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Basis of Presentation (Details) | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Mar. 31, 2018 USD ($) | Sep. 30, 2018 USD ($) reporting_unit | Dec. 31, 2017 USD ($) reporting_unit
Basis Of Presentation [Line Items] |  |  |
Number of reporting units | reporting_unit |  | 3 | 4
Goodwill, impairment loss |  | $ 0 |
Goodwill |  | $ 4,517,000,000 | $ 4,535,000,000
Arconic Engines |  |  |
Basis Of Presentation [Line Items] |  |  |
Goodwill | $ 2,095,000,000 |  |
Arconic Fastening Systems |  |  |
Basis Of Presentation [Line Items] |  |  |
Goodwill | 1,623,000,000 |  |
Arconic Engineered Structures |  |  |
Basis Of Presentation [Line Items] |  |  |
Goodwill | $ 517,000,000 |  |
Minimum |  |  |
Basis Of Presentation [Line Items] |  |  |
Percentage of goodwill allocated to reporting units | 92.00% |  |

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