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Goodwill and Other Intangible Assets - Summary of Goodwill by Reportable Segment (Parenthetical) (Details) - USD ($) $ in Millions | 6 Months Ended |
--- | --- | ---
Mar. 31, 2025 | Sep. 30, 2024
Goodwill [Line Items] |  |  |
Amount allocated to Avoca disposal group | [1] | $ 1 |
Discontinued Operations, Held-for-sale [Member] |  |  |
Goodwill [Line Items] |  |  |
Amount allocated to Avoca disposal group |  | 1 |
Personal Care [Member] |  |  |
Goodwill [Line Items] |  |  |
Accumulated impairment |  | 356 | $ 356
Amount allocated to Avoca disposal group | [1],[2] | 1 |
Specialty Additives [Member] |  |  |
Goodwill [Line Items] |  |  |
Accumulated impairment |  | 174 | 174
Amount allocated to Avoca disposal group | [1],[2] | 0 |
Intermediates [Member] |  |  |
Goodwill [Line Items] |  |  |
Accumulated impairment |  | 90 | $ 90
Amount allocated to Avoca disposal group | [1],[2] | $ 0 |
[1] | Ashland allocated $1 million to the Avoca disposal group during the six months ended March 31, 2025. See Note B of the Notes to the Condensed Consolidated Financial Statements for more information.
[2] | As of March 31, 2025 and September 30, 2024, there were accumulated impairments of $356 million, $174 million and $90 million related to the Personal Care, Specialty Additives and Intermediates reportable segments, respectively.

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