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Reportable Segment Information - Summary of Financial Information for Each Reportable Segment (Parenthetical) (Details) $ in Millions | 6 Months Ended
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Mar. 31, 2021 USD ($)
Specialty Additives [Member] |
Segment Reporting Information [Line Items] |
Capital project impairment | $ 9

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- Definition Amount recognized as an operating expense or loss during the period to reduce the carrying amount of a project that has been impaired but not abandoned. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 45 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=123351718&loc=d3e2420-110228 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=123570139&loc=d3e3602-108585 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=ash_SpecialtyAdditivesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |