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Arconic Inc. Separation Transaction and Discontinued Operations - Narrative (Details) |  |  |  | 3 Months Ended | 6 Months Ended |  |
--- | --- | --- | --- | --- | --- | --- | ---
Apr. 01, 2020 company | Mar. 25, 2020 USD ($) | Feb. 01, 2020 USD ($) | Mar. 31, 2021 USD ($) | Mar. 31, 2020 USD ($) | Dec. 31, 2019 USD ($) | Mar. 01, 2020 USD ($) | Feb. 07, 2020 USD ($)
Restructuring Cost and Reserve [Line Items] |  |  |  |  |  |  |  |
Number of independent, publicly-traded companies resulting from proposed separation transaction | company | 2 |  |  |  |  |  |  |
Debt issuance costs incurred |  |  |  | $ 1,000,000 | $ 45,000,000 |  |  |
Aluminum Rolling Mill |  |  |  |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |  |  |  |
Consideration |  |  | $ 50,000,000 |  |  |  |  |
Gain (loss) on sale of business |  |  | $ (59,000,000) |  | 6,000,000 | $ 53,000,000 |  |
Hard Alloy Extrusions Plant In South Korea |  |  |  |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |  |  |  |
Consideration |  |  |  |  |  |  | $ 62,000,000 |
Gain (loss) on sale of business |  |  |  |  | 27,000,000 |  |  |
Lien Notes |  |  |  |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |  |  |  |
Debt issuance costs incurred |  |  |  |  | $ 45,000,000 |  |  |
Senior Notes | Second Lien Notes Due 2028 |  |  |  |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  | $ 600,000,000
Long-term debt, interest rate |  |  |  |  |  |  |  | 6.125%
Line of Credit | Senior Secured First Lien Term B Loan Facility Due 2027 | Secured Debt |  |  |  |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |  |  |  |
Aggregate principal amount |  | $ 600,000,000 |  |  |  |  |  |
Credit agreement term |  | 7 years |  |  |  |  |  |

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