invested by Alcoa and Aluminum Financing Limited |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 30,000,000 |  | 36,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Capital investment |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 12,000,000 |  | AUD 16,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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