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Acquisitions and Divestitures (Details) $ in Millions |  |  |  |  | 3 Months Ended | 9 Months Ended | 12 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Aug. 15, 2019 USD ($) | Jan. 01, 2019 USD ($) | Oct. 31, 2018 USD ($) | Apr. 02, 2018 USD ($) employee | Mar. 31, 2020 USD ($) | Sep. 30, 2019 USD ($) | Jun. 30, 2019 USD ($) | Dec. 31, 2018 USD ($) | Sep. 30, 2018 USD ($) | Mar. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Sep. 30, 2019 USD ($) | Sep. 30, 2018 USD ($) | Dec. 31, 2018 USD ($) | Oct. 30, 2019 USD ($) | Sep. 13, 2019 USD ($) | Aug. 23, 2019 USD ($) | May 31, 2019 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restructuring and other charges (benefits) |  |  |  |  |  | $ 119 |  |  | $ (2) |  |  | $ 630 | $ 20 |  |  |  |  |
Latin America Extrusions (LAE) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disposal group, consideration |  |  |  | $ 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase price allocation, subsequent years, remaining adjustments |  |  |  |  |  |  |  | $ 2 |  |  |  |  |  | $ 2 |  |  |  |
Number of employees | employee |  |  |  | 612 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Latin America Extrusions (LAE) | Disposed of by Sale |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disposal group, including discontinued operation, revenue |  |  |  |  |  |  |  |  |  | $ 25 |  |  |  |  |  |  |  |
Latin America Extrusions (LAE) | Disposed of by Sale | Divested Businesses |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restructuring and other charges (benefits) |  |  |  |  |  |  |  |  |  |  | $ 41 |  |  |  |  |  |  |
Texarkana, Texas Rolling Mill And Cast House |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (loss) on sale of business |  |  |  |  |  |  |  | 154 |  |  |  |  |  |  |  |  |  |
Period of operationalizing rolling mill equipment of transaction closing date |  |  | 36 months |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Minimum required terms of leases on cast house building and equipment |  |  | 18 months |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Period of supply with aluminum |  |  | 24 months |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Contingent consideration |  |  |  |  |  | 45 |  | 5 |  |  |  | $ 45 |  | 5 |  |  |  |
Deferred gain on sale of property |  |  |  |  |  |  |  | $ 95 |  |  |  |  |  | 95 |  |  |  |
Texarkana, Texas Rolling Mill And Cast House | Ta Chen International, Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Period of services performed, processing of metal |  |  | 6 months |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Texarkana, Texas Rolling Mill And Cast House | Disposed of by Sale |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash received on sale of operations |  |  | $ 302 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net book value of disposal |  |  | 63 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disposal group, consideration |  |  | $ 50 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Standards Update 2016-02 | Property, Plant and Equipment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Effect on retained earnings |  |  |  |  |  |  |  |  |  |  |  |  |  | (24) |  |  |  |
Accounting Standards Update 2016-02 | Other Noncurrent Liabilities and Deferred Credits |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Effect on retained earnings |  |  |  |  |  |  |  |  |  |  |  |  |  | 119 |  |  |  |
Accounting Standards Update 2016-02 | Other Noncurrent Assets |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Effect on retained earnings |  | $ (22) |  |  |  |  |  |  |  |  |  |  |  | $ (22) |  |  |  |
Arconic Engineered Structures | Small Additive Manufacturing Facility |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disposal group, consideration |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1
Gain (loss) on sale of business |  |  |  |  |  |  | $ (13) |  |  |  |  |  |  |  |  |  |  |
Cash received on sale of operations | $ 13 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Asset impairment charges |  |  |  |  |  |  | $ 9 |  |  |  |  |  |  |  |  |  |  |
Global Rolled Products | Aluminum Rolling Mill |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disposal group, consideration |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 50 |
Asset impairment charges |  |  |  |  |  | 59 |  |  |  |  |  |  |  |  |  |  |  |
Engineered Products And Forgings | Forgings Business |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disposal group, consideration |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 62 |  |
Asset impairment charges |  |  |  |  |  | $ 43 |  |  |  |  |  |  |  |  |  |  |  |
Subsequent Event | Global Rolled Products | Alloy Extrusions |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disposal group, consideration |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 61 |  |  |
Scenario, Forecast | Minimum | Global Rolled Products | Alloy Extrusions |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (loss) on sale of business |  |  |  |  | $ 20 |  |  |  |  |  |  |  |  |  |  |  |  |
Scenario, Forecast | Maximum | Global Rolled Products | Alloy Extrusions |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (loss) on sale of business |  |  |  |  | $ 25 |  |  |  |  |  |  |  |  |  |  |  |  |

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