Reportable Segment Information - Additional Information (Details) - ReportableSegment |
9 Months Ended | |
|---|---|---|
Jun. 30, 2026 |
Jun. 30, 2025 |
|
| Segment Reporting [Abstract] | ||
| Number of reportable segments not disclosed flag | 0 | 0 |
| Segment Reporting, CODM, Profit (Loss) Measure, How Used, Description | EBITDA is the primary measures of performance that are reviewed by the chief operating decision maker in assessing each reportable segment's financial performance. Ashland does not aggregate operating segments to arrive at these reportable segments. | |
| Segment Reporting, CODM, Individual Title and Position or Group Name [Extensible Enumeration] | Chair and Chief Executive Officer [Member] | |