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Contingencies and Commitments - Additional Information - 2 (Detail) | 9 Months Ended |  |  | 9 Months Ended |  |  |  | 3 Months Ended | 9 Months Ended | 63 Months Ended |  |  | 9 Months Ended |  | 12 Months Ended | 93 Months Ended |  |  |  |  |  | 9 Months Ended | 3 Months Ended | 9 Months Ended | 33 Months Ended | 0 Months Ended | 9 Months Ended | 1 Months Ended | 0 Months Ended | 1 Months Ended | 0 Months Ended |  | 36 Months Ended |  | 3 Months Ended | 12 Months Ended
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Sep. 30, 2014 USD ($) | Sep. 30, 2014 EUR (€) | Jun. 30, 2012 USD ($) | Jun. 30, 2012 EUR (€) | Sep. 30, 2014 Maaden Alcoa Joint Venture [Member] USD ($) Entity | Sep. 30, 2014 Maaden Alcoa Joint Venture [Member] SAR | Sep. 30, 2014 Maaden Alcoa Joint Venture Smelter And Rolling Mill Companies [Member] USD ($) | Sep. 30, 2014 Maaden Alcoa Joint Venture Mine And Refinery Company [Member] USD ($) | Jan. 31, 2014 Maaden Alcoa Joint Venture Mine And Refinery Company [Member] USD ($) | Sep. 30, 2014 Maaden Alcoa Joint Venture Smelter And Rolling Mill Companies [Member] USD ($) | Sep. 30, 2014 Maaden Alcoa Joint Venture [Member] USD ($) | Sep. 30, 2014 Maaden Alcoa Joint Venture [Member] USD ($) Agency | Sep. 30, 2014 Maaden Alcoa Joint Venture [Member] USD ($) | Apr. 11, 2014 Maaden Alcoa Joint Venture [Member] | Dec. 31, 2013 Maaden Alcoa Joint Venture [Member] USD ($) | Sep. 30, 2014 Maaden Alcoa Joint Venture Mine And Refinery Company [Member] USD ($) | Jan. 31, 2014 Maaden Alcoa Joint Venture Mine And Refinery Company [Member] USD ($) | Dec. 31, 2004 Dampier to Bunbury Natural Gas Pipeline [Member] USD ($) | Dec. 31, 2004 Dampier to Bunbury Natural Gas Pipeline [Member] AUD | Sep. 30, 2011 Dampier to Bunbury Natural Gas Pipeline [Member] USD ($) | Sep. 30, 2011 Dampier to Bunbury Natural Gas Pipeline [Member] AUD | Sep. 30, 2014 Other Noncurrent Liabilities and Deferred Credits [Member] Maaden Alcoa Joint Venture Smelter And Rolling Mill Companies [Member] USD ($) | Dec. 31, 2013 Other Noncurrent Liabilities and Deferred Credits [Member] Maaden Alcoa Joint Venture Smelter And Rolling Mill Companies [Member] USD ($) | Sep. 30, 2014 Other Noncurrent Liabilities and Deferred Credits [Member] Maaden Alcoa Joint Venture Mine And Refinery Company [Member] USD ($) | Dec. 31, 2013 Other Noncurrent Liabilities and Deferred Credits [Member] Maaden Alcoa Joint Venture Mine And Refinery Company [Member] USD ($) | Sep. 30, 2014 State and Local Jurisdiction [Member] USD ($) | Sep. 30, 2014 State and Local Jurisdiction [Member] BRL | Sep. 30, 2014 Financial Guarantee [Member] Maaden Alcoa Joint Venture Smelter And Rolling Mill Companies [Member] USD ($) | Sep. 30, 2014 Financial Guarantee [Member] Maaden Alcoa Joint Venture Mine And Refinery Company [Member] USD ($) | Sep. 30, 2014 Estreito Project [Member] USD ($) MWh | Sep. 30, 2014 Estreito Project [Member] BRL | Sep. 30, 2014 Three-Year Equity Call Plan [Member] Dampier to Bunbury Natural Gas Pipeline [Member] USD ($) | Sep. 30, 2014 Three-Year Equity Call Plan [Member] Dampier to Bunbury Natural Gas Pipeline [Member] AUD | Sep. 30, 2014 Three-Year Equity Call Plan [Member] Dampier to Bunbury Natural Gas Pipeline [Member] USD ($) | Sep. 30, 2014 Three-Year Equity Call Plan [Member] Dampier to Bunbury Natural Gas Pipeline [Member] AUD | Sep. 30, 2014 Three-Year Equity Call Plan [Member] Dampier to Bunbury Natural Gas Pipeline [Member] USD ($) | Sep. 30, 2014 Three-Year Equity Call Plan [Member] Dampier to Bunbury Natural Gas Pipeline [Member] AUD | Feb. 27, 2014 Ligestra [Member] USD ($) | Feb. 27, 2014 Ligestra [Member] EUR (€) | Sep. 30, 2014 Ligestra [Member] | Jan. 31, 2014 Ligestra [Member] Minimum [Member] | Jan. 31, 2014 Ligestra [Member] Maximum [Member] | Feb. 27, 2014 Italian Ministry of Environment and Protection of Land and Sea[Member] USD ($) | Feb. 27, 2014 Italian Ministry of Environment and Protection of Land and Sea[Member] EUR (€) | Apr. 08, 2013 Alcoa World Alumina Brasil [Member] BRL | Mar. 31, 2013 Alcoa World Alumina Brasil [Member] USD ($) | Mar. 31, 2013 Alcoa World Alumina Brasil [Member] BRL | May 31, 2012 Alcoa World Alumina Brasil [Member] USD ($) | May 31, 2012 Alcoa World Alumina Brasil [Member] BRL | Apr. 08, 2013 Alcoa World Alumina Brasil [Member] Minimum [Member] USD ($) | Apr. 08, 2013 Alcoa World Alumina Brasil [Member] Maximum [Member] USD ($) | Apr. 08, 2013 Alcoa World Alumina Brasil [Member] Fixed Assets [Member] BRL | Apr. 08, 2013 Alcoa World Alumina Brasil [Member] Fixed Assets [Member] Minimum [Member] USD ($) | Apr. 08, 2013 Alcoa World Alumina Brasil [Member] Fixed Assets [Member] Maximum [Member] USD ($) | Dec. 31, 2002 Alcoa Aluminio [Member] t | Sep. 30, 2014 Maaden Company [Member] Maaden Alcoa Joint Venture [Member] | Jun. 30, 2014 Portovesme [Member] USD ($) | Dec. 31, 2009 Portovesme [Member] USD ($) | Dec. 31, 2012 Baie Comeau [Member] USD ($) | Dec. 31, 2012 Mosjoen [Member] USD ($) | Dec. 31, 2009 Fusina [Member] USD ($)
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase in remediation reserve | $ 42,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 3,000,000 | $ 3,000,000 | $ 25,000,000 | $ 20,000,000 | $ 12,000,000
Percentage of payments and remediation costs |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 50.00% | 80.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Soil remediation project, estimated cost |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 33,000,000 | 24,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments for emergency action and natural resource damages |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 25,000,000 | 18,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Costs and payments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 17,000,000 | 12,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Costs and payments related to damages |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 9,000,000 | 6,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payment period |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 10 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total combined assessments | 307,000,000 | 242,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 51,000,000 | 125,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disallowed tax credits |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 110,000,000 | 220,000,000 |  |  |  |  | 175,000,000 | 0 | 70,000,000 |  |  |  |  |  |  |
Percentage of penalty of the gross disallowed amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 50.00% | 50.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |
Value added tax receivable |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 41,000,000 | 82,000,000 |  |  |  |  |  |  |  |  |  |  |  |
Estimated range of reasonably possible loss, minimum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 155,000,000 |  |  |  |  | 0 | 60,000,000 |  |  |  |  |  |  |  |  |  |
Administrative appeal, assessment period |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Metal sold per month |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2,000 |  |  |  |  |  |
Ownership interest issued as guarantee for Saudi Arabian Joint Venture |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 74.90% |  |  |  |  |
Investment percentage in Australian Project |  |  |  |  |  |  |  |  |  |  | 25.10% | 25.10% | 25.10% |  |  |  |  | 20.00% | 20.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of companies in Saudi Arabian joint venture |  |  |  |  | 3 | 3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount invested by Alcoa and Aluminum Financing Limited |  |  |  |  | 10,800,000,000 | 40,500,000,000 |  |  |  |  |  | 1,100,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 40,000,000 | 40,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional contributions |  |  |  |  |  |  |  |  |  |  | 4,000,000 | 87,000,000 | 919,000,000 |  |  |  |  | 17,000,000 | 24,000,000 | 141,000,000 | 176,000,000 |  |  |  |  |  |  |  |  |  |  | 5,000,000 | 5,000,000 | 8,000,000 | 9,000,000 | 37,000,000 | 38,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity method investment, carrying value |  |  |  |  |  |  |  |  |  |  | 956,000,000 | 956,000,000 | 956,000,000 |  | 951,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Capital investment |  |  |  |  |  |  | 4,515,000,000 | 1,992,000,000 | 240,000,000 | 1,133,000,000 |  |  |  |  |  | 500,000,000 | 60,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt service requirements, principal |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 193,000,000 | 120,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt service requirements, interest maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 60,000,000 | 30,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Guarantee issued on behalf of smelting and rolling mill companies |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 8,000,000 | 10,000,000 | 3,000,000 | 4,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of amount required to be contributed in the event Alcoa would be required to make payments under the guarantees |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 40.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of agencies needed to downgrade Alcoa's credit rating below investment grade in order to require Alcoa to provide a letter of credit or fund an escrow account |  |  |  |  |  |  |  |  |  |  |  | 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Minimum percentage of equity commitment required |  |  |  |  |  |  |  |  |  |  | 67.00% | 67.00% | 67.00% | 67.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investment percentage in Brazilian project |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 25.49% | 25.49% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Assured power from installed capacity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 150 | 150 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Estimated project costs |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2,110,000,000 | 5,170,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aluminio's share of project |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 540,000,000 | 1,320,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Commitment expended on the project |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 520,000,000 | 1,270,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Prepayments made under the agreement for future gas transmission services |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 312,000,000 | 358,000,000 | 312,000,000 | 358,000,000 | 312,000,000 | 358,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Alcoa's maximum exposure to loss in projects |  |  | $ 385,000,000 | € 303,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 430,000,000 | 500,000,000 | $ 430,000,000 | 500,000,000 | $ 430,000,000 | 500,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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- Definition Cost incurred in providing environmental engineering and consulting; environmental testing and analysis; and remediation services during the reporting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.2(d)) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 + Details
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- Definition The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6382943&loc=d3e33918-111571 + Details
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- Definition The current carrying amount of the liability for the freestanding or embedded guarantor's obligations under the guarantee or each group of similar guarantees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 460 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6851643&loc=d3e12069-110248 + Details
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