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Contingencies - Additional Information - 1 (Detail) € in Millions, $ in Millions |  | 1 Months Ended | 3 Months Ended | 6 Months Ended | 12 Months Ended | 30 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Sep. 17, 2020 USD ($) | Sep. 17, 2020 AUD ($) | Mar. 31, 2013 USD ($) | May 31, 2012 USD ($) | May 31, 2012 BRL (R$) | Jun. 30, 2021 USD ($) | Jun. 30, 2021 AUD ($) | Dec. 31, 2020 USD ($) | Dec. 31, 2020 EUR (€) | Sep. 30, 2020 USD ($) | Sep. 30, 2020 AUD ($) | Sep. 30, 2018 USD ($) | Sep. 30, 2018 EUR (€) | Jun. 30, 2021 USD ($) | Jun. 30, 2021 BRL (R$) | Jun. 30, 2021 AUD ($) | Dec. 31, 2019 USD ($) | Jun. 30, 2021 USD ($) | Jun. 30, 2021 AUD ($) | Sep. 30, 2020 AUD ($) | Jul. 07, 2020 USD ($) | Jul. 07, 2020 AUD ($) | Mar. 31, 2013 BRL (R$)
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other noncurrent liabilities and deferred credits |  |  |  |  |  | $ 550,000,000 |  | $ 515,000,000 |  |  |  |  |  | $ 550,000,000 |  |  |  | $ 550,000,000 |  |  |  |  |
AWAC [Member] | Alumina Limited [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Non-controlling interest, ownership percentage |  |  |  |  |  | 40.00% |  |  |  |  |  |  |  | 40.00% |  |  |  | 40.00% | 40.00% |  |  |  |
Alcoa Corporation [Member] | AWAC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest percentage |  |  |  |  |  | 60.00% |  |  |  |  |  |  |  | 60.00% |  |  |  | 60.00% | 60.00% |  |  |  |
Maximum [Member] | PARTER Capital Group AG [Member] | Spain [Member] | Aviles and La Coruna Smelters [Member] | Avilés and La Coruña Aluminum Facilities [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Financial contributions |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 95,000,000 |  |  |  |  |  |
Financial contributions paid to buyer in relation to divestiture |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 78,000,000 |  |  |  |  |
Tax Authority, Spain [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Charge recorded in provision for income taxes to establish liability for estimated loss |  |  |  |  |  |  |  |  |  |  |  | $ 30,000,000 | € 26 |  |  |  |  |  |  |  |  |  |
Reversal of reserve recorded in provision for income taxes to establish liability for estimated loss |  |  |  |  |  |  |  | $ 32,000,000 | € 26 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Brazilian Federal Revenue Office [Member] | Alcoa World Alumina Brasil [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disallowed tax credits |  |  | $ 110,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | R$ 220,000,000
Percentage of penalty of the gross disallowed amount |  |  | 50.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Value added tax receivable |  |  |  | $ 41,000,000 | R$ 82,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Brazilian Federal Revenue Office [Member] | Minimum [Member] | Alcoa World Alumina Brasil [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Charge recorded in provision for income taxes to establish liability for estimated loss |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0 |  |  |  |  |  |  |  |  |
Brazilian Federal Revenue Office [Member] | Maximum [Member] | Alcoa World Alumina Brasil [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Charge recorded in provision for income taxes to establish liability for estimated loss |  |  |  |  |  |  |  |  |  |  |  |  |  | 44,000,000 | R$ 220,000,000 |  |  |  |  |  |  |  |
Australian Taxation Office [Member] | Foreign Jurisdiction [Member] | AofA [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional income tax payable, exclusive of interest and penalties |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 160,000,000 | $ 214 |
Notices include claims for compounded interest on the tax amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 530,000,000 | $ 707 |
Proposed administrative penalties | $ 96,000,000 | $ 128 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payment of dispute resolution practices income tax percentage |  |  |  |  |  |  |  |  |  | 50.00% | 50.00% |  |  |  |  |  |  |  |  |  |  |  |
Assessed income tax amount exclusive of interest and penalties |  |  |  |  |  |  |  |  |  | $ 74,000,000 |  |  |  |  |  |  |  |  |  | $ 107 |  |  |
Payment amount refund percentage |  |  |  |  |  |  |  |  |  | 50.00% | 50.00% |  |  |  |  |  |  |  |  |  |  |  |
Reduction in current year cash tax payment |  |  |  |  |  | $ 3,000,000 | $ 5 |  |  | $ 165,000,000 | $ 219 |  |  | 7,000,000 |  | $ 10 |  |  |  |  |  |  |
Other noncurrent liabilities and deferred credits |  |  |  |  |  | 172,000,000 |  |  |  |  |  |  |  | 172,000,000 |  |  |  | 172,000,000 | $ 229 |  |  |  |
Tax assessment deposit |  |  |  |  |  | $ 80,000,000 |  |  |  |  |  |  |  | $ 80,000,000 |  |  |  | $ 80,000,000 | $ 107 |  |  |  |

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