Reportable Segment Information - Additional Information (Details) - ReportableSegment	3 Months Ended	6 Months Ended
Mar. 31, 2026	Mar. 31, 2026	Mar. 31, 2025
Segment Reporting [Abstract]
Number of reportable segments not disclosed flag	0		0
Segment Reporting, CODM, Profit (Loss) Measure, How Used, Description		Operating income (loss) before income taxes, depreciation and amortization ("EBITDA") are 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]

Indicates title and position of individual or name of group identified as chief operating decision maker (CODM) for segment reporting.

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 54
-Subparagraph (f)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482785/280-10-55-54

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 47
-Subparagraph (e)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482785/280-10-55-47

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 21
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482810/280-10-50-21

Name:	us-gaap_SegmentReportingCodmIndividualTitleAndPositionOrGroupOrCommitteeNameExtensibleEnumeration
Namespace Prefix:	us-gaap_
Data Type:	enum2:enumerationSetItemType
Balance Type:	na
Period Type:	duration